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Source file src/runtime/mgcmark.go

Documentation: runtime

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Garbage collector: marking and scanning
     6  
     7  package runtime
     8  
     9  import (
    10  	"runtime/internal/atomic"
    11  	"runtime/internal/sys"
    12  	"unsafe"
    13  )
    14  
    15  const (
    16  	fixedRootFinalizers = iota
    17  	fixedRootFreeGStacks
    18  	fixedRootCount
    19  
    20  	// rootBlockBytes is the number of bytes to scan per data or
    21  	// BSS root.
    22  	rootBlockBytes = 256 << 10
    23  
    24  	// maxObletBytes is the maximum bytes of an object to scan at
    25  	// once. Larger objects will be split up into "oblets" of at
    26  	// most this size. Since we can scan 1–2 MB/ms, 128 KB bounds
    27  	// scan preemption at ~100 µs.
    28  	//
    29  	// This must be > _MaxSmallSize so that the object base is the
    30  	// span base.
    31  	maxObletBytes = 128 << 10
    32  
    33  	// drainCheckThreshold specifies how many units of work to do
    34  	// between self-preemption checks in gcDrain. Assuming a scan
    35  	// rate of 1 MB/ms, this is ~100 µs. Lower values have higher
    36  	// overhead in the scan loop (the scheduler check may perform
    37  	// a syscall, so its overhead is nontrivial). Higher values
    38  	// make the system less responsive to incoming work.
    39  	drainCheckThreshold = 100000
    40  
    41  	// pagesPerSpanRoot indicates how many pages to scan from a span root
    42  	// at a time. Used by special root marking.
    43  	//
    44  	// Higher values improve throughput by increasing locality, but
    45  	// increase the minimum latency of a marking operation.
    46  	//
    47  	// Must be a multiple of the pageInUse bitmap element size and
    48  	// must also evenly divide pagesPerArena.
    49  	pagesPerSpanRoot = 512
    50  )
    51  
    52  // gcMarkRootPrepare queues root scanning jobs (stacks, globals, and
    53  // some miscellany) and initializes scanning-related state.
    54  //
    55  // The world must be stopped.
    56  func gcMarkRootPrepare() {
    57  	assertWorldStopped()
    58  
    59  	work.nFlushCacheRoots = 0
    60  
    61  	// Compute how many data and BSS root blocks there are.
    62  	nBlocks := func(bytes uintptr) int {
    63  		return int(divRoundUp(bytes, rootBlockBytes))
    64  	}
    65  
    66  	work.nDataRoots = 0
    67  	work.nBSSRoots = 0
    68  
    69  	// Scan globals.
    70  	for _, datap := range activeModules() {
    71  		nDataRoots := nBlocks(datap.edata - datap.data)
    72  		if nDataRoots > work.nDataRoots {
    73  			work.nDataRoots = nDataRoots
    74  		}
    75  	}
    76  
    77  	for _, datap := range activeModules() {
    78  		nBSSRoots := nBlocks(datap.ebss - datap.bss)
    79  		if nBSSRoots > work.nBSSRoots {
    80  			work.nBSSRoots = nBSSRoots
    81  		}
    82  	}
    83  
    84  	// Scan span roots for finalizer specials.
    85  	//
    86  	// We depend on addfinalizer to mark objects that get
    87  	// finalizers after root marking.
    88  	//
    89  	// We're going to scan the whole heap (that was available at the time the
    90  	// mark phase started, i.e. markArenas) for in-use spans which have specials.
    91  	//
    92  	// Break up the work into arenas, and further into chunks.
    93  	//
    94  	// Snapshot allArenas as markArenas. This snapshot is safe because allArenas
    95  	// is append-only.
    96  	mheap_.markArenas = mheap_.allArenas[:len(mheap_.allArenas):len(mheap_.allArenas)]
    97  	work.nSpanRoots = len(mheap_.markArenas) * (pagesPerArena / pagesPerSpanRoot)
    98  
    99  	// Scan stacks.
   100  	//
   101  	// Gs may be created after this point, but it's okay that we
   102  	// ignore them because they begin life without any roots, so
   103  	// there's nothing to scan, and any roots they create during
   104  	// the concurrent phase will be caught by the write barrier.
   105  	work.nStackRoots = int(atomic.Loaduintptr(&allglen))
   106  
   107  	work.markrootNext = 0
   108  	work.markrootJobs = uint32(fixedRootCount + work.nFlushCacheRoots + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots)
   109  }
   110  
   111  // gcMarkRootCheck checks that all roots have been scanned. It is
   112  // purely for debugging.
   113  func gcMarkRootCheck() {
   114  	if work.markrootNext < work.markrootJobs {
   115  		print(work.markrootNext, " of ", work.markrootJobs, " markroot jobs done\n")
   116  		throw("left over markroot jobs")
   117  	}
   118  
   119  	lock(&allglock)
   120  	// Check that stacks have been scanned.
   121  	var gp *g
   122  	for i := 0; i < work.nStackRoots; i++ {
   123  		gp = allgs[i]
   124  		if !gp.gcscandone {
   125  			goto fail
   126  		}
   127  	}
   128  	unlock(&allglock)
   129  	return
   130  
   131  fail:
   132  	println("gp", gp, "goid", gp.goid,
   133  		"status", readgstatus(gp),
   134  		"gcscandone", gp.gcscandone)
   135  	throw("scan missed a g")
   136  }
   137  
   138  // ptrmask for an allocation containing a single pointer.
   139  var oneptrmask = [...]uint8{1}
   140  
   141  // markroot scans the i'th root.
   142  //
   143  // Preemption must be disabled (because this uses a gcWork).
   144  //
   145  // nowritebarrier is only advisory here.
   146  //
   147  //go:nowritebarrier
   148  func markroot(gcw *gcWork, i uint32) {
   149  	// TODO(austin): This is a bit ridiculous. Compute and store
   150  	// the bases in gcMarkRootPrepare instead of the counts.
   151  	baseFlushCache := uint32(fixedRootCount)
   152  	baseData := baseFlushCache + uint32(work.nFlushCacheRoots)
   153  	baseBSS := baseData + uint32(work.nDataRoots)
   154  	baseSpans := baseBSS + uint32(work.nBSSRoots)
   155  	baseStacks := baseSpans + uint32(work.nSpanRoots)
   156  	end := baseStacks + uint32(work.nStackRoots)
   157  
   158  	// Note: if you add a case here, please also update heapdump.go:dumproots.
   159  	switch {
   160  	case baseFlushCache <= i && i < baseData:
   161  		flushmcache(int(i - baseFlushCache))
   162  
   163  	case baseData <= i && i < baseBSS:
   164  		for _, datap := range activeModules() {
   165  			markrootBlock(datap.data, datap.edata-datap.data, datap.gcdatamask.bytedata, gcw, int(i-baseData))
   166  		}
   167  
   168  	case baseBSS <= i && i < baseSpans:
   169  		for _, datap := range activeModules() {
   170  			markrootBlock(datap.bss, datap.ebss-datap.bss, datap.gcbssmask.bytedata, gcw, int(i-baseBSS))
   171  		}
   172  
   173  	case i == fixedRootFinalizers:
   174  		for fb := allfin; fb != nil; fb = fb.alllink {
   175  			cnt := uintptr(atomic.Load(&fb.cnt))
   176  			scanblock(uintptr(unsafe.Pointer(&fb.fin[0])), cnt*unsafe.Sizeof(fb.fin[0]), &finptrmask[0], gcw, nil)
   177  		}
   178  
   179  	case i == fixedRootFreeGStacks:
   180  		// Switch to the system stack so we can call
   181  		// stackfree.
   182  		systemstack(markrootFreeGStacks)
   183  
   184  	case baseSpans <= i && i < baseStacks:
   185  		// mark mspan.specials
   186  		markrootSpans(gcw, int(i-baseSpans))
   187  
   188  	default:
   189  		// the rest is scanning goroutine stacks
   190  		var gp *g
   191  		if baseStacks <= i && i < end {
   192  			gp = allgs[i-baseStacks]
   193  		} else {
   194  			throw("markroot: bad index")
   195  		}
   196  
   197  		// remember when we've first observed the G blocked
   198  		// needed only to output in traceback
   199  		status := readgstatus(gp) // We are not in a scan state
   200  		if (status == _Gwaiting || status == _Gsyscall) && gp.waitsince == 0 {
   201  			gp.waitsince = work.tstart
   202  		}
   203  
   204  		// scanstack must be done on the system stack in case
   205  		// we're trying to scan our own stack.
   206  		systemstack(func() {
   207  			// If this is a self-scan, put the user G in
   208  			// _Gwaiting to prevent self-deadlock. It may
   209  			// already be in _Gwaiting if this is a mark
   210  			// worker or we're in mark termination.
   211  			userG := getg().m.curg
   212  			selfScan := gp == userG && readgstatus(userG) == _Grunning
   213  			if selfScan {
   214  				casgstatus(userG, _Grunning, _Gwaiting)
   215  				userG.waitreason = waitReasonGarbageCollectionScan
   216  			}
   217  
   218  			// TODO: suspendG blocks (and spins) until gp
   219  			// stops, which may take a while for
   220  			// running goroutines. Consider doing this in
   221  			// two phases where the first is non-blocking:
   222  			// we scan the stacks we can and ask running
   223  			// goroutines to scan themselves; and the
   224  			// second blocks.
   225  			stopped := suspendG(gp)
   226  			if stopped.dead {
   227  				gp.gcscandone = true
   228  				return
   229  			}
   230  			if gp.gcscandone {
   231  				throw("g already scanned")
   232  			}
   233  			scanstack(gp, gcw)
   234  			gp.gcscandone = true
   235  			resumeG(stopped)
   236  
   237  			if selfScan {
   238  				casgstatus(userG, _Gwaiting, _Grunning)
   239  			}
   240  		})
   241  	}
   242  }
   243  
   244  // markrootBlock scans the shard'th shard of the block of memory [b0,
   245  // b0+n0), with the given pointer mask.
   246  //
   247  //go:nowritebarrier
   248  func markrootBlock(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard int) {
   249  	if rootBlockBytes%(8*sys.PtrSize) != 0 {
   250  		// This is necessary to pick byte offsets in ptrmask0.
   251  		throw("rootBlockBytes must be a multiple of 8*ptrSize")
   252  	}
   253  
   254  	// Note that if b0 is toward the end of the address space,
   255  	// then b0 + rootBlockBytes might wrap around.
   256  	// These tests are written to avoid any possible overflow.
   257  	off := uintptr(shard) * rootBlockBytes
   258  	if off >= n0 {
   259  		return
   260  	}
   261  	b := b0 + off
   262  	ptrmask := (*uint8)(add(unsafe.Pointer(ptrmask0), uintptr(shard)*(rootBlockBytes/(8*sys.PtrSize))))
   263  	n := uintptr(rootBlockBytes)
   264  	if off+n > n0 {
   265  		n = n0 - off
   266  	}
   267  
   268  	// Scan this shard.
   269  	scanblock(b, n, ptrmask, gcw, nil)
   270  }
   271  
   272  // markrootFreeGStacks frees stacks of dead Gs.
   273  //
   274  // This does not free stacks of dead Gs cached on Ps, but having a few
   275  // cached stacks around isn't a problem.
   276  func markrootFreeGStacks() {
   277  	// Take list of dead Gs with stacks.
   278  	lock(&sched.gFree.lock)
   279  	list := sched.gFree.stack
   280  	sched.gFree.stack = gList{}
   281  	unlock(&sched.gFree.lock)
   282  	if list.empty() {
   283  		return
   284  	}
   285  
   286  	// Free stacks.
   287  	q := gQueue{list.head, list.head}
   288  	for gp := list.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
   289  		stackfree(gp.stack)
   290  		gp.stack.lo = 0
   291  		gp.stack.hi = 0
   292  		// Manipulate the queue directly since the Gs are
   293  		// already all linked the right way.
   294  		q.tail.set(gp)
   295  	}
   296  
   297  	// Put Gs back on the free list.
   298  	lock(&sched.gFree.lock)
   299  	sched.gFree.noStack.pushAll(q)
   300  	unlock(&sched.gFree.lock)
   301  }
   302  
   303  // markrootSpans marks roots for one shard of markArenas.
   304  //
   305  //go:nowritebarrier
   306  func markrootSpans(gcw *gcWork, shard int) {
   307  	// Objects with finalizers have two GC-related invariants:
   308  	//
   309  	// 1) Everything reachable from the object must be marked.
   310  	// This ensures that when we pass the object to its finalizer,
   311  	// everything the finalizer can reach will be retained.
   312  	//
   313  	// 2) Finalizer specials (which are not in the garbage
   314  	// collected heap) are roots. In practice, this means the fn
   315  	// field must be scanned.
   316  	sg := mheap_.sweepgen
   317  
   318  	// Find the arena and page index into that arena for this shard.
   319  	ai := mheap_.markArenas[shard/(pagesPerArena/pagesPerSpanRoot)]
   320  	ha := mheap_.arenas[ai.l1()][ai.l2()]
   321  	arenaPage := uint(uintptr(shard) * pagesPerSpanRoot % pagesPerArena)
   322  
   323  	// Construct slice of bitmap which we'll iterate over.
   324  	specialsbits := ha.pageSpecials[arenaPage/8:]
   325  	specialsbits = specialsbits[:pagesPerSpanRoot/8]
   326  	for i := range specialsbits {
   327  		// Find set bits, which correspond to spans with specials.
   328  		specials := atomic.Load8(&specialsbits[i])
   329  		if specials == 0 {
   330  			continue
   331  		}
   332  		for j := uint(0); j < 8; j++ {
   333  			if specials&(1<<j) == 0 {
   334  				continue
   335  			}
   336  			// Find the span for this bit.
   337  			//
   338  			// This value is guaranteed to be non-nil because having
   339  			// specials implies that the span is in-use, and since we're
   340  			// currently marking we can be sure that we don't have to worry
   341  			// about the span being freed and re-used.
   342  			s := ha.spans[arenaPage+uint(i)*8+j]
   343  
   344  			// The state must be mSpanInUse if the specials bit is set, so
   345  			// sanity check that.
   346  			if state := s.state.get(); state != mSpanInUse {
   347  				print("s.state = ", state, "\n")
   348  				throw("non in-use span found with specials bit set")
   349  			}
   350  			// Check that this span was swept (it may be cached or uncached).
   351  			if !useCheckmark && !(s.sweepgen == sg || s.sweepgen == sg+3) {
   352  				// sweepgen was updated (+2) during non-checkmark GC pass
   353  				print("sweep ", s.sweepgen, " ", sg, "\n")
   354  				throw("gc: unswept span")
   355  			}
   356  
   357  			// Lock the specials to prevent a special from being
   358  			// removed from the list while we're traversing it.
   359  			lock(&s.speciallock)
   360  			for sp := s.specials; sp != nil; sp = sp.next {
   361  				if sp.kind != _KindSpecialFinalizer {
   362  					continue
   363  				}
   364  				// don't mark finalized object, but scan it so we
   365  				// retain everything it points to.
   366  				spf := (*specialfinalizer)(unsafe.Pointer(sp))
   367  				// A finalizer can be set for an inner byte of an object, find object beginning.
   368  				p := s.base() + uintptr(spf.special.offset)/s.elemsize*s.elemsize
   369  
   370  				// Mark everything that can be reached from
   371  				// the object (but *not* the object itself or
   372  				// we'll never collect it).
   373  				scanobject(p, gcw)
   374  
   375  				// The special itself is a root.
   376  				scanblock(uintptr(unsafe.Pointer(&spf.fn)), sys.PtrSize, &oneptrmask[0], gcw, nil)
   377  			}
   378  			unlock(&s.speciallock)
   379  		}
   380  	}
   381  }
   382  
   383  // gcAssistAlloc performs GC work to make gp's assist debt positive.
   384  // gp must be the calling user gorountine.
   385  //
   386  // This must be called with preemption enabled.
   387  func gcAssistAlloc(gp *g) {
   388  	// Don't assist in non-preemptible contexts. These are
   389  	// generally fragile and won't allow the assist to block.
   390  	if getg() == gp.m.g0 {
   391  		return
   392  	}
   393  	if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" {
   394  		return
   395  	}
   396  
   397  	traced := false
   398  retry:
   399  	// Compute the amount of scan work we need to do to make the
   400  	// balance positive. When the required amount of work is low,
   401  	// we over-assist to build up credit for future allocations
   402  	// and amortize the cost of assisting.
   403  	assistWorkPerByte := float64frombits(atomic.Load64(&gcController.assistWorkPerByte))
   404  	assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
   405  	debtBytes := -gp.gcAssistBytes
   406  	scanWork := int64(assistWorkPerByte * float64(debtBytes))
   407  	if scanWork < gcOverAssistWork {
   408  		scanWork = gcOverAssistWork
   409  		debtBytes = int64(assistBytesPerWork * float64(scanWork))
   410  	}
   411  
   412  	// Steal as much credit as we can from the background GC's
   413  	// scan credit. This is racy and may drop the background
   414  	// credit below 0 if two mutators steal at the same time. This
   415  	// will just cause steals to fail until credit is accumulated
   416  	// again, so in the long run it doesn't really matter, but we
   417  	// do have to handle the negative credit case.
   418  	bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit)
   419  	stolen := int64(0)
   420  	if bgScanCredit > 0 {
   421  		if bgScanCredit < scanWork {
   422  			stolen = bgScanCredit
   423  			gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(stolen))
   424  		} else {
   425  			stolen = scanWork
   426  			gp.gcAssistBytes += debtBytes
   427  		}
   428  		atomic.Xaddint64(&gcController.bgScanCredit, -stolen)
   429  
   430  		scanWork -= stolen
   431  
   432  		if scanWork == 0 {
   433  			// We were able to steal all of the credit we
   434  			// needed.
   435  			if traced {
   436  				traceGCMarkAssistDone()
   437  			}
   438  			return
   439  		}
   440  	}
   441  
   442  	if trace.enabled && !traced {
   443  		traced = true
   444  		traceGCMarkAssistStart()
   445  	}
   446  
   447  	// Perform assist work
   448  	systemstack(func() {
   449  		gcAssistAlloc1(gp, scanWork)
   450  		// The user stack may have moved, so this can't touch
   451  		// anything on it until it returns from systemstack.
   452  	})
   453  
   454  	completed := gp.param != nil
   455  	gp.param = nil
   456  	if completed {
   457  		gcMarkDone()
   458  	}
   459  
   460  	if gp.gcAssistBytes < 0 {
   461  		// We were unable steal enough credit or perform
   462  		// enough work to pay off the assist debt. We need to
   463  		// do one of these before letting the mutator allocate
   464  		// more to prevent over-allocation.
   465  		//
   466  		// If this is because we were preempted, reschedule
   467  		// and try some more.
   468  		if gp.preempt {
   469  			Gosched()
   470  			goto retry
   471  		}
   472  
   473  		// Add this G to an assist queue and park. When the GC
   474  		// has more background credit, it will satisfy queued
   475  		// assists before flushing to the global credit pool.
   476  		//
   477  		// Note that this does *not* get woken up when more
   478  		// work is added to the work list. The theory is that
   479  		// there wasn't enough work to do anyway, so we might
   480  		// as well let background marking take care of the
   481  		// work that is available.
   482  		if !gcParkAssist() {
   483  			goto retry
   484  		}
   485  
   486  		// At this point either background GC has satisfied
   487  		// this G's assist debt, or the GC cycle is over.
   488  	}
   489  	if traced {
   490  		traceGCMarkAssistDone()
   491  	}
   492  }
   493  
   494  // gcAssistAlloc1 is the part of gcAssistAlloc that runs on the system
   495  // stack. This is a separate function to make it easier to see that
   496  // we're not capturing anything from the user stack, since the user
   497  // stack may move while we're in this function.
   498  //
   499  // gcAssistAlloc1 indicates whether this assist completed the mark
   500  // phase by setting gp.param to non-nil. This can't be communicated on
   501  // the stack since it may move.
   502  //
   503  //go:systemstack
   504  func gcAssistAlloc1(gp *g, scanWork int64) {
   505  	// Clear the flag indicating that this assist completed the
   506  	// mark phase.
   507  	gp.param = nil
   508  
   509  	if atomic.Load(&gcBlackenEnabled) == 0 {
   510  		// The gcBlackenEnabled check in malloc races with the
   511  		// store that clears it but an atomic check in every malloc
   512  		// would be a performance hit.
   513  		// Instead we recheck it here on the non-preemptable system
   514  		// stack to determine if we should perform an assist.
   515  
   516  		// GC is done, so ignore any remaining debt.
   517  		gp.gcAssistBytes = 0
   518  		return
   519  	}
   520  	// Track time spent in this assist. Since we're on the
   521  	// system stack, this is non-preemptible, so we can
   522  	// just measure start and end time.
   523  	startTime := nanotime()
   524  
   525  	decnwait := atomic.Xadd(&work.nwait, -1)
   526  	if decnwait == work.nproc {
   527  		println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc)
   528  		throw("nwait > work.nprocs")
   529  	}
   530  
   531  	// gcDrainN requires the caller to be preemptible.
   532  	casgstatus(gp, _Grunning, _Gwaiting)
   533  	gp.waitreason = waitReasonGCAssistMarking
   534  
   535  	// drain own cached work first in the hopes that it
   536  	// will be more cache friendly.
   537  	gcw := &getg().m.p.ptr().gcw
   538  	workDone := gcDrainN(gcw, scanWork)
   539  
   540  	casgstatus(gp, _Gwaiting, _Grunning)
   541  
   542  	// Record that we did this much scan work.
   543  	//
   544  	// Back out the number of bytes of assist credit that
   545  	// this scan work counts for. The "1+" is a poor man's
   546  	// round-up, to ensure this adds credit even if
   547  	// assistBytesPerWork is very low.
   548  	assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
   549  	gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(workDone))
   550  
   551  	// If this is the last worker and we ran out of work,
   552  	// signal a completion point.
   553  	incnwait := atomic.Xadd(&work.nwait, +1)
   554  	if incnwait > work.nproc {
   555  		println("runtime: work.nwait=", incnwait,
   556  			"work.nproc=", work.nproc)
   557  		throw("work.nwait > work.nproc")
   558  	}
   559  
   560  	if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
   561  		// This has reached a background completion point. Set
   562  		// gp.param to a non-nil value to indicate this. It
   563  		// doesn't matter what we set it to (it just has to be
   564  		// a valid pointer).
   565  		gp.param = unsafe.Pointer(gp)
   566  	}
   567  	duration := nanotime() - startTime
   568  	_p_ := gp.m.p.ptr()
   569  	_p_.gcAssistTime += duration
   570  	if _p_.gcAssistTime > gcAssistTimeSlack {
   571  		atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime)
   572  		_p_.gcAssistTime = 0
   573  	}
   574  }
   575  
   576  // gcWakeAllAssists wakes all currently blocked assists. This is used
   577  // at the end of a GC cycle. gcBlackenEnabled must be false to prevent
   578  // new assists from going to sleep after this point.
   579  func gcWakeAllAssists() {
   580  	lock(&work.assistQueue.lock)
   581  	list := work.assistQueue.q.popList()
   582  	injectglist(&list)
   583  	unlock(&work.assistQueue.lock)
   584  }
   585  
   586  // gcParkAssist puts the current goroutine on the assist queue and parks.
   587  //
   588  // gcParkAssist reports whether the assist is now satisfied. If it
   589  // returns false, the caller must retry the assist.
   590  //
   591  //go:nowritebarrier
   592  func gcParkAssist() bool {
   593  	lock(&work.assistQueue.lock)
   594  	// If the GC cycle finished while we were getting the lock,
   595  	// exit the assist. The cycle can't finish while we hold the
   596  	// lock.
   597  	if atomic.Load(&gcBlackenEnabled) == 0 {
   598  		unlock(&work.assistQueue.lock)
   599  		return true
   600  	}
   601  
   602  	gp := getg()
   603  	oldList := work.assistQueue.q
   604  	work.assistQueue.q.pushBack(gp)
   605  
   606  	// Recheck for background credit now that this G is in
   607  	// the queue, but can still back out. This avoids a
   608  	// race in case background marking has flushed more
   609  	// credit since we checked above.
   610  	if atomic.Loadint64(&gcController.bgScanCredit) > 0 {
   611  		work.assistQueue.q = oldList
   612  		if oldList.tail != 0 {
   613  			oldList.tail.ptr().schedlink.set(nil)
   614  		}
   615  		unlock(&work.assistQueue.lock)
   616  		return false
   617  	}
   618  	// Park.
   619  	goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceEvGoBlockGC, 2)
   620  	return true
   621  }
   622  
   623  // gcFlushBgCredit flushes scanWork units of background scan work
   624  // credit. This first satisfies blocked assists on the
   625  // work.assistQueue and then flushes any remaining credit to
   626  // gcController.bgScanCredit.
   627  //
   628  // Write barriers are disallowed because this is used by gcDrain after
   629  // it has ensured that all work is drained and this must preserve that
   630  // condition.
   631  //
   632  //go:nowritebarrierrec
   633  func gcFlushBgCredit(scanWork int64) {
   634  	if work.assistQueue.q.empty() {
   635  		// Fast path; there are no blocked assists. There's a
   636  		// small window here where an assist may add itself to
   637  		// the blocked queue and park. If that happens, we'll
   638  		// just get it on the next flush.
   639  		atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
   640  		return
   641  	}
   642  
   643  	assistBytesPerWork := float64frombits(atomic.Load64(&gcController.assistBytesPerWork))
   644  	scanBytes := int64(float64(scanWork) * assistBytesPerWork)
   645  
   646  	lock(&work.assistQueue.lock)
   647  	for !work.assistQueue.q.empty() && scanBytes > 0 {
   648  		gp := work.assistQueue.q.pop()
   649  		// Note that gp.gcAssistBytes is negative because gp
   650  		// is in debt. Think carefully about the signs below.
   651  		if scanBytes+gp.gcAssistBytes >= 0 {
   652  			// Satisfy this entire assist debt.
   653  			scanBytes += gp.gcAssistBytes
   654  			gp.gcAssistBytes = 0
   655  			// It's important that we *not* put gp in
   656  			// runnext. Otherwise, it's possible for user
   657  			// code to exploit the GC worker's high
   658  			// scheduler priority to get itself always run
   659  			// before other goroutines and always in the
   660  			// fresh quantum started by GC.
   661  			ready(gp, 0, false)
   662  		} else {
   663  			// Partially satisfy this assist.
   664  			gp.gcAssistBytes += scanBytes
   665  			scanBytes = 0
   666  			// As a heuristic, we move this assist to the
   667  			// back of the queue so that large assists
   668  			// can't clog up the assist queue and
   669  			// substantially delay small assists.
   670  			work.assistQueue.q.pushBack(gp)
   671  			break
   672  		}
   673  	}
   674  
   675  	if scanBytes > 0 {
   676  		// Convert from scan bytes back to work.
   677  		assistWorkPerByte := float64frombits(atomic.Load64(&gcController.assistWorkPerByte))
   678  		scanWork = int64(float64(scanBytes) * assistWorkPerByte)
   679  		atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
   680  	}
   681  	unlock(&work.assistQueue.lock)
   682  }
   683  
   684  // scanstack scans gp's stack, greying all pointers found on the stack.
   685  //
   686  // scanstack will also shrink the stack if it is safe to do so. If it
   687  // is not, it schedules a stack shrink for the next synchronous safe
   688  // point.
   689  //
   690  // scanstack is marked go:systemstack because it must not be preempted
   691  // while using a workbuf.
   692  //
   693  //go:nowritebarrier
   694  //go:systemstack
   695  func scanstack(gp *g, gcw *gcWork) {
   696  	if readgstatus(gp)&_Gscan == 0 {
   697  		print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n")
   698  		throw("scanstack - bad status")
   699  	}
   700  
   701  	switch readgstatus(gp) &^ _Gscan {
   702  	default:
   703  		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
   704  		throw("mark - bad status")
   705  	case _Gdead:
   706  		return
   707  	case _Grunning:
   708  		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
   709  		throw("scanstack: goroutine not stopped")
   710  	case _Grunnable, _Gsyscall, _Gwaiting:
   711  		// ok
   712  	}
   713  
   714  	if gp == getg() {
   715  		throw("can't scan our own stack")
   716  	}
   717  
   718  	if isShrinkStackSafe(gp) {
   719  		// Shrink the stack if not much of it is being used.
   720  		shrinkstack(gp)
   721  	} else {
   722  		// Otherwise, shrink the stack at the next sync safe point.
   723  		gp.preemptShrink = true
   724  	}
   725  
   726  	var state stackScanState
   727  	state.stack = gp.stack
   728  
   729  	if stackTraceDebug {
   730  		println("stack trace goroutine", gp.goid)
   731  	}
   732  
   733  	if debugScanConservative && gp.asyncSafePoint {
   734  		print("scanning async preempted goroutine ", gp.goid, " stack [", hex(gp.stack.lo), ",", hex(gp.stack.hi), ")\n")
   735  	}
   736  
   737  	// Scan the saved context register. This is effectively a live
   738  	// register that gets moved back and forth between the
   739  	// register and sched.ctxt without a write barrier.
   740  	if gp.sched.ctxt != nil {
   741  		scanblock(uintptr(unsafe.Pointer(&gp.sched.ctxt)), sys.PtrSize, &oneptrmask[0], gcw, &state)
   742  	}
   743  
   744  	// Scan the stack. Accumulate a list of stack objects.
   745  	scanframe := func(frame *stkframe, unused unsafe.Pointer) bool {
   746  		scanframeworker(frame, &state, gcw)
   747  		return true
   748  	}
   749  	gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, scanframe, nil, 0)
   750  
   751  	// Find additional pointers that point into the stack from the heap.
   752  	// Currently this includes defers and panics. See also function copystack.
   753  
   754  	// Find and trace all defer arguments.
   755  	tracebackdefers(gp, scanframe, nil)
   756  
   757  	// Find and trace other pointers in defer records.
   758  	for d := gp._defer; d != nil; d = d.link {
   759  		if d.fn != nil {
   760  			// tracebackdefers above does not scan the func value, which could
   761  			// be a stack allocated closure. See issue 30453.
   762  			scanblock(uintptr(unsafe.Pointer(&d.fn)), sys.PtrSize, &oneptrmask[0], gcw, &state)
   763  		}
   764  		if d.link != nil {
   765  			// The link field of a stack-allocated defer record might point
   766  			// to a heap-allocated defer record. Keep that heap record live.
   767  			scanblock(uintptr(unsafe.Pointer(&d.link)), sys.PtrSize, &oneptrmask[0], gcw, &state)
   768  		}
   769  		// Retain defers records themselves.
   770  		// Defer records might not be reachable from the G through regular heap
   771  		// tracing because the defer linked list might weave between the stack and the heap.
   772  		if d.heap {
   773  			scanblock(uintptr(unsafe.Pointer(&d)), sys.PtrSize, &oneptrmask[0], gcw, &state)
   774  		}
   775  	}
   776  	if gp._panic != nil {
   777  		// Panics are always stack allocated.
   778  		state.putPtr(uintptr(unsafe.Pointer(gp._panic)), false)
   779  	}
   780  
   781  	// Find and scan all reachable stack objects.
   782  	//
   783  	// The state's pointer queue prioritizes precise pointers over
   784  	// conservative pointers so that we'll prefer scanning stack
   785  	// objects precisely.
   786  	state.buildIndex()
   787  	for {
   788  		p, conservative := state.getPtr()
   789  		if p == 0 {
   790  			break
   791  		}
   792  		obj := state.findObject(p)
   793  		if obj == nil {
   794  			continue
   795  		}
   796  		t := obj.typ
   797  		if t == nil {
   798  			// We've already scanned this object.
   799  			continue
   800  		}
   801  		obj.setType(nil) // Don't scan it again.
   802  		if stackTraceDebug {
   803  			printlock()
   804  			print("  live stkobj at", hex(state.stack.lo+uintptr(obj.off)), "of type", t.string())
   805  			if conservative {
   806  				print(" (conservative)")
   807  			}
   808  			println()
   809  			printunlock()
   810  		}
   811  		gcdata := t.gcdata
   812  		var s *mspan
   813  		if t.kind&kindGCProg != 0 {
   814  			// This path is pretty unlikely, an object large enough
   815  			// to have a GC program allocated on the stack.
   816  			// We need some space to unpack the program into a straight
   817  			// bitmask, which we allocate/free here.
   818  			// TODO: it would be nice if there were a way to run a GC
   819  			// program without having to store all its bits. We'd have
   820  			// to change from a Lempel-Ziv style program to something else.
   821  			// Or we can forbid putting objects on stacks if they require
   822  			// a gc program (see issue 27447).
   823  			s = materializeGCProg(t.ptrdata, gcdata)
   824  			gcdata = (*byte)(unsafe.Pointer(s.startAddr))
   825  		}
   826  
   827  		b := state.stack.lo + uintptr(obj.off)
   828  		if conservative {
   829  			scanConservative(b, t.ptrdata, gcdata, gcw, &state)
   830  		} else {
   831  			scanblock(b, t.ptrdata, gcdata, gcw, &state)
   832  		}
   833  
   834  		if s != nil {
   835  			dematerializeGCProg(s)
   836  		}
   837  	}
   838  
   839  	// Deallocate object buffers.
   840  	// (Pointer buffers were all deallocated in the loop above.)
   841  	for state.head != nil {
   842  		x := state.head
   843  		state.head = x.next
   844  		if stackTraceDebug {
   845  			for i := 0; i < x.nobj; i++ {
   846  				obj := &x.obj[i]
   847  				if obj.typ == nil { // reachable
   848  					continue
   849  				}
   850  				println("  dead stkobj at", hex(gp.stack.lo+uintptr(obj.off)), "of type", obj.typ.string())
   851  				// Note: not necessarily really dead - only reachable-from-ptr dead.
   852  			}
   853  		}
   854  		x.nobj = 0
   855  		putempty((*workbuf)(unsafe.Pointer(x)))
   856  	}
   857  	if state.buf != nil || state.cbuf != nil || state.freeBuf != nil {
   858  		throw("remaining pointer buffers")
   859  	}
   860  }
   861  
   862  // Scan a stack frame: local variables and function arguments/results.
   863  //go:nowritebarrier
   864  func scanframeworker(frame *stkframe, state *stackScanState, gcw *gcWork) {
   865  	if _DebugGC > 1 && frame.continpc != 0 {
   866  		print("scanframe ", funcname(frame.fn), "\n")
   867  	}
   868  
   869  	isAsyncPreempt := frame.fn.valid() && frame.fn.funcID == funcID_asyncPreempt
   870  	isDebugCall := frame.fn.valid() && frame.fn.funcID == funcID_debugCallV1
   871  	if state.conservative || isAsyncPreempt || isDebugCall {
   872  		if debugScanConservative {
   873  			println("conservatively scanning function", funcname(frame.fn), "at PC", hex(frame.continpc))
   874  		}
   875  
   876  		// Conservatively scan the frame. Unlike the precise
   877  		// case, this includes the outgoing argument space
   878  		// since we may have stopped while this function was
   879  		// setting up a call.
   880  		//
   881  		// TODO: We could narrow this down if the compiler
   882  		// produced a single map per function of stack slots
   883  		// and registers that ever contain a pointer.
   884  		if frame.varp != 0 {
   885  			size := frame.varp - frame.sp
   886  			if size > 0 {
   887  				scanConservative(frame.sp, size, nil, gcw, state)
   888  			}
   889  		}
   890  
   891  		// Scan arguments to this frame.
   892  		if frame.arglen != 0 {
   893  			// TODO: We could pass the entry argument map
   894  			// to narrow this down further.
   895  			scanConservative(frame.argp, frame.arglen, nil, gcw, state)
   896  		}
   897  
   898  		if isAsyncPreempt || isDebugCall {
   899  			// This function's frame contained the
   900  			// registers for the asynchronously stopped
   901  			// parent frame. Scan the parent
   902  			// conservatively.
   903  			state.conservative = true
   904  		} else {
   905  			// We only wanted to scan those two frames
   906  			// conservatively. Clear the flag for future
   907  			// frames.
   908  			state.conservative = false
   909  		}
   910  		return
   911  	}
   912  
   913  	locals, args, objs := getStackMap(frame, &state.cache, false)
   914  
   915  	// Scan local variables if stack frame has been allocated.
   916  	if locals.n > 0 {
   917  		size := uintptr(locals.n) * sys.PtrSize
   918  		scanblock(frame.varp-size, size, locals.bytedata, gcw, state)
   919  	}
   920  
   921  	// Scan arguments.
   922  	if args.n > 0 {
   923  		scanblock(frame.argp, uintptr(args.n)*sys.PtrSize, args.bytedata, gcw, state)
   924  	}
   925  
   926  	// Add all stack objects to the stack object list.
   927  	if frame.varp != 0 {
   928  		// varp is 0 for defers, where there are no locals.
   929  		// In that case, there can't be a pointer to its args, either.
   930  		// (And all args would be scanned above anyway.)
   931  		for _, obj := range objs {
   932  			off := obj.off
   933  			base := frame.varp // locals base pointer
   934  			if off >= 0 {
   935  				base = frame.argp // arguments and return values base pointer
   936  			}
   937  			ptr := base + uintptr(off)
   938  			if ptr < frame.sp {
   939  				// object hasn't been allocated in the frame yet.
   940  				continue
   941  			}
   942  			if stackTraceDebug {
   943  				println("stkobj at", hex(ptr), "of type", obj.typ.string())
   944  			}
   945  			state.addObject(ptr, obj.typ)
   946  		}
   947  	}
   948  }
   949  
   950  type gcDrainFlags int
   951  
   952  const (
   953  	gcDrainUntilPreempt gcDrainFlags = 1 << iota
   954  	gcDrainFlushBgCredit
   955  	gcDrainIdle
   956  	gcDrainFractional
   957  )
   958  
   959  // gcDrain scans roots and objects in work buffers, blackening grey
   960  // objects until it is unable to get more work. It may return before
   961  // GC is done; it's the caller's responsibility to balance work from
   962  // other Ps.
   963  //
   964  // If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt
   965  // is set.
   966  //
   967  // If flags&gcDrainIdle != 0, gcDrain returns when there is other work
   968  // to do.
   969  //
   970  // If flags&gcDrainFractional != 0, gcDrain self-preempts when
   971  // pollFractionalWorkerExit() returns true. This implies
   972  // gcDrainNoBlock.
   973  //
   974  // If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
   975  // credit to gcController.bgScanCredit every gcCreditSlack units of
   976  // scan work.
   977  //
   978  // gcDrain will always return if there is a pending STW.
   979  //
   980  //go:nowritebarrier
   981  func gcDrain(gcw *gcWork, flags gcDrainFlags) {
   982  	if !writeBarrier.needed {
   983  		throw("gcDrain phase incorrect")
   984  	}
   985  
   986  	gp := getg().m.curg
   987  	preemptible := flags&gcDrainUntilPreempt != 0
   988  	flushBgCredit := flags&gcDrainFlushBgCredit != 0
   989  	idle := flags&gcDrainIdle != 0
   990  
   991  	initScanWork := gcw.scanWork
   992  
   993  	// checkWork is the scan work before performing the next
   994  	// self-preempt check.
   995  	checkWork := int64(1<<63 - 1)
   996  	var check func() bool
   997  	if flags&(gcDrainIdle|gcDrainFractional) != 0 {
   998  		checkWork = initScanWork + drainCheckThreshold
   999  		if idle {
  1000  			check = pollWork
  1001  		} else if flags&gcDrainFractional != 0 {
  1002  			check = pollFractionalWorkerExit
  1003  		}
  1004  	}
  1005  
  1006  	// Drain root marking jobs.
  1007  	if work.markrootNext < work.markrootJobs {
  1008  		// Stop if we're preemptible or if someone wants to STW.
  1009  		for !(gp.preempt && (preemptible || atomic.Load(&sched.gcwaiting) != 0)) {
  1010  			job := atomic.Xadd(&work.markrootNext, +1) - 1
  1011  			if job >= work.markrootJobs {
  1012  				break
  1013  			}
  1014  			markroot(gcw, job)
  1015  			if check != nil && check() {
  1016  				goto done
  1017  			}
  1018  		}
  1019  	}
  1020  
  1021  	// Drain heap marking jobs.
  1022  	// Stop if we're preemptible or if someone wants to STW.
  1023  	for !(gp.preempt && (preemptible || atomic.Load(&sched.gcwaiting) != 0)) {
  1024  		// Try to keep work available on the global queue. We used to
  1025  		// check if there were waiting workers, but it's better to
  1026  		// just keep work available than to make workers wait. In the
  1027  		// worst case, we'll do O(log(_WorkbufSize)) unnecessary
  1028  		// balances.
  1029  		if work.full == 0 {
  1030  			gcw.balance()
  1031  		}
  1032  
  1033  		b := gcw.tryGetFast()
  1034  		if b == 0 {
  1035  			b = gcw.tryGet()
  1036  			if b == 0 {
  1037  				// Flush the write barrier
  1038  				// buffer; this may create
  1039  				// more work.
  1040  				wbBufFlush(nil, 0)
  1041  				b = gcw.tryGet()
  1042  			}
  1043  		}
  1044  		if b == 0 {
  1045  			// Unable to get work.
  1046  			break
  1047  		}
  1048  		scanobject(b, gcw)
  1049  
  1050  		// Flush background scan work credit to the global
  1051  		// account if we've accumulated enough locally so
  1052  		// mutator assists can draw on it.
  1053  		if gcw.scanWork >= gcCreditSlack {
  1054  			atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
  1055  			if flushBgCredit {
  1056  				gcFlushBgCredit(gcw.scanWork - initScanWork)
  1057  				initScanWork = 0
  1058  			}
  1059  			checkWork -= gcw.scanWork
  1060  			gcw.scanWork = 0
  1061  
  1062  			if checkWork <= 0 {
  1063  				checkWork += drainCheckThreshold
  1064  				if check != nil && check() {
  1065  					break
  1066  				}
  1067  			}
  1068  		}
  1069  	}
  1070  
  1071  done:
  1072  	// Flush remaining scan work credit.
  1073  	if gcw.scanWork > 0 {
  1074  		atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
  1075  		if flushBgCredit {
  1076  			gcFlushBgCredit(gcw.scanWork - initScanWork)
  1077  		}
  1078  		gcw.scanWork = 0
  1079  	}
  1080  }
  1081  
  1082  // gcDrainN blackens grey objects until it has performed roughly
  1083  // scanWork units of scan work or the G is preempted. This is
  1084  // best-effort, so it may perform less work if it fails to get a work
  1085  // buffer. Otherwise, it will perform at least n units of work, but
  1086  // may perform more because scanning is always done in whole object
  1087  // increments. It returns the amount of scan work performed.
  1088  //
  1089  // The caller goroutine must be in a preemptible state (e.g.,
  1090  // _Gwaiting) to prevent deadlocks during stack scanning. As a
  1091  // consequence, this must be called on the system stack.
  1092  //
  1093  //go:nowritebarrier
  1094  //go:systemstack
  1095  func gcDrainN(gcw *gcWork, scanWork int64) int64 {
  1096  	if !writeBarrier.needed {
  1097  		throw("gcDrainN phase incorrect")
  1098  	}
  1099  
  1100  	// There may already be scan work on the gcw, which we don't
  1101  	// want to claim was done by this call.
  1102  	workFlushed := -gcw.scanWork
  1103  
  1104  	gp := getg().m.curg
  1105  	for !gp.preempt && workFlushed+gcw.scanWork < scanWork {
  1106  		// See gcDrain comment.
  1107  		if work.full == 0 {
  1108  			gcw.balance()
  1109  		}
  1110  
  1111  		// This might be a good place to add prefetch code...
  1112  		// if(wbuf.nobj > 4) {
  1113  		//         PREFETCH(wbuf->obj[wbuf.nobj - 3];
  1114  		//  }
  1115  		//
  1116  		b := gcw.tryGetFast()
  1117  		if b == 0 {
  1118  			b = gcw.tryGet()
  1119  			if b == 0 {
  1120  				// Flush the write barrier buffer;
  1121  				// this may create more work.
  1122  				wbBufFlush(nil, 0)
  1123  				b = gcw.tryGet()
  1124  			}
  1125  		}
  1126  
  1127  		if b == 0 {
  1128  			// Try to do a root job.
  1129  			//
  1130  			// TODO: Assists should get credit for this
  1131  			// work.
  1132  			if work.markrootNext < work.markrootJobs {
  1133  				job := atomic.Xadd(&work.markrootNext, +1) - 1
  1134  				if job < work.markrootJobs {
  1135  					markroot(gcw, job)
  1136  					continue
  1137  				}
  1138  			}
  1139  			// No heap or root jobs.
  1140  			break
  1141  		}
  1142  		scanobject(b, gcw)
  1143  
  1144  		// Flush background scan work credit.
  1145  		if gcw.scanWork >= gcCreditSlack {
  1146  			atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
  1147  			workFlushed += gcw.scanWork
  1148  			gcw.scanWork = 0
  1149  		}
  1150  	}
  1151  
  1152  	// Unlike gcDrain, there's no need to flush remaining work
  1153  	// here because this never flushes to bgScanCredit and
  1154  	// gcw.dispose will flush any remaining work to scanWork.
  1155  
  1156  	return workFlushed + gcw.scanWork
  1157  }
  1158  
  1159  // scanblock scans b as scanobject would, but using an explicit
  1160  // pointer bitmap instead of the heap bitmap.
  1161  //
  1162  // This is used to scan non-heap roots, so it does not update
  1163  // gcw.bytesMarked or gcw.scanWork.
  1164  //
  1165  // If stk != nil, possible stack pointers are also reported to stk.putPtr.
  1166  //go:nowritebarrier
  1167  func scanblock(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork, stk *stackScanState) {
  1168  	// Use local copies of original parameters, so that a stack trace
  1169  	// due to one of the throws below shows the original block
  1170  	// base and extent.
  1171  	b := b0
  1172  	n := n0
  1173  
  1174  	for i := uintptr(0); i < n; {
  1175  		// Find bits for the next word.
  1176  		bits := uint32(*addb(ptrmask, i/(sys.PtrSize*8)))
  1177  		if bits == 0 {
  1178  			i += sys.PtrSize * 8
  1179  			continue
  1180  		}
  1181  		for j := 0; j < 8 && i < n; j++ {
  1182  			if bits&1 != 0 {
  1183  				// Same work as in scanobject; see comments there.
  1184  				p := *(*uintptr)(unsafe.Pointer(b + i))
  1185  				if p != 0 {
  1186  					if obj, span, objIndex := findObject(p, b, i); obj != 0 {
  1187  						greyobject(obj, b, i, span, gcw, objIndex)
  1188  					} else if stk != nil && p >= stk.stack.lo && p < stk.stack.hi {
  1189  						stk.putPtr(p, false)
  1190  					}
  1191  				}
  1192  			}
  1193  			bits >>= 1
  1194  			i += sys.PtrSize
  1195  		}
  1196  	}
  1197  }
  1198  
  1199  // scanobject scans the object starting at b, adding pointers to gcw.
  1200  // b must point to the beginning of a heap object or an oblet.
  1201  // scanobject consults the GC bitmap for the pointer mask and the
  1202  // spans for the size of the object.
  1203  //
  1204  //go:nowritebarrier
  1205  func scanobject(b uintptr, gcw *gcWork) {
  1206  	// Find the bits for b and the size of the object at b.
  1207  	//
  1208  	// b is either the beginning of an object, in which case this
  1209  	// is the size of the object to scan, or it points to an
  1210  	// oblet, in which case we compute the size to scan below.
  1211  	hbits := heapBitsForAddr(b)
  1212  	s := spanOfUnchecked(b)
  1213  	n := s.elemsize
  1214  	if n == 0 {
  1215  		throw("scanobject n == 0")
  1216  	}
  1217  
  1218  	if n > maxObletBytes {
  1219  		// Large object. Break into oblets for better
  1220  		// parallelism and lower latency.
  1221  		if b == s.base() {
  1222  			// It's possible this is a noscan object (not
  1223  			// from greyobject, but from other code
  1224  			// paths), in which case we must *not* enqueue
  1225  			// oblets since their bitmaps will be
  1226  			// uninitialized.
  1227  			if s.spanclass.noscan() {
  1228  				// Bypass the whole scan.
  1229  				gcw.bytesMarked += uint64(n)
  1230  				return
  1231  			}
  1232  
  1233  			// Enqueue the other oblets to scan later.
  1234  			// Some oblets may be in b's scalar tail, but
  1235  			// these will be marked as "no more pointers",
  1236  			// so we'll drop out immediately when we go to
  1237  			// scan those.
  1238  			for oblet := b + maxObletBytes; oblet < s.base()+s.elemsize; oblet += maxObletBytes {
  1239  				if !gcw.putFast(oblet) {
  1240  					gcw.put(oblet)
  1241  				}
  1242  			}
  1243  		}
  1244  
  1245  		// Compute the size of the oblet. Since this object
  1246  		// must be a large object, s.base() is the beginning
  1247  		// of the object.
  1248  		n = s.base() + s.elemsize - b
  1249  		if n > maxObletBytes {
  1250  			n = maxObletBytes
  1251  		}
  1252  	}
  1253  
  1254  	var i uintptr
  1255  	for i = 0; i < n; i += sys.PtrSize {
  1256  		// Find bits for this word.
  1257  		if i != 0 {
  1258  			// Avoid needless hbits.next() on last iteration.
  1259  			hbits = hbits.next()
  1260  		}
  1261  		// Load bits once. See CL 22712 and issue 16973 for discussion.
  1262  		bits := hbits.bits()
  1263  		if bits&bitScan == 0 {
  1264  			break // no more pointers in this object
  1265  		}
  1266  		if bits&bitPointer == 0 {
  1267  			continue // not a pointer
  1268  		}
  1269  
  1270  		// Work here is duplicated in scanblock and above.
  1271  		// If you make changes here, make changes there too.
  1272  		obj := *(*uintptr)(unsafe.Pointer(b + i))
  1273  
  1274  		// At this point we have extracted the next potential pointer.
  1275  		// Quickly filter out nil and pointers back to the current object.
  1276  		if obj != 0 && obj-b >= n {
  1277  			// Test if obj points into the Go heap and, if so,
  1278  			// mark the object.
  1279  			//
  1280  			// Note that it's possible for findObject to
  1281  			// fail if obj points to a just-allocated heap
  1282  			// object because of a race with growing the
  1283  			// heap. In this case, we know the object was
  1284  			// just allocated and hence will be marked by
  1285  			// allocation itself.
  1286  			if obj, span, objIndex := findObject(obj, b, i); obj != 0 {
  1287  				greyobject(obj, b, i, span, gcw, objIndex)
  1288  			}
  1289  		}
  1290  	}
  1291  	gcw.bytesMarked += uint64(n)
  1292  	gcw.scanWork += int64(i)
  1293  }
  1294  
  1295  // scanConservative scans block [b, b+n) conservatively, treating any
  1296  // pointer-like value in the block as a pointer.
  1297  //
  1298  // If ptrmask != nil, only words that are marked in ptrmask are
  1299  // considered as potential pointers.
  1300  //
  1301  // If state != nil, it's assumed that [b, b+n) is a block in the stack
  1302  // and may contain pointers to stack objects.
  1303  func scanConservative(b, n uintptr, ptrmask *uint8, gcw *gcWork, state *stackScanState) {
  1304  	if debugScanConservative {
  1305  		printlock()
  1306  		print("conservatively scanning [", hex(b), ",", hex(b+n), ")\n")
  1307  		hexdumpWords(b, b+n, func(p uintptr) byte {
  1308  			if ptrmask != nil {
  1309  				word := (p - b) / sys.PtrSize
  1310  				bits := *addb(ptrmask, word/8)
  1311  				if (bits>>(word%8))&1 == 0 {
  1312  					return '$'
  1313  				}
  1314  			}
  1315  
  1316  			val := *(*uintptr)(unsafe.Pointer(p))
  1317  			if state != nil && state.stack.lo <= val && val < state.stack.hi {
  1318  				return '@'
  1319  			}
  1320  
  1321  			span := spanOfHeap(val)
  1322  			if span == nil {
  1323  				return ' '
  1324  			}
  1325  			idx := span.objIndex(val)
  1326  			if span.isFree(idx) {
  1327  				return ' '
  1328  			}
  1329  			return '*'
  1330  		})
  1331  		printunlock()
  1332  	}
  1333  
  1334  	for i := uintptr(0); i < n; i += sys.PtrSize {
  1335  		if ptrmask != nil {
  1336  			word := i / sys.PtrSize
  1337  			bits := *addb(ptrmask, word/8)
  1338  			if bits == 0 {
  1339  				// Skip 8 words (the loop increment will do the 8th)
  1340  				//
  1341  				// This must be the first time we've
  1342  				// seen this word of ptrmask, so i
  1343  				// must be 8-word-aligned, but check
  1344  				// our reasoning just in case.
  1345  				if i%(sys.PtrSize*8) != 0 {
  1346  					throw("misaligned mask")
  1347  				}
  1348  				i += sys.PtrSize*8 - sys.PtrSize
  1349  				continue
  1350  			}
  1351  			if (bits>>(word%8))&1 == 0 {
  1352  				continue
  1353  			}
  1354  		}
  1355  
  1356  		val := *(*uintptr)(unsafe.Pointer(b + i))
  1357  
  1358  		// Check if val points into the stack.
  1359  		if state != nil && state.stack.lo <= val && val < state.stack.hi {
  1360  			// val may point to a stack object. This
  1361  			// object may be dead from last cycle and
  1362  			// hence may contain pointers to unallocated
  1363  			// objects, but unlike heap objects we can't
  1364  			// tell if it's already dead. Hence, if all
  1365  			// pointers to this object are from
  1366  			// conservative scanning, we have to scan it
  1367  			// defensively, too.
  1368  			state.putPtr(val, true)
  1369  			continue
  1370  		}
  1371  
  1372  		// Check if val points to a heap span.
  1373  		span := spanOfHeap(val)
  1374  		if span == nil {
  1375  			continue
  1376  		}
  1377  
  1378  		// Check if val points to an allocated object.
  1379  		idx := span.objIndex(val)
  1380  		if span.isFree(idx) {
  1381  			continue
  1382  		}
  1383  
  1384  		// val points to an allocated object. Mark it.
  1385  		obj := span.base() + idx*span.elemsize
  1386  		greyobject(obj, b, i, span, gcw, idx)
  1387  	}
  1388  }
  1389  
  1390  // Shade the object if it isn't already.
  1391  // The object is not nil and known to be in the heap.
  1392  // Preemption must be disabled.
  1393  //go:nowritebarrier
  1394  func shade(b uintptr) {
  1395  	if obj, span, objIndex := findObject(b, 0, 0); obj != 0 {
  1396  		gcw := &getg().m.p.ptr().gcw
  1397  		greyobject(obj, 0, 0, span, gcw, objIndex)
  1398  	}
  1399  }
  1400  
  1401  // obj is the start of an object with mark mbits.
  1402  // If it isn't already marked, mark it and enqueue into gcw.
  1403  // base and off are for debugging only and could be removed.
  1404  //
  1405  // See also wbBufFlush1, which partially duplicates this logic.
  1406  //
  1407  //go:nowritebarrierrec
  1408  func greyobject(obj, base, off uintptr, span *mspan, gcw *gcWork, objIndex uintptr) {
  1409  	// obj should be start of allocation, and so must be at least pointer-aligned.
  1410  	if obj&(sys.PtrSize-1) != 0 {
  1411  		throw("greyobject: obj not pointer-aligned")
  1412  	}
  1413  	mbits := span.markBitsForIndex(objIndex)
  1414  
  1415  	if useCheckmark {
  1416  		if setCheckmark(obj, base, off, mbits) {
  1417  			// Already marked.
  1418  			return
  1419  		}
  1420  	} else {
  1421  		if debug.gccheckmark > 0 && span.isFree(objIndex) {
  1422  			print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")
  1423  			gcDumpObject("base", base, off)
  1424  			gcDumpObject("obj", obj, ^uintptr(0))
  1425  			getg().m.traceback = 2
  1426  			throw("marking free object")
  1427  		}
  1428  
  1429  		// If marked we have nothing to do.
  1430  		if mbits.isMarked() {
  1431  			return
  1432  		}
  1433  		mbits.setMarked()
  1434  
  1435  		// Mark span.
  1436  		arena, pageIdx, pageMask := pageIndexOf(span.base())
  1437  		if arena.pageMarks[pageIdx]&pageMask == 0 {
  1438  			atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
  1439  		}
  1440  
  1441  		// If this is a noscan object, fast-track it to black
  1442  		// instead of greying it.
  1443  		if span.spanclass.noscan() {
  1444  			gcw.bytesMarked += uint64(span.elemsize)
  1445  			return
  1446  		}
  1447  	}
  1448  
  1449  	// Queue the obj for scanning. The PREFETCH(obj) logic has been removed but
  1450  	// seems like a nice optimization that can be added back in.
  1451  	// There needs to be time between the PREFETCH and the use.
  1452  	// Previously we put the obj in an 8 element buffer that is drained at a rate
  1453  	// to give the PREFETCH time to do its work.
  1454  	// Use of PREFETCHNTA might be more appropriate than PREFETCH
  1455  	if !gcw.putFast(obj) {
  1456  		gcw.put(obj)
  1457  	}
  1458  }
  1459  
  1460  // gcDumpObject dumps the contents of obj for debugging and marks the
  1461  // field at byte offset off in obj.
  1462  func gcDumpObject(label string, obj, off uintptr) {
  1463  	s := spanOf(obj)
  1464  	print(label, "=", hex(obj))
  1465  	if s == nil {
  1466  		print(" s=nil\n")
  1467  		return
  1468  	}
  1469  	print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.spanclass=", s.spanclass, " s.elemsize=", s.elemsize, " s.state=")
  1470  	if state := s.state.get(); 0 <= state && int(state) < len(mSpanStateNames) {
  1471  		print(mSpanStateNames[state], "\n")
  1472  	} else {
  1473  		print("unknown(", state, ")\n")
  1474  	}
  1475  
  1476  	skipped := false
  1477  	size := s.elemsize
  1478  	if s.state.get() == mSpanManual && size == 0 {
  1479  		// We're printing something from a stack frame. We
  1480  		// don't know how big it is, so just show up to an
  1481  		// including off.
  1482  		size = off + sys.PtrSize
  1483  	}
  1484  	for i := uintptr(0); i < size; i += sys.PtrSize {
  1485  		// For big objects, just print the beginning (because
  1486  		// that usually hints at the object's type) and the
  1487  		// fields around off.
  1488  		if !(i < 128*sys.PtrSize || off-16*sys.PtrSize < i && i < off+16*sys.PtrSize) {
  1489  			skipped = true
  1490  			continue
  1491  		}
  1492  		if skipped {
  1493  			print(" ...\n")
  1494  			skipped = false
  1495  		}
  1496  		print(" *(", label, "+", i, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + i))))
  1497  		if i == off {
  1498  			print(" <==")
  1499  		}
  1500  		print("\n")
  1501  	}
  1502  	if skipped {
  1503  		print(" ...\n")
  1504  	}
  1505  }
  1506  
  1507  // gcmarknewobject marks a newly allocated object black. obj must
  1508  // not contain any non-nil pointers.
  1509  //
  1510  // This is nosplit so it can manipulate a gcWork without preemption.
  1511  //
  1512  //go:nowritebarrier
  1513  //go:nosplit
  1514  func gcmarknewobject(span *mspan, obj, size, scanSize uintptr) {
  1515  	if useCheckmark { // The world should be stopped so this should not happen.
  1516  		throw("gcmarknewobject called while doing checkmark")
  1517  	}
  1518  
  1519  	// Mark object.
  1520  	objIndex := span.objIndex(obj)
  1521  	span.markBitsForIndex(objIndex).setMarked()
  1522  
  1523  	// Mark span.
  1524  	arena, pageIdx, pageMask := pageIndexOf(span.base())
  1525  	if arena.pageMarks[pageIdx]&pageMask == 0 {
  1526  		atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
  1527  	}
  1528  
  1529  	gcw := &getg().m.p.ptr().gcw
  1530  	gcw.bytesMarked += uint64(size)
  1531  	gcw.scanWork += int64(scanSize)
  1532  }
  1533  
  1534  // gcMarkTinyAllocs greys all active tiny alloc blocks.
  1535  //
  1536  // The world must be stopped.
  1537  func gcMarkTinyAllocs() {
  1538  	assertWorldStopped()
  1539  
  1540  	for _, p := range allp {
  1541  		c := p.mcache
  1542  		if c == nil || c.tiny == 0 {
  1543  			continue
  1544  		}
  1545  		_, span, objIndex := findObject(c.tiny, 0, 0)
  1546  		gcw := &p.gcw
  1547  		greyobject(c.tiny, 0, 0, span, gcw, objIndex)
  1548  	}
  1549  }
  1550  

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