src/runtime/sema.go - go - Git at Google

 // 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, true)
 }

 //go:linkname net_runtime_Semacquire net.runtime_Semacquire
 func net_runtime_Semacquire(addr *uint32) {
 	semacquire(addr, true)
 }

 //go:linkname sync_runtime_Semrelease sync.runtime_Semrelease
 func sync_runtime_Semrelease(addr *uint32) {
 	semrelease(addr)
 }

 //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)
 }

 // Called from runtime.
 func semacquire(addr *uint32, profile bool) {
 	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
 	if profile && blockprofilerate > 0 {
 		t0 = cputicks()
 		s.releasetime = -1
 	}
 	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
 		}
 	}
 	unlock(&root.lock)
 	if s != nil {
 		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")
 	}
 }
	// 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, true)
	}

	//go:linkname net_runtime_Semacquire net.runtime_Semacquire
	func net_runtime_Semacquire(addr *uint32) {
	semacquire(addr, true)
	}

	//go:linkname sync_runtime_Semrelease sync.runtime_Semrelease
	func sync_runtime_Semrelease(addr *uint32) {
	semrelease(addr)
	}

	//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)
	}

	// Called from runtime.
	func semacquire(addr *uint32, profile bool) {
	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
	if profile && blockprofilerate > 0 {
	t0 = cputicks()
	s.releasetime = -1
	}
	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
	}
	}
	unlock(&root.lock)
	if s != nil {
	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")
	}
	}