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Source file src/runtime/sema.go

  // Copyright 2009 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.
  
  // Semaphore implementation exposed to Go.
  // Intended use is provide a sleep and wakeup
  // primitive that can be used in the contended case
  // of other synchronization primitives.
  // Thus it targets the same goal as Linux's futex,
  // but it has much simpler semantics.
  //
  // That is, don't think of these as semaphores.
  // Think of them as a way to implement sleep and wakeup
  // such that every sleep is paired with a single wakeup,
  // even if, due to races, the wakeup happens before the sleep.
  //
  // See Mullender and Cox, ``Semaphores in Plan 9,''
  // http://swtch.com/semaphore.pdf
  
  package runtime
  
  import (
  	"runtime/internal/atomic"
  	"runtime/internal/sys"
  	"unsafe"
  )
  
  // Asynchronous semaphore for sync.Mutex.
  
  type semaRoot struct {
  	lock  mutex
  	head  *sudog
  	tail  *sudog
  	nwait uint32 // Number of waiters. Read w/o the lock.
  }
  
  // Prime to not correlate with any user patterns.
  const semTabSize = 251
  
  var semtable [semTabSize]struct {
  	root semaRoot
  	pad  [sys.CacheLineSize - unsafe.Sizeof(semaRoot{})]byte
  }
  
  //go:linkname sync_runtime_Semacquire sync.runtime_Semacquire
  func sync_runtime_Semacquire(addr *uint32) {
  	semacquire(addr, semaBlockProfile)
  }
  
  //go:linkname net_runtime_Semacquire net.runtime_Semacquire
  func net_runtime_Semacquire(addr *uint32) {
  	semacquire(addr, semaBlockProfile)
  }
  
  //go:linkname sync_runtime_Semrelease sync.runtime_Semrelease
  func sync_runtime_Semrelease(addr *uint32) {
  	semrelease(addr)
  }
  
  //go:linkname sync_runtime_SemacquireMutex sync.runtime_SemacquireMutex
  func sync_runtime_SemacquireMutex(addr *uint32) {
  	semacquire(addr, semaBlockProfile|semaMutexProfile)
  }
  
  //go:linkname net_runtime_Semrelease net.runtime_Semrelease
  func net_runtime_Semrelease(addr *uint32) {
  	semrelease(addr)
  }
  
  func readyWithTime(s *sudog, traceskip int) {
  	if s.releasetime != 0 {
  		s.releasetime = cputicks()
  	}
  	goready(s.g, traceskip)
  }
  
  type semaProfileFlags int
  
  const (
  	semaBlockProfile semaProfileFlags = 1 << iota
  	semaMutexProfile
  )
  
  // Called from runtime.
  func semacquire(addr *uint32, profile semaProfileFlags) {
  	gp := getg()
  	if gp != gp.m.curg {
  		throw("semacquire not on the G stack")
  	}
  
  	// Easy case.
  	if cansemacquire(addr) {
  		return
  	}
  
  	// Harder case:
  	//	increment waiter count
  	//	try cansemacquire one more time, return if succeeded
  	//	enqueue itself as a waiter
  	//	sleep
  	//	(waiter descriptor is dequeued by signaler)
  	s := acquireSudog()
  	root := semroot(addr)
  	t0 := int64(0)
  	s.releasetime = 0
  	s.acquiretime = 0
  	if profile&semaBlockProfile != 0 && blockprofilerate > 0 {
  		t0 = cputicks()
  		s.releasetime = -1
  	}
  	if profile&semaMutexProfile != 0 && mutexprofilerate > 0 {
  		if t0 == 0 {
  			t0 = cputicks()
  		}
  		s.acquiretime = t0
  	}
  	for {
  		lock(&root.lock)
  		// Add ourselves to nwait to disable "easy case" in semrelease.
  		atomic.Xadd(&root.nwait, 1)
  		// Check cansemacquire to avoid missed wakeup.
  		if cansemacquire(addr) {
  			atomic.Xadd(&root.nwait, -1)
  			unlock(&root.lock)
  			break
  		}
  		// Any semrelease after the cansemacquire knows we're waiting
  		// (we set nwait above), so go to sleep.
  		root.queue(addr, s)
  		goparkunlock(&root.lock, "semacquire", traceEvGoBlockSync, 4)
  		if cansemacquire(addr) {
  			break
  		}
  	}
  	if s.releasetime > 0 {
  		blockevent(s.releasetime-t0, 3)
  	}
  	releaseSudog(s)
  }
  
  func semrelease(addr *uint32) {
  	root := semroot(addr)
  	atomic.Xadd(addr, 1)
  
  	// Easy case: no waiters?
  	// This check must happen after the xadd, to avoid a missed wakeup
  	// (see loop in semacquire).
  	if atomic.Load(&root.nwait) == 0 {
  		return
  	}
  
  	// Harder case: search for a waiter and wake it.
  	lock(&root.lock)
  	if atomic.Load(&root.nwait) == 0 {
  		// The count is already consumed by another goroutine,
  		// so no need to wake up another goroutine.
  		unlock(&root.lock)
  		return
  	}
  	s := root.head
  	for ; s != nil; s = s.next {
  		if s.elem == unsafe.Pointer(addr) {
  			atomic.Xadd(&root.nwait, -1)
  			root.dequeue(s)
  			break
  		}
  	}
  	if s != nil {
  		if s.acquiretime != 0 {
  			t0 := cputicks()
  			for x := root.head; x != nil; x = x.next {
  				if x.elem == unsafe.Pointer(addr) {
  					x.acquiretime = t0
  					break
  				}
  			}
  			mutexevent(t0-s.acquiretime, 3)
  		}
  	}
  	unlock(&root.lock)
  	if s != nil { // May be slow, so unlock first
  		readyWithTime(s, 5)
  	}
  }
  
  func semroot(addr *uint32) *semaRoot {
  	return &semtable[(uintptr(unsafe.Pointer(addr))>>3)%semTabSize].root
  }
  
  func cansemacquire(addr *uint32) bool {
  	for {
  		v := atomic.Load(addr)
  		if v == 0 {
  			return false
  		}
  		if atomic.Cas(addr, v, v-1) {
  			return true
  		}
  	}
  }
  
  func (root *semaRoot) queue(addr *uint32, s *sudog) {
  	s.g = getg()
  	s.elem = unsafe.Pointer(addr)
  	s.next = nil
  	s.prev = root.tail
  	if root.tail != nil {
  		root.tail.next = s
  	} else {
  		root.head = s
  	}
  	root.tail = s
  }
  
  func (root *semaRoot) dequeue(s *sudog) {
  	if s.next != nil {
  		s.next.prev = s.prev
  	} else {
  		root.tail = s.prev
  	}
  	if s.prev != nil {
  		s.prev.next = s.next
  	} else {
  		root.head = s.next
  	}
  	s.elem = nil
  	s.next = nil
  	s.prev = nil
  }
  
  // notifyList is a ticket-based notification list used to implement sync.Cond.
  //
  // It must be kept in sync with the sync package.
  type notifyList struct {
  	// wait is the ticket number of the next waiter. It is atomically
  	// incremented outside the lock.
  	wait uint32
  
  	// notify is the ticket number of the next waiter to be notified. It can
  	// be read outside the lock, but is only written to with lock held.
  	//
  	// Both wait & notify can wrap around, and such cases will be correctly
  	// handled as long as their "unwrapped" difference is bounded by 2^31.
  	// For this not to be the case, we'd need to have 2^31+ goroutines
  	// blocked on the same condvar, which is currently not possible.
  	notify uint32
  
  	// List of parked waiters.
  	lock mutex
  	head *sudog
  	tail *sudog
  }
  
  // less checks if a < b, considering a & b running counts that may overflow the
  // 32-bit range, and that their "unwrapped" difference is always less than 2^31.
  func less(a, b uint32) bool {
  	return int32(a-b) < 0
  }
  
  // notifyListAdd adds the caller to a notify list such that it can receive
  // notifications. The caller must eventually call notifyListWait to wait for
  // such a notification, passing the returned ticket number.
  //go:linkname notifyListAdd sync.runtime_notifyListAdd
  func notifyListAdd(l *notifyList) uint32 {
  	// This may be called concurrently, for example, when called from
  	// sync.Cond.Wait while holding a RWMutex in read mode.
  	return atomic.Xadd(&l.wait, 1) - 1
  }
  
  // notifyListWait waits for a notification. If one has been sent since
  // notifyListAdd was called, it returns immediately. Otherwise, it blocks.
  //go:linkname notifyListWait sync.runtime_notifyListWait
  func notifyListWait(l *notifyList, t uint32) {
  	lock(&l.lock)
  
  	// Return right away if this ticket has already been notified.
  	if less(t, l.notify) {
  		unlock(&l.lock)
  		return
  	}
  
  	// Enqueue itself.
  	s := acquireSudog()
  	s.g = getg()
  	s.ticket = t
  	s.releasetime = 0
  	t0 := int64(0)
  	if blockprofilerate > 0 {
  		t0 = cputicks()
  		s.releasetime = -1
  	}
  	if l.tail == nil {
  		l.head = s
  	} else {
  		l.tail.next = s
  	}
  	l.tail = s
  	goparkunlock(&l.lock, "semacquire", traceEvGoBlockCond, 3)
  	if t0 != 0 {
  		blockevent(s.releasetime-t0, 2)
  	}
  	releaseSudog(s)
  }
  
  // notifyListNotifyAll notifies all entries in the list.
  //go:linkname notifyListNotifyAll sync.runtime_notifyListNotifyAll
  func notifyListNotifyAll(l *notifyList) {
  	// Fast-path: if there are no new waiters since the last notification
  	// we don't need to acquire the lock.
  	if atomic.Load(&l.wait) == atomic.Load(&l.notify) {
  		return
  	}
  
  	// Pull the list out into a local variable, waiters will be readied
  	// outside the lock.
  	lock(&l.lock)
  	s := l.head
  	l.head = nil
  	l.tail = nil
  
  	// Update the next ticket to be notified. We can set it to the current
  	// value of wait because any previous waiters are already in the list
  	// or will notice that they have already been notified when trying to
  	// add themselves to the list.
  	atomic.Store(&l.notify, atomic.Load(&l.wait))
  	unlock(&l.lock)
  
  	// Go through the local list and ready all waiters.
  	for s != nil {
  		next := s.next
  		s.next = nil
  		readyWithTime(s, 4)
  		s = next
  	}
  }
  
  // notifyListNotifyOne notifies one entry in the list.
  //go:linkname notifyListNotifyOne sync.runtime_notifyListNotifyOne
  func notifyListNotifyOne(l *notifyList) {
  	// Fast-path: if there are no new waiters since the last notification
  	// we don't need to acquire the lock at all.
  	if atomic.Load(&l.wait) == atomic.Load(&l.notify) {
  		return
  	}
  
  	lock(&l.lock)
  
  	// Re-check under the lock if we need to do anything.
  	t := l.notify
  	if t == atomic.Load(&l.wait) {
  		unlock(&l.lock)
  		return
  	}
  
  	// Update the next notify ticket number, and try to find the G that
  	// needs to be notified. If it hasn't made it to the list yet we won't
  	// find it, but it won't park itself once it sees the new notify number.
  	atomic.Store(&l.notify, t+1)
  	for p, s := (*sudog)(nil), l.head; s != nil; p, s = s, s.next {
  		if s.ticket == t {
  			n := s.next
  			if p != nil {
  				p.next = n
  			} else {
  				l.head = n
  			}
  			if n == nil {
  				l.tail = p
  			}
  			unlock(&l.lock)
  			s.next = nil
  			readyWithTime(s, 4)
  			return
  		}
  	}
  	unlock(&l.lock)
  }
  
  //go:linkname notifyListCheck sync.runtime_notifyListCheck
  func notifyListCheck(sz uintptr) {
  	if sz != unsafe.Sizeof(notifyList{}) {
  		print("runtime: bad notifyList size - sync=", sz, " runtime=", unsafe.Sizeof(notifyList{}), "\n")
  		throw("bad notifyList size")
  	}
  }
  

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