sema.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  // Semaphore implementation exposed to Go.
     6  // Intended use is provide a sleep and wakeup
     7  // primitive that can be used in the contended case
     8  // of other synchronization primitives.
     9  // Thus it targets the same goal as Linux's futex,
    10  // but it has much simpler semantics.
    11  //
    12  // That is, don't think of these as semaphores.
    13  // Think of them as a way to implement sleep and wakeup
    14  // such that every sleep is paired with a single wakeup,
    15  // even if, due to races, the wakeup happens before the sleep.
    16  //
    17  // See Mullender and Cox, ``Semaphores in Plan 9,''
    18  // https://swtch.com/semaphore.pdf
    19  
    20  package runtime
    21  
    22  import (
    23  	"internal/cpu"
    24  	"runtime/internal/atomic"
    25  	"unsafe"
    26  )
    27  
    28  // Asynchronous semaphore for sync.Mutex.
    29  
    30  // A semaRoot holds a balanced tree of sudog with distinct addresses (s.elem).
    31  // Each of those sudog may in turn point (through s.waitlink) to a list
    32  // of other sudogs waiting on the same address.
    33  // The operations on the inner lists of sudogs with the same address
    34  // are all O(1). The scanning of the top-level semaRoot list is O(log n),
    35  // where n is the number of distinct addresses with goroutines blocked
    36  // on them that hash to the given semaRoot.
    37  // See golang.org/issue/17953 for a program that worked badly
    38  // before we introduced the second level of list, and
    39  // BenchmarkSemTable/OneAddrCollision/* for a benchmark that exercises this.
    40  type semaRoot struct {
    41  	lock  mutex
    42  	treap *sudog        // root of balanced tree of unique waiters.
    43  	nwait atomic.Uint32 // Number of waiters. Read w/o the lock.
    44  }
    45  
    46  var semtable semTable
    47  
    48  // Prime to not correlate with any user patterns.
    49  const semTabSize = 251
    50  
    51  type semTable [semTabSize]struct {
    52  	root semaRoot
    53  	pad  [cpu.CacheLinePadSize - unsafe.Sizeof(semaRoot{})]byte
    54  }
    55  
    56  func (t *semTable) rootFor(addr *uint32) *semaRoot {
    57  	return &t[(uintptr(unsafe.Pointer(addr))>>3)%semTabSize].root
    58  }
    59  
    60  //go:linkname sync_runtime_Semacquire sync.runtime_Semacquire
    61  func sync_runtime_Semacquire(addr *uint32) {
    62  	semacquire1(addr, false, semaBlockProfile, 0, waitReasonSemacquire)
    63  }
    64  
    65  //go:linkname poll_runtime_Semacquire internal/poll.runtime_Semacquire
    66  func poll_runtime_Semacquire(addr *uint32) {
    67  	semacquire1(addr, false, semaBlockProfile, 0, waitReasonSemacquire)
    68  }
    69  
    70  //go:linkname sync_runtime_Semrelease sync.runtime_Semrelease
    71  func sync_runtime_Semrelease(addr *uint32, handoff bool, skipframes int) {
    72  	semrelease1(addr, handoff, skipframes)
    73  }
    74  
    75  //go:linkname sync_runtime_SemacquireMutex sync.runtime_SemacquireMutex
    76  func sync_runtime_SemacquireMutex(addr *uint32, lifo bool, skipframes int) {
    77  	semacquire1(addr, lifo, semaBlockProfile|semaMutexProfile, skipframes, waitReasonSyncMutexLock)
    78  }
    79  
    80  //go:linkname sync_runtime_SemacquireRWMutexR sync.runtime_SemacquireRWMutexR
    81  func sync_runtime_SemacquireRWMutexR(addr *uint32, lifo bool, skipframes int) {
    82  	semacquire1(addr, lifo, semaBlockProfile|semaMutexProfile, skipframes, waitReasonSyncRWMutexRLock)
    83  }
    84  
    85  //go:linkname sync_runtime_SemacquireRWMutex sync.runtime_SemacquireRWMutex
    86  func sync_runtime_SemacquireRWMutex(addr *uint32, lifo bool, skipframes int) {
    87  	semacquire1(addr, lifo, semaBlockProfile|semaMutexProfile, skipframes, waitReasonSyncRWMutexLock)
    88  }
    89  
    90  //go:linkname poll_runtime_Semrelease internal/poll.runtime_Semrelease
    91  func poll_runtime_Semrelease(addr *uint32) {
    92  	semrelease(addr)
    93  }
    94  
    95  func readyWithTime(s *sudog, traceskip int) {
    96  	if s.releasetime != 0 {
    97  		s.releasetime = cputicks()
    98  	}
    99  	goready(s.g, traceskip)
   100  }
   101  
   102  type semaProfileFlags int
   103  
   104  const (
   105  	semaBlockProfile semaProfileFlags = 1 << iota
   106  	semaMutexProfile
   107  )
   108  
   109  // Called from runtime.
   110  func semacquire(addr *uint32) {
   111  	semacquire1(addr, false, 0, 0, waitReasonSemacquire)
   112  }
   113  
   114  func semacquire1(addr *uint32, lifo bool, profile semaProfileFlags, skipframes int, reason waitReason) {
   115  	gp := getg()
   116  	if gp != gp.m.curg {
   117  		throw("semacquire not on the G stack")
   118  	}
   119  
   120  	// Easy case.
   121  	if cansemacquire(addr) {
   122  		return
   123  	}
   124  
   125  	// Harder case:
   126  	//	increment waiter count
   127  	//	try cansemacquire one more time, return if succeeded
   128  	//	enqueue itself as a waiter
   129  	//	sleep
   130  	//	(waiter descriptor is dequeued by signaler)
   131  	s := acquireSudog()
   132  	root := semtable.rootFor(addr)
   133  	t0 := int64(0)
   134  	s.releasetime = 0
   135  	s.acquiretime = 0
   136  	s.ticket = 0
   137  	if profile&semaBlockProfile != 0 && blockprofilerate > 0 {
   138  		t0 = cputicks()
   139  		s.releasetime = -1
   140  	}
   141  	if profile&semaMutexProfile != 0 && mutexprofilerate > 0 {
   142  		if t0 == 0 {
   143  			t0 = cputicks()
   144  		}
   145  		s.acquiretime = t0
   146  	}
   147  	for {
   148  		lockWithRank(&root.lock, lockRankRoot)
   149  		// Add ourselves to nwait to disable "easy case" in semrelease.
   150  		root.nwait.Add(1)
   151  		// Check cansemacquire to avoid missed wakeup.
   152  		if cansemacquire(addr) {
   153  			root.nwait.Add(-1)
   154  			unlock(&root.lock)
   155  			break
   156  		}
   157  		// Any semrelease after the cansemacquire knows we're waiting
   158  		// (we set nwait above), so go to sleep.
   159  		root.queue(addr, s, lifo)
   160  		goparkunlock(&root.lock, reason, traceBlockSync, 4+skipframes)
   161  		if s.ticket != 0 || cansemacquire(addr) {
   162  			break
   163  		}
   164  	}
   165  	if s.releasetime > 0 {
   166  		blockevent(s.releasetime-t0, 3+skipframes)
   167  	}
   168  	releaseSudog(s)
   169  }
   170  
   171  func semrelease(addr *uint32) {
   172  	semrelease1(addr, false, 0)
   173  }
   174  
   175  func semrelease1(addr *uint32, handoff bool, skipframes int) {
   176  	root := semtable.rootFor(addr)
   177  	atomic.Xadd(addr, 1)
   178  
   179  	// Easy case: no waiters?
   180  	// This check must happen after the xadd, to avoid a missed wakeup
   181  	// (see loop in semacquire).
   182  	if root.nwait.Load() == 0 {
   183  		return
   184  	}
   185  
   186  	// Harder case: search for a waiter and wake it.
   187  	lockWithRank(&root.lock, lockRankRoot)
   188  	if root.nwait.Load() == 0 {
   189  		// The count is already consumed by another goroutine,
   190  		// so no need to wake up another goroutine.
   191  		unlock(&root.lock)
   192  		return
   193  	}
   194  	s, t0, tailtime := root.dequeue(addr)
   195  	if s != nil {
   196  		root.nwait.Add(-1)
   197  	}
   198  	unlock(&root.lock)
   199  	if s != nil { // May be slow or even yield, so unlock first
   200  		acquiretime := s.acquiretime
   201  		if acquiretime != 0 {
   202  			// Charge contention that this (delayed) unlock caused.
   203  			// If there are N more goroutines waiting beyond the
   204  			// one that's waking up, charge their delay as well, so that
   205  			// contention holding up many goroutines shows up as
   206  			// more costly than contention holding up a single goroutine.
   207  			// It would take O(N) time to calculate how long each goroutine
   208  			// has been waiting, so instead we charge avg(head-wait, tail-wait)*N.
   209  			// head-wait is the longest wait and tail-wait is the shortest.
   210  			// (When we do a lifo insertion, we preserve this property by
   211  			// copying the old head's acquiretime into the inserted new head.
   212  			// In that case the overall average may be slightly high, but that's fine:
   213  			// the average of the ends is only an approximation to the actual
   214  			// average anyway.)
   215  			// The root.dequeue above changed the head and tail acquiretime
   216  			// to the current time, so the next unlock will not re-count this contention.
   217  			dt0 := t0 - acquiretime
   218  			dt := dt0
   219  			if s.waiters != 0 {
   220  				dtail := t0 - tailtime
   221  				dt += (dtail + dt0) / 2 * int64(s.waiters)
   222  			}
   223  			mutexevent(dt, 3+skipframes)
   224  		}
   225  		if s.ticket != 0 {
   226  			throw("corrupted semaphore ticket")
   227  		}
   228  		if handoff && cansemacquire(addr) {
   229  			s.ticket = 1
   230  		}
   231  		readyWithTime(s, 5+skipframes)
   232  		if s.ticket == 1 && getg().m.locks == 0 {
   233  			// Direct G handoff
   234  			// readyWithTime has added the waiter G as runnext in the
   235  			// current P; we now call the scheduler so that we start running
   236  			// the waiter G immediately.
   237  			// Note that waiter inherits our time slice: this is desirable
   238  			// to avoid having a highly contended semaphore hog the P
   239  			// indefinitely. goyield is like Gosched, but it emits a
   240  			// "preempted" trace event instead and, more importantly, puts
   241  			// the current G on the local runq instead of the global one.
   242  			// We only do this in the starving regime (handoff=true), as in
   243  			// the non-starving case it is possible for a different waiter
   244  			// to acquire the semaphore while we are yielding/scheduling,
   245  			// and this would be wasteful. We wait instead to enter starving
   246  			// regime, and then we start to do direct handoffs of ticket and
   247  			// P.
   248  			// See issue 33747 for discussion.
   249  			goyield()
   250  		}
   251  	}
   252  }
   253  
   254  func cansemacquire(addr *uint32) bool {
   255  	for {
   256  		v := atomic.Load(addr)
   257  		if v == 0 {
   258  			return false
   259  		}
   260  		if atomic.Cas(addr, v, v-1) {
   261  			return true
   262  		}
   263  	}
   264  }
   265  
   266  // queue adds s to the blocked goroutines in semaRoot.
   267  func (root *semaRoot) queue(addr *uint32, s *sudog, lifo bool) {
   268  	s.g = getg()
   269  	s.elem = unsafe.Pointer(addr)
   270  	s.next = nil
   271  	s.prev = nil
   272  	s.waiters = 0
   273  
   274  	var last *sudog
   275  	pt := &root.treap
   276  	for t := *pt; t != nil; t = *pt {
   277  		if t.elem == unsafe.Pointer(addr) {
   278  			// Already have addr in list.
   279  			if lifo {
   280  				// Substitute s in t's place in treap.
   281  				*pt = s
   282  				s.ticket = t.ticket
   283  				s.acquiretime = t.acquiretime // preserve head acquiretime as oldest time
   284  				s.parent = t.parent
   285  				s.prev = t.prev
   286  				s.next = t.next
   287  				if s.prev != nil {
   288  					s.prev.parent = s
   289  				}
   290  				if s.next != nil {
   291  					s.next.parent = s
   292  				}
   293  				// Add t first in s's wait list.
   294  				s.waitlink = t
   295  				s.waittail = t.waittail
   296  				if s.waittail == nil {
   297  					s.waittail = t
   298  				}
   299  				s.waiters = t.waiters
   300  				if s.waiters+1 != 0 {
   301  					s.waiters++
   302  				}
   303  				t.parent = nil
   304  				t.prev = nil
   305  				t.next = nil
   306  				t.waittail = nil
   307  			} else {
   308  				// Add s to end of t's wait list.
   309  				if t.waittail == nil {
   310  					t.waitlink = s
   311  				} else {
   312  					t.waittail.waitlink = s
   313  				}
   314  				t.waittail = s
   315  				s.waitlink = nil
   316  				if t.waiters+1 != 0 {
   317  					t.waiters++
   318  				}
   319  			}
   320  			return
   321  		}
   322  		last = t
   323  		if uintptr(unsafe.Pointer(addr)) < uintptr(t.elem) {
   324  			pt = &t.prev
   325  		} else {
   326  			pt = &t.next
   327  		}
   328  	}
   329  
   330  	// Add s as new leaf in tree of unique addrs.
   331  	// The balanced tree is a treap using ticket as the random heap priority.
   332  	// That is, it is a binary tree ordered according to the elem addresses,
   333  	// but then among the space of possible binary trees respecting those
   334  	// addresses, it is kept balanced on average by maintaining a heap ordering
   335  	// on the ticket: s.ticket <= both s.prev.ticket and s.next.ticket.
   336  	// https://en.wikipedia.org/wiki/Treap
   337  	// https://faculty.washington.edu/aragon/pubs/rst89.pdf
   338  	//
   339  	// s.ticket compared with zero in couple of places, therefore set lowest bit.
   340  	// It will not affect treap's quality noticeably.
   341  	s.ticket = cheaprand() | 1
   342  	s.parent = last
   343  	*pt = s
   344  
   345  	// Rotate up into tree according to ticket (priority).
   346  	for s.parent != nil && s.parent.ticket > s.ticket {
   347  		if s.parent.prev == s {
   348  			root.rotateRight(s.parent)
   349  		} else {
   350  			if s.parent.next != s {
   351  				panic("semaRoot queue")
   352  			}
   353  			root.rotateLeft(s.parent)
   354  		}
   355  	}
   356  }
   357  
   358  // dequeue searches for and finds the first goroutine
   359  // in semaRoot blocked on addr.
   360  // If the sudog was being profiled, dequeue returns the time
   361  // at which it was woken up as now. Otherwise now is 0.
   362  // If there are additional entries in the wait list, dequeue
   363  // returns tailtime set to the last entry's acquiretime.
   364  // Otherwise tailtime is found.acquiretime.
   365  func (root *semaRoot) dequeue(addr *uint32) (found *sudog, now, tailtime int64) {
   366  	ps := &root.treap
   367  	s := *ps
   368  	for ; s != nil; s = *ps {
   369  		if s.elem == unsafe.Pointer(addr) {
   370  			goto Found
   371  		}
   372  		if uintptr(unsafe.Pointer(addr)) < uintptr(s.elem) {
   373  			ps = &s.prev
   374  		} else {
   375  			ps = &s.next
   376  		}
   377  	}
   378  	return nil, 0, 0
   379  
   380  Found:
   381  	now = int64(0)
   382  	if s.acquiretime != 0 {
   383  		now = cputicks()
   384  	}
   385  	if t := s.waitlink; t != nil {
   386  		// Substitute t, also waiting on addr, for s in root tree of unique addrs.
   387  		*ps = t
   388  		t.ticket = s.ticket
   389  		t.parent = s.parent
   390  		t.prev = s.prev
   391  		if t.prev != nil {
   392  			t.prev.parent = t
   393  		}
   394  		t.next = s.next
   395  		if t.next != nil {
   396  			t.next.parent = t
   397  		}
   398  		if t.waitlink != nil {
   399  			t.waittail = s.waittail
   400  		} else {
   401  			t.waittail = nil
   402  		}
   403  		t.waiters = s.waiters
   404  		if t.waiters > 1 {
   405  			t.waiters--
   406  		}
   407  		// Set head and tail acquire time to 'now',
   408  		// because the caller will take care of charging
   409  		// the delays before now for all entries in the list.
   410  		t.acquiretime = now
   411  		tailtime = s.waittail.acquiretime
   412  		s.waittail.acquiretime = now
   413  		s.waitlink = nil
   414  		s.waittail = nil
   415  	} else {
   416  		// Rotate s down to be leaf of tree for removal, respecting priorities.
   417  		for s.next != nil || s.prev != nil {
   418  			if s.next == nil || s.prev != nil && s.prev.ticket < s.next.ticket {
   419  				root.rotateRight(s)
   420  			} else {
   421  				root.rotateLeft(s)
   422  			}
   423  		}
   424  		// Remove s, now a leaf.
   425  		if s.parent != nil {
   426  			if s.parent.prev == s {
   427  				s.parent.prev = nil
   428  			} else {
   429  				s.parent.next = nil
   430  			}
   431  		} else {
   432  			root.treap = nil
   433  		}
   434  		tailtime = s.acquiretime
   435  	}
   436  	s.parent = nil
   437  	s.elem = nil
   438  	s.next = nil
   439  	s.prev = nil
   440  	s.ticket = 0
   441  	return s, now, tailtime
   442  }
   443  
   444  // rotateLeft rotates the tree rooted at node x.
   445  // turning (x a (y b c)) into (y (x a b) c).
   446  func (root *semaRoot) rotateLeft(x *sudog) {
   447  	// p -> (x a (y b c))
   448  	p := x.parent
   449  	y := x.next
   450  	b := y.prev
   451  
   452  	y.prev = x
   453  	x.parent = y
   454  	x.next = b
   455  	if b != nil {
   456  		b.parent = x
   457  	}
   458  
   459  	y.parent = p
   460  	if p == nil {
   461  		root.treap = y
   462  	} else if p.prev == x {
   463  		p.prev = y
   464  	} else {
   465  		if p.next != x {
   466  			throw("semaRoot rotateLeft")
   467  		}
   468  		p.next = y
   469  	}
   470  }
   471  
   472  // rotateRight rotates the tree rooted at node y.
   473  // turning (y (x a b) c) into (x a (y b c)).
   474  func (root *semaRoot) rotateRight(y *sudog) {
   475  	// p -> (y (x a b) c)
   476  	p := y.parent
   477  	x := y.prev
   478  	b := x.next
   479  
   480  	x.next = y
   481  	y.parent = x
   482  	y.prev = b
   483  	if b != nil {
   484  		b.parent = y
   485  	}
   486  
   487  	x.parent = p
   488  	if p == nil {
   489  		root.treap = x
   490  	} else if p.prev == y {
   491  		p.prev = x
   492  	} else {
   493  		if p.next != y {
   494  			throw("semaRoot rotateRight")
   495  		}
   496  		p.next = x
   497  	}
   498  }
   499  
   500  // notifyList is a ticket-based notification list used to implement sync.Cond.
   501  //
   502  // It must be kept in sync with the sync package.
   503  type notifyList struct {
   504  	// wait is the ticket number of the next waiter. It is atomically
   505  	// incremented outside the lock.
   506  	wait atomic.Uint32
   507  
   508  	// notify is the ticket number of the next waiter to be notified. It can
   509  	// be read outside the lock, but is only written to with lock held.
   510  	//
   511  	// Both wait & notify can wrap around, and such cases will be correctly
   512  	// handled as long as their "unwrapped" difference is bounded by 2^31.
   513  	// For this not to be the case, we'd need to have 2^31+ goroutines
   514  	// blocked on the same condvar, which is currently not possible.
   515  	notify uint32
   516  
   517  	// List of parked waiters.
   518  	lock mutex
   519  	head *sudog
   520  	tail *sudog
   521  }
   522  
   523  // less checks if a < b, considering a & b running counts that may overflow the
   524  // 32-bit range, and that their "unwrapped" difference is always less than 2^31.
   525  func less(a, b uint32) bool {
   526  	return int32(a-b) < 0
   527  }
   528  
   529  // notifyListAdd adds the caller to a notify list such that it can receive
   530  // notifications. The caller must eventually call notifyListWait to wait for
   531  // such a notification, passing the returned ticket number.
   532  //
   533  //go:linkname notifyListAdd sync.runtime_notifyListAdd
   534  func notifyListAdd(l *notifyList) uint32 {
   535  	// This may be called concurrently, for example, when called from
   536  	// sync.Cond.Wait while holding a RWMutex in read mode.
   537  	return l.wait.Add(1) - 1
   538  }
   539  
   540  // notifyListWait waits for a notification. If one has been sent since
   541  // notifyListAdd was called, it returns immediately. Otherwise, it blocks.
   542  //
   543  //go:linkname notifyListWait sync.runtime_notifyListWait
   544  func notifyListWait(l *notifyList, t uint32) {
   545  	lockWithRank(&l.lock, lockRankNotifyList)
   546  
   547  	// Return right away if this ticket has already been notified.
   548  	if less(t, l.notify) {
   549  		unlock(&l.lock)
   550  		return
   551  	}
   552  
   553  	// Enqueue itself.
   554  	s := acquireSudog()
   555  	s.g = getg()
   556  	s.ticket = t
   557  	s.releasetime = 0
   558  	t0 := int64(0)
   559  	if blockprofilerate > 0 {
   560  		t0 = cputicks()
   561  		s.releasetime = -1
   562  	}
   563  	if l.tail == nil {
   564  		l.head = s
   565  	} else {
   566  		l.tail.next = s
   567  	}
   568  	l.tail = s
   569  	goparkunlock(&l.lock, waitReasonSyncCondWait, traceBlockCondWait, 3)
   570  	if t0 != 0 {
   571  		blockevent(s.releasetime-t0, 2)
   572  	}
   573  	releaseSudog(s)
   574  }
   575  
   576  // notifyListNotifyAll notifies all entries in the list.
   577  //
   578  //go:linkname notifyListNotifyAll sync.runtime_notifyListNotifyAll
   579  func notifyListNotifyAll(l *notifyList) {
   580  	// Fast-path: if there are no new waiters since the last notification
   581  	// we don't need to acquire the lock.
   582  	if l.wait.Load() == atomic.Load(&l.notify) {
   583  		return
   584  	}
   585  
   586  	// Pull the list out into a local variable, waiters will be readied
   587  	// outside the lock.
   588  	lockWithRank(&l.lock, lockRankNotifyList)
   589  	s := l.head
   590  	l.head = nil
   591  	l.tail = nil
   592  
   593  	// Update the next ticket to be notified. We can set it to the current
   594  	// value of wait because any previous waiters are already in the list
   595  	// or will notice that they have already been notified when trying to
   596  	// add themselves to the list.
   597  	atomic.Store(&l.notify, l.wait.Load())
   598  	unlock(&l.lock)
   599  
   600  	// Go through the local list and ready all waiters.
   601  	for s != nil {
   602  		next := s.next
   603  		s.next = nil
   604  		readyWithTime(s, 4)
   605  		s = next
   606  	}
   607  }
   608  
   609  // notifyListNotifyOne notifies one entry in the list.
   610  //
   611  //go:linkname notifyListNotifyOne sync.runtime_notifyListNotifyOne
   612  func notifyListNotifyOne(l *notifyList) {
   613  	// Fast-path: if there are no new waiters since the last notification
   614  	// we don't need to acquire the lock at all.
   615  	if l.wait.Load() == atomic.Load(&l.notify) {
   616  		return
   617  	}
   618  
   619  	lockWithRank(&l.lock, lockRankNotifyList)
   620  
   621  	// Re-check under the lock if we need to do anything.
   622  	t := l.notify
   623  	if t == l.wait.Load() {
   624  		unlock(&l.lock)
   625  		return
   626  	}
   627  
   628  	// Update the next notify ticket number.
   629  	atomic.Store(&l.notify, t+1)
   630  
   631  	// Try to find the g that needs to be notified.
   632  	// If it hasn't made it to the list yet we won't find it,
   633  	// but it won't park itself once it sees the new notify number.
   634  	//
   635  	// This scan looks linear but essentially always stops quickly.
   636  	// Because g's queue separately from taking numbers,
   637  	// there may be minor reorderings in the list, but we
   638  	// expect the g we're looking for to be near the front.
   639  	// The g has others in front of it on the list only to the
   640  	// extent that it lost the race, so the iteration will not
   641  	// be too long. This applies even when the g is missing:
   642  	// it hasn't yet gotten to sleep and has lost the race to
   643  	// the (few) other g's that we find on the list.
   644  	for p, s := (*sudog)(nil), l.head; s != nil; p, s = s, s.next {
   645  		if s.ticket == t {
   646  			n := s.next
   647  			if p != nil {
   648  				p.next = n
   649  			} else {
   650  				l.head = n
   651  			}
   652  			if n == nil {
   653  				l.tail = p
   654  			}
   655  			unlock(&l.lock)
   656  			s.next = nil
   657  			readyWithTime(s, 4)
   658  			return
   659  		}
   660  	}
   661  	unlock(&l.lock)
   662  }
   663  
   664  //go:linkname notifyListCheck sync.runtime_notifyListCheck
   665  func notifyListCheck(sz uintptr) {
   666  	if sz != unsafe.Sizeof(notifyList{}) {
   667  		print("runtime: bad notifyList size - sync=", sz, " runtime=", unsafe.Sizeof(notifyList{}), "\n")
   668  		throw("bad notifyList size")
   669  	}
   670  }
   671  
   672  //go:linkname sync_nanotime sync.runtime_nanotime
   673  func sync_nanotime() int64 {
   674  	return nanotime()
   675  }
   676
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