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Source file src/runtime/mbarrier.go

Documentation: runtime

  // Copyright 2015 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Garbage collector: write barriers.
  //
  // For the concurrent garbage collector, the Go compiler implements
  // updates to pointer-valued fields that may be in heap objects by
  // emitting calls to write barriers. The main write barrier for
  // individual pointer writes is gcWriteBarrier and is implemented in
  // assembly. This file contains write barrier entry points for bulk
  // operations. See also mwbbuf.go.
  
  package runtime
  
  import (
  	"runtime/internal/sys"
  	"unsafe"
  )
  
  // Go uses a hybrid barrier that combines a Yuasa-style deletion
  // barrier—which shades the object whose reference is being
  // overwritten—with Dijkstra insertion barrier—which shades the object
  // whose reference is being written. The insertion part of the barrier
  // is necessary while the calling goroutine's stack is grey. In
  // pseudocode, the barrier is:
  //
  //     writePointer(slot, ptr):
  //         shade(*slot)
  //         if current stack is grey:
  //             shade(ptr)
  //         *slot = ptr
  //
  // slot is the destination in Go code.
  // ptr is the value that goes into the slot in Go code.
  //
  // Shade indicates that it has seen a white pointer by adding the referent
  // to wbuf as well as marking it.
  //
  // The two shades and the condition work together to prevent a mutator
  // from hiding an object from the garbage collector:
  //
  // 1. shade(*slot) prevents a mutator from hiding an object by moving
  // the sole pointer to it from the heap to its stack. If it attempts
  // to unlink an object from the heap, this will shade it.
  //
  // 2. shade(ptr) prevents a mutator from hiding an object by moving
  // the sole pointer to it from its stack into a black object in the
  // heap. If it attempts to install the pointer into a black object,
  // this will shade it.
  //
  // 3. Once a goroutine's stack is black, the shade(ptr) becomes
  // unnecessary. shade(ptr) prevents hiding an object by moving it from
  // the stack to the heap, but this requires first having a pointer
  // hidden on the stack. Immediately after a stack is scanned, it only
  // points to shaded objects, so it's not hiding anything, and the
  // shade(*slot) prevents it from hiding any other pointers on its
  // stack.
  //
  // For a detailed description of this barrier and proof of
  // correctness, see https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md
  //
  //
  //
  // Dealing with memory ordering:
  //
  // Both the Yuasa and Dijkstra barriers can be made conditional on the
  // color of the object containing the slot. We chose not to make these
  // conditional because the cost of ensuring that the object holding
  // the slot doesn't concurrently change color without the mutator
  // noticing seems prohibitive.
  //
  // Consider the following example where the mutator writes into
  // a slot and then loads the slot's mark bit while the GC thread
  // writes to the slot's mark bit and then as part of scanning reads
  // the slot.
  //
  // Initially both [slot] and [slotmark] are 0 (nil)
  // Mutator thread          GC thread
  // st [slot], ptr          st [slotmark], 1
  //
  // ld r1, [slotmark]       ld r2, [slot]
  //
  // Without an expensive memory barrier between the st and the ld, the final
  // result on most HW (including 386/amd64) can be r1==r2==0. This is a classic
  // example of what can happen when loads are allowed to be reordered with older
  // stores (avoiding such reorderings lies at the heart of the classic
  // Peterson/Dekker algorithms for mutual exclusion). Rather than require memory
  // barriers, which will slow down both the mutator and the GC, we always grey
  // the ptr object regardless of the slot's color.
  //
  // Another place where we intentionally omit memory barriers is when
  // accessing mheap_.arena_used to check if a pointer points into the
  // heap. On relaxed memory machines, it's possible for a mutator to
  // extend the size of the heap by updating arena_used, allocate an
  // object from this new region, and publish a pointer to that object,
  // but for tracing running on another processor to observe the pointer
  // but use the old value of arena_used. In this case, tracing will not
  // mark the object, even though it's reachable. However, the mutator
  // is guaranteed to execute a write barrier when it publishes the
  // pointer, so it will take care of marking the object. A general
  // consequence of this is that the garbage collector may cache the
  // value of mheap_.arena_used. (See issue #9984.)
  //
  //
  // Stack writes:
  //
  // The compiler omits write barriers for writes to the current frame,
  // but if a stack pointer has been passed down the call stack, the
  // compiler will generate a write barrier for writes through that
  // pointer (because it doesn't know it's not a heap pointer).
  //
  // One might be tempted to ignore the write barrier if slot points
  // into to the stack. Don't do it! Mark termination only re-scans
  // frames that have potentially been active since the concurrent scan,
  // so it depends on write barriers to track changes to pointers in
  // stack frames that have not been active.
  //
  //
  // Global writes:
  //
  // The Go garbage collector requires write barriers when heap pointers
  // are stored in globals. Many garbage collectors ignore writes to
  // globals and instead pick up global -> heap pointers during
  // termination. This increases pause time, so we instead rely on write
  // barriers for writes to globals so that we don't have to rescan
  // global during mark termination.
  //
  //
  // Publication ordering:
  //
  // The write barrier is *pre-publication*, meaning that the write
  // barrier happens prior to the *slot = ptr write that may make ptr
  // reachable by some goroutine that currently cannot reach it.
  //
  //
  // Signal handler pointer writes:
  //
  // In general, the signal handler cannot safely invoke the write
  // barrier because it may run without a P or even during the write
  // barrier.
  //
  // There is exactly one exception: profbuf.go omits a barrier during
  // signal handler profile logging. That's safe only because of the
  // deletion barrier. See profbuf.go for a detailed argument. If we
  // remove the deletion barrier, we'll have to work out a new way to
  // handle the profile logging.
  
  // typedmemmove copies a value of type t to dst from src.
  // Must be nosplit, see #16026.
  //
  // TODO: Perfect for go:nosplitrec since we can't have a safe point
  // anywhere in the bulk barrier or memmove.
  //
  //go:nosplit
  func typedmemmove(typ *_type, dst, src unsafe.Pointer) {
  	if dst == src {
  		return
  	}
  	if typ.kind&kindNoPointers == 0 {
  		bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.size)
  	}
  	// There's a race here: if some other goroutine can write to
  	// src, it may change some pointer in src after we've
  	// performed the write barrier but before we perform the
  	// memory copy. This safe because the write performed by that
  	// other goroutine must also be accompanied by a write
  	// barrier, so at worst we've unnecessarily greyed the old
  	// pointer that was in src.
  	memmove(dst, src, typ.size)
  	if writeBarrier.cgo {
  		cgoCheckMemmove(typ, dst, src, 0, typ.size)
  	}
  }
  
  //go:linkname reflect_typedmemmove reflect.typedmemmove
  func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
  	if raceenabled {
  		raceWriteObjectPC(typ, dst, getcallerpc(), funcPC(reflect_typedmemmove))
  		raceReadObjectPC(typ, src, getcallerpc(), funcPC(reflect_typedmemmove))
  	}
  	if msanenabled {
  		msanwrite(dst, typ.size)
  		msanread(src, typ.size)
  	}
  	typedmemmove(typ, dst, src)
  }
  
  // typedmemmovepartial is like typedmemmove but assumes that
  // dst and src point off bytes into the value and only copies size bytes.
  //go:linkname reflect_typedmemmovepartial reflect.typedmemmovepartial
  func reflect_typedmemmovepartial(typ *_type, dst, src unsafe.Pointer, off, size uintptr) {
  	if writeBarrier.needed && typ.kind&kindNoPointers == 0 && size >= sys.PtrSize {
  		// Pointer-align start address for bulk barrier.
  		adst, asrc, asize := dst, src, size
  		if frag := -off & (sys.PtrSize - 1); frag != 0 {
  			adst = add(dst, frag)
  			asrc = add(src, frag)
  			asize -= frag
  		}
  		bulkBarrierPreWrite(uintptr(adst), uintptr(asrc), asize&^(sys.PtrSize-1))
  	}
  
  	memmove(dst, src, size)
  	if writeBarrier.cgo {
  		cgoCheckMemmove(typ, dst, src, off, size)
  	}
  }
  
  // reflectcallmove is invoked by reflectcall to copy the return values
  // out of the stack and into the heap, invoking the necessary write
  // barriers. dst, src, and size describe the return value area to
  // copy. typ describes the entire frame (not just the return values).
  // typ may be nil, which indicates write barriers are not needed.
  //
  // It must be nosplit and must only call nosplit functions because the
  // stack map of reflectcall is wrong.
  //
  //go:nosplit
  func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr) {
  	if writeBarrier.needed && typ != nil && typ.kind&kindNoPointers == 0 && size >= sys.PtrSize {
  		bulkBarrierPreWrite(uintptr(dst), uintptr(src), size)
  	}
  	memmove(dst, src, size)
  }
  
  //go:nosplit
  func typedslicecopy(typ *_type, dst, src slice) int {
  	// TODO(rsc): If typedslicecopy becomes faster than calling
  	// typedmemmove repeatedly, consider using during func growslice.
  	n := dst.len
  	if n > src.len {
  		n = src.len
  	}
  	if n == 0 {
  		return 0
  	}
  	dstp := dst.array
  	srcp := src.array
  
  	// The compiler emits calls to typedslicecopy before
  	// instrumentation runs, so unlike the other copying and
  	// assignment operations, it's not instrumented in the calling
  	// code and needs its own instrumentation.
  	if raceenabled {
  		callerpc := getcallerpc()
  		pc := funcPC(slicecopy)
  		racewriterangepc(dstp, uintptr(n)*typ.size, callerpc, pc)
  		racereadrangepc(srcp, uintptr(n)*typ.size, callerpc, pc)
  	}
  	if msanenabled {
  		msanwrite(dstp, uintptr(n)*typ.size)
  		msanread(srcp, uintptr(n)*typ.size)
  	}
  
  	if writeBarrier.cgo {
  		cgoCheckSliceCopy(typ, dst, src, n)
  	}
  
  	if dstp == srcp {
  		return n
  	}
  
  	// Note: No point in checking typ.kind&kindNoPointers here:
  	// compiler only emits calls to typedslicecopy for types with pointers,
  	// and growslice and reflect_typedslicecopy check for pointers
  	// before calling typedslicecopy.
  	size := uintptr(n) * typ.size
  	if writeBarrier.needed {
  		bulkBarrierPreWrite(uintptr(dstp), uintptr(srcp), size)
  	}
  	// See typedmemmove for a discussion of the race between the
  	// barrier and memmove.
  	memmove(dstp, srcp, size)
  	return n
  }
  
  //go:linkname reflect_typedslicecopy reflect.typedslicecopy
  func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
  	if elemType.kind&kindNoPointers != 0 {
  		n := dst.len
  		if n > src.len {
  			n = src.len
  		}
  		if n == 0 {
  			return 0
  		}
  
  		size := uintptr(n) * elemType.size
  		if raceenabled {
  			callerpc := getcallerpc()
  			pc := funcPC(reflect_typedslicecopy)
  			racewriterangepc(dst.array, size, callerpc, pc)
  			racereadrangepc(src.array, size, callerpc, pc)
  		}
  		if msanenabled {
  			msanwrite(dst.array, size)
  			msanread(src.array, size)
  		}
  
  		memmove(dst.array, src.array, size)
  		return n
  	}
  	return typedslicecopy(elemType, dst, src)
  }
  
  // typedmemclr clears the typed memory at ptr with type typ. The
  // memory at ptr must already be initialized (and hence in type-safe
  // state). If the memory is being initialized for the first time, see
  // memclrNoHeapPointers.
  //
  // If the caller knows that typ has pointers, it can alternatively
  // call memclrHasPointers.
  //
  //go:nosplit
  func typedmemclr(typ *_type, ptr unsafe.Pointer) {
  	if typ.kind&kindNoPointers == 0 {
  		bulkBarrierPreWrite(uintptr(ptr), 0, typ.size)
  	}
  	memclrNoHeapPointers(ptr, typ.size)
  }
  
  // memclrHasPointers clears n bytes of typed memory starting at ptr.
  // The caller must ensure that the type of the object at ptr has
  // pointers, usually by checking typ.kind&kindNoPointers. However, ptr
  // does not have to point to the start of the allocation.
  //
  //go:nosplit
  func memclrHasPointers(ptr unsafe.Pointer, n uintptr) {
  	bulkBarrierPreWrite(uintptr(ptr), 0, n)
  	memclrNoHeapPointers(ptr, n)
  }
  

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