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// 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,''
// https://swtch.com/semaphore.pdf
package runtime
import (
"internal/cpu"
"runtime/internal/atomic"
"unsafe"
)
// Asynchronous semaphore for sync.Mutex.
// A semaRoot holds a balanced tree of sudog with distinct addresses (s.elem).
// Each of those sudog may in turn point (through s.waitlink) to a list
// of other sudogs waiting on the same address.
// The operations on the inner lists of sudogs with the same address
// are all O(1). The scanning of the top-level semaRoot list is O(log n),
// where n is the number of distinct addresses with goroutines blocked
// on them that hash to the given semaRoot.
// See golang.org/issue/17953 for a program that worked badly
// before we introduced the second level of list, and test/locklinear.go
// for a test that exercises this.
type semaRoot struct {
lock mutex
treap *sudog // root of balanced tree of unique waiters.
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 [cpu.CacheLinePadSize - unsafe.Sizeof(semaRoot{})]byte
}
//go:linkname sync_runtime_Semacquire sync.runtime_Semacquire
func sync_runtime_Semacquire(addr *uint32) {
semacquire1(addr, false, semaBlockProfile, 0)
}
//go:linkname poll_runtime_Semacquire internal/poll.runtime_Semacquire
func poll_runtime_Semacquire(addr *uint32) {
semacquire1(addr, false, semaBlockProfile, 0)
}
//go:linkname sync_runtime_Semrelease sync.runtime_Semrelease
func sync_runtime_Semrelease(addr *uint32, handoff bool, skipframes int) {
semrelease1(addr, handoff, skipframes)
}
//go:linkname sync_runtime_SemacquireMutex sync.runtime_SemacquireMutex
func sync_runtime_SemacquireMutex(addr *uint32, lifo bool, skipframes int) {
semacquire1(addr, lifo, semaBlockProfile|semaMutexProfile, skipframes)
}
//go:linkname poll_runtime_Semrelease internal/poll.runtime_Semrelease
func poll_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) {
semacquire1(addr, false, 0, 0)
}
func semacquire1(addr *uint32, lifo bool, profile semaProfileFlags, skipframes int) {
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
s.ticket = 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, lifo)
goparkunlock(&root.lock, waitReasonSemacquire, traceEvGoBlockSync, 4+skipframes)
if s.ticket != 0 || cansemacquire(addr) {
break
}
}
if s.releasetime > 0 {
blockevent(s.releasetime-t0, 3+skipframes)
}
releaseSudog(s)
}
func semrelease(addr *uint32) {
semrelease1(addr, false, 0)
}
func semrelease1(addr *uint32, handoff bool, skipframes int) {
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, t0 := root.dequeue(addr)
if s != nil {
atomic.Xadd(&root.nwait, -1)
}
unlock(&root.lock)
if s != nil { // May be slow or even yield, so unlock first
acquiretime := s.acquiretime
if acquiretime != 0 {
mutexevent(t0-acquiretime, 3+skipframes)
}
if s.ticket != 0 {
throw("corrupted semaphore ticket")
}
if handoff && cansemacquire(addr) {
s.ticket = 1
}
readyWithTime(s, 5+skipframes)
if s.ticket == 1 && getg().m.locks == 0 {
// Direct G handoff
// readyWithTime has added the waiter G as runnext in the
// current P; we now call the scheduler so that we start running
// the waiter G immediately.
// Note that waiter inherits our time slice: this is desirable
// to avoid having a highly contended semaphore hog the P
// indefinitely. goyield is like Gosched, but it emits a
// "preempted" trace event instead and, more importantly, puts
// the current G on the local runq instead of the global one.
// We only do this in the starving regime (handoff=true), as in
// the non-starving case it is possible for a different waiter
// to acquire the semaphore while we are yielding/scheduling,
// and this would be wasteful. We wait instead to enter starving
// regime, and then we start to do direct handoffs of ticket and
// P.
// See issue 33747 for discussion.
goyield()
}
}
}
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
}
}
}
// queue adds s to the blocked goroutines in semaRoot.
func (root *semaRoot) queue(addr *uint32, s *sudog, lifo bool) {
s.g = getg()
s.elem = unsafe.Pointer(addr)
s.next = nil
s.prev = nil
var last *sudog
pt := &root.treap
for t := *pt; t != nil; t = *pt {
if t.elem == unsafe.Pointer(addr) {
// Already have addr in list.
if lifo {
// Substitute s in t's place in treap.
*pt = s
s.ticket = t.ticket
s.acquiretime = t.acquiretime
s.parent = t.parent
s.prev = t.prev
s.next = t.next
if s.prev != nil {
s.prev.parent = s
}
if s.next != nil {
s.next.parent = s
}
// Add t first in s's wait list.
s.waitlink = t
s.waittail = t.waittail
if s.waittail == nil {
s.waittail = t
}
t.parent = nil
t.prev = nil
t.next = nil
t.waittail = nil
} else {
// Add s to end of t's wait list.
if t.waittail == nil {
t.waitlink = s
} else {
t.waittail.waitlink = s
}
t.waittail = s
s.waitlink = nil
}
return
}
last = t
if uintptr(unsafe.Pointer(addr)) < uintptr(t.elem) {
pt = &t.prev
} else {
pt = &t.next
}
}
// Add s as new leaf in tree of unique addrs.
// The balanced tree is a treap using ticket as the random heap priority.
// That is, it is a binary tree ordered according to the elem addresses,
// but then among the space of possible binary trees respecting those
// addresses, it is kept balanced on average by maintaining a heap ordering
// on the ticket: s.ticket <= both s.prev.ticket and s.next.ticket.
// https://en.wikipedia.org/wiki/Treap
// https://faculty.washington.edu/aragon/pubs/rst89.pdf
//
// s.ticket compared with zero in couple of places, therefore set lowest bit.
// It will not affect treap's quality noticeably.
s.ticket = fastrand() | 1
s.parent = last
*pt = s
// Rotate up into tree according to ticket (priority).
for s.parent != nil && s.parent.ticket > s.ticket {
if s.parent.prev == s {
root.rotateRight(s.parent)
} else {
if s.parent.next != s {
panic("semaRoot queue")
}
root.rotateLeft(s.parent)
}
}
}
// dequeue searches for and finds the first goroutine
// in semaRoot blocked on addr.
// If the sudog was being profiled, dequeue returns the time
// at which it was woken up as now. Otherwise now is 0.
func (root *semaRoot) dequeue(addr *uint32) (found *sudog, now int64) {
ps := &root.treap
s := *ps
for ; s != nil; s = *ps {
if s.elem == unsafe.Pointer(addr) {
goto Found
}
if uintptr(unsafe.Pointer(addr)) < uintptr(s.elem) {
ps = &s.prev
} else {
ps = &s.next
}
}
return nil, 0
Found:
now = int64(0)
if s.acquiretime != 0 {
now = cputicks()
}
if t := s.waitlink; t != nil {
// Substitute t, also waiting on addr, for s in root tree of unique addrs.
*ps = t
t.ticket = s.ticket
t.parent = s.parent
t.prev = s.prev
if t.prev != nil {
t.prev.parent = t
}
t.next = s.next
if t.next != nil {
t.next.parent = t
}
if t.waitlink != nil {
t.waittail = s.waittail
} else {
t.waittail = nil
}
t.acquiretime = now
s.waitlink = nil
s.waittail = nil
} else {
// Rotate s down to be leaf of tree for removal, respecting priorities.
for s.next != nil || s.prev != nil {
if s.next == nil || s.prev != nil && s.prev.ticket < s.next.ticket {
root.rotateRight(s)
} else {
root.rotateLeft(s)
}
}
// Remove s, now a leaf.
if s.parent != nil {
if s.parent.prev == s {
s.parent.prev = nil
} else {
s.parent.next = nil
}
} else {
root.treap = nil
}
}
s.parent = nil
s.elem = nil
s.next = nil
s.prev = nil
s.ticket = 0
return s, now
}
// rotateLeft rotates the tree rooted at node x.
// turning (x a (y b c)) into (y (x a b) c).
func (root *semaRoot) rotateLeft(x *sudog) {
// p -> (x a (y b c))
p := x.parent
y := x.next
b := y.prev
y.prev = x
x.parent = y
x.next = b
if b != nil {
b.parent = x
}
y.parent = p
if p == nil {
root.treap = y
} else if p.prev == x {
p.prev = y
} else {
if p.next != x {
throw("semaRoot rotateLeft")
}
p.next = y
}
}
// rotateRight rotates the tree rooted at node y.
// turning (y (x a b) c) into (x a (y b c)).
func (root *semaRoot) rotateRight(y *sudog) {
// p -> (y (x a b) c)
p := y.parent
x := y.prev
b := x.next
x.next = y
y.parent = x
y.prev = b
if b != nil {
b.parent = y
}
x.parent = p
if p == nil {
root.treap = x
} else if p.prev == y {
p.prev = x
} else {
if p.next != y {
throw("semaRoot rotateRight")
}
p.next = x
}
}
// 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, waitReasonSyncCondWait, 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.
atomic.Store(&l.notify, t+1)
// 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.
//
// This scan looks linear but essentially always stops quickly.
// Because g's queue separately from taking numbers,
// there may be minor reorderings in the list, but we
// expect the g we're looking for to be near the front.
// The g has others in front of it on the list only to the
// extent that it lost the race, so the iteration will not
// be too long. This applies even when the g is missing:
// it hasn't yet gotten to sleep and has lost the race to
// the (few) other g's that we find on the list.
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")
}
}
//go:linkname sync_nanotime sync.runtime_nanotime
func sync_nanotime() int64 {
return nanotime()
}