Source file src/runtime/time.go

     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  // Time-related runtime and pieces of package time.
     6  
     7  package runtime
     8  
     9  import (
    10  	"runtime/internal/atomic"
    11  	"runtime/internal/sys"
    12  	"unsafe"
    13  )
    14  
    15  // Package time knows the layout of this structure.
    16  // If this struct changes, adjust ../time/sleep.go:/runtimeTimer.
    17  type timer struct {
    18  	// If this timer is on a heap, which P's heap it is on.
    19  	// puintptr rather than *p to match uintptr in the versions
    20  	// of this struct defined in other packages.
    21  	pp puintptr
    22  
    23  	// Timer wakes up at when, and then at when+period, ... (period > 0 only)
    24  	// each time calling f(arg, now) in the timer goroutine, so f must be
    25  	// a well-behaved function and not block.
    26  	//
    27  	// when must be positive on an active timer.
    28  	when   int64
    29  	period int64
    30  	f      func(interface{}, uintptr)
    31  	arg    interface{}
    32  	seq    uintptr
    33  
    34  	// What to set the when field to in timerModifiedXX status.
    35  	nextwhen int64
    36  
    37  	// The status field holds one of the values below.
    38  	status uint32
    39  }
    40  
    41  // Code outside this file has to be careful in using a timer value.
    42  //
    43  // The pp, status, and nextwhen fields may only be used by code in this file.
    44  //
    45  // Code that creates a new timer value can set the when, period, f,
    46  // arg, and seq fields.
    47  // A new timer value may be passed to addtimer (called by time.startTimer).
    48  // After doing that no fields may be touched.
    49  //
    50  // An active timer (one that has been passed to addtimer) may be
    51  // passed to deltimer (time.stopTimer), after which it is no longer an
    52  // active timer. It is an inactive timer.
    53  // In an inactive timer the period, f, arg, and seq fields may be modified,
    54  // but not the when field.
    55  // It's OK to just drop an inactive timer and let the GC collect it.
    56  // It's not OK to pass an inactive timer to addtimer.
    57  // Only newly allocated timer values may be passed to addtimer.
    58  //
    59  // An active timer may be passed to modtimer. No fields may be touched.
    60  // It remains an active timer.
    61  //
    62  // An inactive timer may be passed to resettimer to turn into an
    63  // active timer with an updated when field.
    64  // It's OK to pass a newly allocated timer value to resettimer.
    65  //
    66  // Timer operations are addtimer, deltimer, modtimer, resettimer,
    67  // cleantimers, adjusttimers, and runtimer.
    68  //
    69  // We don't permit calling addtimer/deltimer/modtimer/resettimer simultaneously,
    70  // but adjusttimers and runtimer can be called at the same time as any of those.
    71  //
    72  // Active timers live in heaps attached to P, in the timers field.
    73  // Inactive timers live there too temporarily, until they are removed.
    74  //
    75  // addtimer:
    76  //   timerNoStatus   -> timerWaiting
    77  //   anything else   -> panic: invalid value
    78  // deltimer:
    79  //   timerWaiting         -> timerModifying -> timerDeleted
    80  //   timerModifiedEarlier -> timerModifying -> timerDeleted
    81  //   timerModifiedLater   -> timerModifying -> timerDeleted
    82  //   timerNoStatus        -> do nothing
    83  //   timerDeleted         -> do nothing
    84  //   timerRemoving        -> do nothing
    85  //   timerRemoved         -> do nothing
    86  //   timerRunning         -> wait until status changes
    87  //   timerMoving          -> wait until status changes
    88  //   timerModifying       -> wait until status changes
    89  // modtimer:
    90  //   timerWaiting    -> timerModifying -> timerModifiedXX
    91  //   timerModifiedXX -> timerModifying -> timerModifiedYY
    92  //   timerNoStatus   -> timerModifying -> timerWaiting
    93  //   timerRemoved    -> timerModifying -> timerWaiting
    94  //   timerDeleted    -> timerModifying -> timerModifiedXX
    95  //   timerRunning    -> wait until status changes
    96  //   timerMoving     -> wait until status changes
    97  //   timerRemoving   -> wait until status changes
    98  //   timerModifying  -> wait until status changes
    99  // cleantimers (looks in P's timer heap):
   100  //   timerDeleted    -> timerRemoving -> timerRemoved
   101  //   timerModifiedXX -> timerMoving -> timerWaiting
   102  // adjusttimers (looks in P's timer heap):
   103  //   timerDeleted    -> timerRemoving -> timerRemoved
   104  //   timerModifiedXX -> timerMoving -> timerWaiting
   105  // runtimer (looks in P's timer heap):
   106  //   timerNoStatus   -> panic: uninitialized timer
   107  //   timerWaiting    -> timerWaiting or
   108  //   timerWaiting    -> timerRunning -> timerNoStatus or
   109  //   timerWaiting    -> timerRunning -> timerWaiting
   110  //   timerModifying  -> wait until status changes
   111  //   timerModifiedXX -> timerMoving -> timerWaiting
   112  //   timerDeleted    -> timerRemoving -> timerRemoved
   113  //   timerRunning    -> panic: concurrent runtimer calls
   114  //   timerRemoved    -> panic: inconsistent timer heap
   115  //   timerRemoving   -> panic: inconsistent timer heap
   116  //   timerMoving     -> panic: inconsistent timer heap
   117  
   118  // Values for the timer status field.
   119  const (
   120  	// Timer has no status set yet.
   121  	timerNoStatus = iota
   122  
   123  	// Waiting for timer to fire.
   124  	// The timer is in some P's heap.
   125  	timerWaiting
   126  
   127  	// Running the timer function.
   128  	// A timer will only have this status briefly.
   129  	timerRunning
   130  
   131  	// The timer is deleted and should be removed.
   132  	// It should not be run, but it is still in some P's heap.
   133  	timerDeleted
   134  
   135  	// The timer is being removed.
   136  	// The timer will only have this status briefly.
   137  	timerRemoving
   138  
   139  	// The timer has been stopped.
   140  	// It is not in any P's heap.
   141  	timerRemoved
   142  
   143  	// The timer is being modified.
   144  	// The timer will only have this status briefly.
   145  	timerModifying
   146  
   147  	// The timer has been modified to an earlier time.
   148  	// The new when value is in the nextwhen field.
   149  	// The timer is in some P's heap, possibly in the wrong place.
   150  	timerModifiedEarlier
   151  
   152  	// The timer has been modified to the same or a later time.
   153  	// The new when value is in the nextwhen field.
   154  	// The timer is in some P's heap, possibly in the wrong place.
   155  	timerModifiedLater
   156  
   157  	// The timer has been modified and is being moved.
   158  	// The timer will only have this status briefly.
   159  	timerMoving
   160  )
   161  
   162  // maxWhen is the maximum value for timer's when field.
   163  const maxWhen = 1<<63 - 1
   164  
   165  // verifyTimers can be set to true to add debugging checks that the
   166  // timer heaps are valid.
   167  const verifyTimers = false
   168  
   169  // Package time APIs.
   170  // Godoc uses the comments in package time, not these.
   171  
   172  // time.now is implemented in assembly.
   173  
   174  // timeSleep puts the current goroutine to sleep for at least ns nanoseconds.
   175  //go:linkname timeSleep time.Sleep
   176  func timeSleep(ns int64) {
   177  	if ns <= 0 {
   178  		return
   179  	}
   180  
   181  	gp := getg()
   182  	t := gp.timer
   183  	if t == nil {
   184  		t = new(timer)
   185  		gp.timer = t
   186  	}
   187  	t.f = goroutineReady
   188  	t.arg = gp
   189  	t.nextwhen = nanotime() + ns
   190  	if t.nextwhen < 0 { // check for overflow.
   191  		t.nextwhen = maxWhen
   192  	}
   193  	gopark(resetForSleep, unsafe.Pointer(t), waitReasonSleep, traceEvGoSleep, 1)
   194  }
   195  
   196  // resetForSleep is called after the goroutine is parked for timeSleep.
   197  // We can't call resettimer in timeSleep itself because if this is a short
   198  // sleep and there are many goroutines then the P can wind up running the
   199  // timer function, goroutineReady, before the goroutine has been parked.
   200  func resetForSleep(gp *g, ut unsafe.Pointer) bool {
   201  	t := (*timer)(ut)
   202  	resettimer(t, t.nextwhen)
   203  	return true
   204  }
   205  
   206  // startTimer adds t to the timer heap.
   207  //go:linkname startTimer time.startTimer
   208  func startTimer(t *timer) {
   209  	if raceenabled {
   210  		racerelease(unsafe.Pointer(t))
   211  	}
   212  	addtimer(t)
   213  }
   214  
   215  // stopTimer stops a timer.
   216  // It reports whether t was stopped before being run.
   217  //go:linkname stopTimer time.stopTimer
   218  func stopTimer(t *timer) bool {
   219  	return deltimer(t)
   220  }
   221  
   222  // resetTimer resets an inactive timer, adding it to the heap.
   223  //go:linkname resetTimer time.resetTimer
   224  // Reports whether the timer was modified before it was run.
   225  func resetTimer(t *timer, when int64) bool {
   226  	if raceenabled {
   227  		racerelease(unsafe.Pointer(t))
   228  	}
   229  	return resettimer(t, when)
   230  }
   231  
   232  // modTimer modifies an existing timer.
   233  //go:linkname modTimer time.modTimer
   234  func modTimer(t *timer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr) {
   235  	modtimer(t, when, period, f, arg, seq)
   236  }
   237  
   238  // Go runtime.
   239  
   240  // Ready the goroutine arg.
   241  func goroutineReady(arg interface{}, seq uintptr) {
   242  	goready(arg.(*g), 0)
   243  }
   244  
   245  // addtimer adds a timer to the current P.
   246  // This should only be called with a newly created timer.
   247  // That avoids the risk of changing the when field of a timer in some P's heap,
   248  // which could cause the heap to become unsorted.
   249  func addtimer(t *timer) {
   250  	// when must be positive. A negative value will cause runtimer to
   251  	// overflow during its delta calculation and never expire other runtime
   252  	// timers. Zero will cause checkTimers to fail to notice the timer.
   253  	if t.when <= 0 {
   254  		throw("timer when must be positive")
   255  	}
   256  	if t.period < 0 {
   257  		throw("timer period must be non-negative")
   258  	}
   259  	if t.status != timerNoStatus {
   260  		throw("addtimer called with initialized timer")
   261  	}
   262  	t.status = timerWaiting
   263  
   264  	when := t.when
   265  
   266  	// Disable preemption while using pp to avoid changing another P's heap.
   267  	mp := acquirem()
   268  
   269  	pp := getg().m.p.ptr()
   270  	lock(&pp.timersLock)
   271  	cleantimers(pp)
   272  	doaddtimer(pp, t)
   273  	unlock(&pp.timersLock)
   274  
   275  	wakeNetPoller(when)
   276  
   277  	releasem(mp)
   278  }
   279  
   280  // doaddtimer adds t to the current P's heap.
   281  // The caller must have locked the timers for pp.
   282  func doaddtimer(pp *p, t *timer) {
   283  	// Timers rely on the network poller, so make sure the poller
   284  	// has started.
   285  	if netpollInited == 0 {
   286  		netpollGenericInit()
   287  	}
   288  
   289  	if t.pp != 0 {
   290  		throw("doaddtimer: P already set in timer")
   291  	}
   292  	t.pp.set(pp)
   293  	i := len(pp.timers)
   294  	pp.timers = append(pp.timers, t)
   295  	siftupTimer(pp.timers, i)
   296  	if t == pp.timers[0] {
   297  		atomic.Store64(&pp.timer0When, uint64(t.when))
   298  	}
   299  	atomic.Xadd(&pp.numTimers, 1)
   300  }
   301  
   302  // deltimer deletes the timer t. It may be on some other P, so we can't
   303  // actually remove it from the timers heap. We can only mark it as deleted.
   304  // It will be removed in due course by the P whose heap it is on.
   305  // Reports whether the timer was removed before it was run.
   306  func deltimer(t *timer) bool {
   307  	for {
   308  		switch s := atomic.Load(&t.status); s {
   309  		case timerWaiting, timerModifiedLater:
   310  			// Prevent preemption while the timer is in timerModifying.
   311  			// This could lead to a self-deadlock. See #38070.
   312  			mp := acquirem()
   313  			if atomic.Cas(&t.status, s, timerModifying) {
   314  				// Must fetch t.pp before changing status,
   315  				// as cleantimers in another goroutine
   316  				// can clear t.pp of a timerDeleted timer.
   317  				tpp := t.pp.ptr()
   318  				if !atomic.Cas(&t.status, timerModifying, timerDeleted) {
   319  					badTimer()
   320  				}
   321  				releasem(mp)
   322  				atomic.Xadd(&tpp.deletedTimers, 1)
   323  				// Timer was not yet run.
   324  				return true
   325  			} else {
   326  				releasem(mp)
   327  			}
   328  		case timerModifiedEarlier:
   329  			// Prevent preemption while the timer is in timerModifying.
   330  			// This could lead to a self-deadlock. See #38070.
   331  			mp := acquirem()
   332  			if atomic.Cas(&t.status, s, timerModifying) {
   333  				// Must fetch t.pp before setting status
   334  				// to timerDeleted.
   335  				tpp := t.pp.ptr()
   336  				if !atomic.Cas(&t.status, timerModifying, timerDeleted) {
   337  					badTimer()
   338  				}
   339  				releasem(mp)
   340  				atomic.Xadd(&tpp.deletedTimers, 1)
   341  				// Timer was not yet run.
   342  				return true
   343  			} else {
   344  				releasem(mp)
   345  			}
   346  		case timerDeleted, timerRemoving, timerRemoved:
   347  			// Timer was already run.
   348  			return false
   349  		case timerRunning, timerMoving:
   350  			// The timer is being run or moved, by a different P.
   351  			// Wait for it to complete.
   352  			osyield()
   353  		case timerNoStatus:
   354  			// Removing timer that was never added or
   355  			// has already been run. Also see issue 21874.
   356  			return false
   357  		case timerModifying:
   358  			// Simultaneous calls to deltimer and modtimer.
   359  			// Wait for the other call to complete.
   360  			osyield()
   361  		default:
   362  			badTimer()
   363  		}
   364  	}
   365  }
   366  
   367  // dodeltimer removes timer i from the current P's heap.
   368  // We are locked on the P when this is called.
   369  // It returns the smallest changed index in pp.timers.
   370  // The caller must have locked the timers for pp.
   371  func dodeltimer(pp *p, i int) int {
   372  	if t := pp.timers[i]; t.pp.ptr() != pp {
   373  		throw("dodeltimer: wrong P")
   374  	} else {
   375  		t.pp = 0
   376  	}
   377  	last := len(pp.timers) - 1
   378  	if i != last {
   379  		pp.timers[i] = pp.timers[last]
   380  	}
   381  	pp.timers[last] = nil
   382  	pp.timers = pp.timers[:last]
   383  	smallestChanged := i
   384  	if i != last {
   385  		// Moving to i may have moved the last timer to a new parent,
   386  		// so sift up to preserve the heap guarantee.
   387  		smallestChanged = siftupTimer(pp.timers, i)
   388  		siftdownTimer(pp.timers, i)
   389  	}
   390  	if i == 0 {
   391  		updateTimer0When(pp)
   392  	}
   393  	atomic.Xadd(&pp.numTimers, -1)
   394  	return smallestChanged
   395  }
   396  
   397  // dodeltimer0 removes timer 0 from the current P's heap.
   398  // We are locked on the P when this is called.
   399  // It reports whether it saw no problems due to races.
   400  // The caller must have locked the timers for pp.
   401  func dodeltimer0(pp *p) {
   402  	if t := pp.timers[0]; t.pp.ptr() != pp {
   403  		throw("dodeltimer0: wrong P")
   404  	} else {
   405  		t.pp = 0
   406  	}
   407  	last := len(pp.timers) - 1
   408  	if last > 0 {
   409  		pp.timers[0] = pp.timers[last]
   410  	}
   411  	pp.timers[last] = nil
   412  	pp.timers = pp.timers[:last]
   413  	if last > 0 {
   414  		siftdownTimer(pp.timers, 0)
   415  	}
   416  	updateTimer0When(pp)
   417  	atomic.Xadd(&pp.numTimers, -1)
   418  }
   419  
   420  // modtimer modifies an existing timer.
   421  // This is called by the netpoll code or time.Ticker.Reset or time.Timer.Reset.
   422  // Reports whether the timer was modified before it was run.
   423  func modtimer(t *timer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr) bool {
   424  	if when <= 0 {
   425  		throw("timer when must be positive")
   426  	}
   427  	if period < 0 {
   428  		throw("timer period must be non-negative")
   429  	}
   430  
   431  	status := uint32(timerNoStatus)
   432  	wasRemoved := false
   433  	var pending bool
   434  	var mp *m
   435  loop:
   436  	for {
   437  		switch status = atomic.Load(&t.status); status {
   438  		case timerWaiting, timerModifiedEarlier, timerModifiedLater:
   439  			// Prevent preemption while the timer is in timerModifying.
   440  			// This could lead to a self-deadlock. See #38070.
   441  			mp = acquirem()
   442  			if atomic.Cas(&t.status, status, timerModifying) {
   443  				pending = true // timer not yet run
   444  				break loop
   445  			}
   446  			releasem(mp)
   447  		case timerNoStatus, timerRemoved:
   448  			// Prevent preemption while the timer is in timerModifying.
   449  			// This could lead to a self-deadlock. See #38070.
   450  			mp = acquirem()
   451  
   452  			// Timer was already run and t is no longer in a heap.
   453  			// Act like addtimer.
   454  			if atomic.Cas(&t.status, status, timerModifying) {
   455  				wasRemoved = true
   456  				pending = false // timer already run or stopped
   457  				break loop
   458  			}
   459  			releasem(mp)
   460  		case timerDeleted:
   461  			// Prevent preemption while the timer is in timerModifying.
   462  			// This could lead to a self-deadlock. See #38070.
   463  			mp = acquirem()
   464  			if atomic.Cas(&t.status, status, timerModifying) {
   465  				atomic.Xadd(&t.pp.ptr().deletedTimers, -1)
   466  				pending = false // timer already stopped
   467  				break loop
   468  			}
   469  			releasem(mp)
   470  		case timerRunning, timerRemoving, timerMoving:
   471  			// The timer is being run or moved, by a different P.
   472  			// Wait for it to complete.
   473  			osyield()
   474  		case timerModifying:
   475  			// Multiple simultaneous calls to modtimer.
   476  			// Wait for the other call to complete.
   477  			osyield()
   478  		default:
   479  			badTimer()
   480  		}
   481  	}
   482  
   483  	t.period = period
   484  	t.f = f
   485  	t.arg = arg
   486  	t.seq = seq
   487  
   488  	if wasRemoved {
   489  		t.when = when
   490  		pp := getg().m.p.ptr()
   491  		lock(&pp.timersLock)
   492  		doaddtimer(pp, t)
   493  		unlock(&pp.timersLock)
   494  		if !atomic.Cas(&t.status, timerModifying, timerWaiting) {
   495  			badTimer()
   496  		}
   497  		releasem(mp)
   498  		wakeNetPoller(when)
   499  	} else {
   500  		// The timer is in some other P's heap, so we can't change
   501  		// the when field. If we did, the other P's heap would
   502  		// be out of order. So we put the new when value in the
   503  		// nextwhen field, and let the other P set the when field
   504  		// when it is prepared to resort the heap.
   505  		t.nextwhen = when
   506  
   507  		newStatus := uint32(timerModifiedLater)
   508  		if when < t.when {
   509  			newStatus = timerModifiedEarlier
   510  		}
   511  
   512  		tpp := t.pp.ptr()
   513  
   514  		if newStatus == timerModifiedEarlier {
   515  			updateTimerModifiedEarliest(tpp, when)
   516  		}
   517  
   518  		// Set the new status of the timer.
   519  		if !atomic.Cas(&t.status, timerModifying, newStatus) {
   520  			badTimer()
   521  		}
   522  		releasem(mp)
   523  
   524  		// If the new status is earlier, wake up the poller.
   525  		if newStatus == timerModifiedEarlier {
   526  			wakeNetPoller(when)
   527  		}
   528  	}
   529  
   530  	return pending
   531  }
   532  
   533  // resettimer resets the time when a timer should fire.
   534  // If used for an inactive timer, the timer will become active.
   535  // This should be called instead of addtimer if the timer value has been,
   536  // or may have been, used previously.
   537  // Reports whether the timer was modified before it was run.
   538  func resettimer(t *timer, when int64) bool {
   539  	return modtimer(t, when, t.period, t.f, t.arg, t.seq)
   540  }
   541  
   542  // cleantimers cleans up the head of the timer queue. This speeds up
   543  // programs that create and delete timers; leaving them in the heap
   544  // slows down addtimer. Reports whether no timer problems were found.
   545  // The caller must have locked the timers for pp.
   546  func cleantimers(pp *p) {
   547  	gp := getg()
   548  	for {
   549  		if len(pp.timers) == 0 {
   550  			return
   551  		}
   552  
   553  		// This loop can theoretically run for a while, and because
   554  		// it is holding timersLock it cannot be preempted.
   555  		// If someone is trying to preempt us, just return.
   556  		// We can clean the timers later.
   557  		if gp.preemptStop {
   558  			return
   559  		}
   560  
   561  		t := pp.timers[0]
   562  		if t.pp.ptr() != pp {
   563  			throw("cleantimers: bad p")
   564  		}
   565  		switch s := atomic.Load(&t.status); s {
   566  		case timerDeleted:
   567  			if !atomic.Cas(&t.status, s, timerRemoving) {
   568  				continue
   569  			}
   570  			dodeltimer0(pp)
   571  			if !atomic.Cas(&t.status, timerRemoving, timerRemoved) {
   572  				badTimer()
   573  			}
   574  			atomic.Xadd(&pp.deletedTimers, -1)
   575  		case timerModifiedEarlier, timerModifiedLater:
   576  			if !atomic.Cas(&t.status, s, timerMoving) {
   577  				continue
   578  			}
   579  			// Now we can change the when field.
   580  			t.when = t.nextwhen
   581  			// Move t to the right position.
   582  			dodeltimer0(pp)
   583  			doaddtimer(pp, t)
   584  			if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
   585  				badTimer()
   586  			}
   587  		default:
   588  			// Head of timers does not need adjustment.
   589  			return
   590  		}
   591  	}
   592  }
   593  
   594  // moveTimers moves a slice of timers to pp. The slice has been taken
   595  // from a different P.
   596  // This is currently called when the world is stopped, but the caller
   597  // is expected to have locked the timers for pp.
   598  func moveTimers(pp *p, timers []*timer) {
   599  	for _, t := range timers {
   600  	loop:
   601  		for {
   602  			switch s := atomic.Load(&t.status); s {
   603  			case timerWaiting:
   604  				if !atomic.Cas(&t.status, s, timerMoving) {
   605  					continue
   606  				}
   607  				t.pp = 0
   608  				doaddtimer(pp, t)
   609  				if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
   610  					badTimer()
   611  				}
   612  				break loop
   613  			case timerModifiedEarlier, timerModifiedLater:
   614  				if !atomic.Cas(&t.status, s, timerMoving) {
   615  					continue
   616  				}
   617  				t.when = t.nextwhen
   618  				t.pp = 0
   619  				doaddtimer(pp, t)
   620  				if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
   621  					badTimer()
   622  				}
   623  				break loop
   624  			case timerDeleted:
   625  				if !atomic.Cas(&t.status, s, timerRemoved) {
   626  					continue
   627  				}
   628  				t.pp = 0
   629  				// We no longer need this timer in the heap.
   630  				break loop
   631  			case timerModifying:
   632  				// Loop until the modification is complete.
   633  				osyield()
   634  			case timerNoStatus, timerRemoved:
   635  				// We should not see these status values in a timers heap.
   636  				badTimer()
   637  			case timerRunning, timerRemoving, timerMoving:
   638  				// Some other P thinks it owns this timer,
   639  				// which should not happen.
   640  				badTimer()
   641  			default:
   642  				badTimer()
   643  			}
   644  		}
   645  	}
   646  }
   647  
   648  // adjusttimers looks through the timers in the current P's heap for
   649  // any timers that have been modified to run earlier, and puts them in
   650  // the correct place in the heap. While looking for those timers,
   651  // it also moves timers that have been modified to run later,
   652  // and removes deleted timers. The caller must have locked the timers for pp.
   653  func adjusttimers(pp *p, now int64) {
   654  	// If we haven't yet reached the time of the first timerModifiedEarlier
   655  	// timer, don't do anything. This speeds up programs that adjust
   656  	// a lot of timers back and forth if the timers rarely expire.
   657  	// We'll postpone looking through all the adjusted timers until
   658  	// one would actually expire.
   659  	first := atomic.Load64(&pp.timerModifiedEarliest)
   660  	if first == 0 || int64(first) > now {
   661  		if verifyTimers {
   662  			verifyTimerHeap(pp)
   663  		}
   664  		return
   665  	}
   666  
   667  	// We are going to clear all timerModifiedEarlier timers.
   668  	atomic.Store64(&pp.timerModifiedEarliest, 0)
   669  
   670  	var moved []*timer
   671  	for i := 0; i < len(pp.timers); i++ {
   672  		t := pp.timers[i]
   673  		if t.pp.ptr() != pp {
   674  			throw("adjusttimers: bad p")
   675  		}
   676  		switch s := atomic.Load(&t.status); s {
   677  		case timerDeleted:
   678  			if atomic.Cas(&t.status, s, timerRemoving) {
   679  				changed := dodeltimer(pp, i)
   680  				if !atomic.Cas(&t.status, timerRemoving, timerRemoved) {
   681  					badTimer()
   682  				}
   683  				atomic.Xadd(&pp.deletedTimers, -1)
   684  				// Go back to the earliest changed heap entry.
   685  				// "- 1" because the loop will add 1.
   686  				i = changed - 1
   687  			}
   688  		case timerModifiedEarlier, timerModifiedLater:
   689  			if atomic.Cas(&t.status, s, timerMoving) {
   690  				// Now we can change the when field.
   691  				t.when = t.nextwhen
   692  				// Take t off the heap, and hold onto it.
   693  				// We don't add it back yet because the
   694  				// heap manipulation could cause our
   695  				// loop to skip some other timer.
   696  				changed := dodeltimer(pp, i)
   697  				moved = append(moved, t)
   698  				// Go back to the earliest changed heap entry.
   699  				// "- 1" because the loop will add 1.
   700  				i = changed - 1
   701  			}
   702  		case timerNoStatus, timerRunning, timerRemoving, timerRemoved, timerMoving:
   703  			badTimer()
   704  		case timerWaiting:
   705  			// OK, nothing to do.
   706  		case timerModifying:
   707  			// Check again after modification is complete.
   708  			osyield()
   709  			i--
   710  		default:
   711  			badTimer()
   712  		}
   713  	}
   714  
   715  	if len(moved) > 0 {
   716  		addAdjustedTimers(pp, moved)
   717  	}
   718  
   719  	if verifyTimers {
   720  		verifyTimerHeap(pp)
   721  	}
   722  }
   723  
   724  // addAdjustedTimers adds any timers we adjusted in adjusttimers
   725  // back to the timer heap.
   726  func addAdjustedTimers(pp *p, moved []*timer) {
   727  	for _, t := range moved {
   728  		doaddtimer(pp, t)
   729  		if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
   730  			badTimer()
   731  		}
   732  	}
   733  }
   734  
   735  // nobarrierWakeTime looks at P's timers and returns the time when we
   736  // should wake up the netpoller. It returns 0 if there are no timers.
   737  // This function is invoked when dropping a P, and must run without
   738  // any write barriers.
   739  //go:nowritebarrierrec
   740  func nobarrierWakeTime(pp *p) int64 {
   741  	next := int64(atomic.Load64(&pp.timer0When))
   742  	nextAdj := int64(atomic.Load64(&pp.timerModifiedEarliest))
   743  	if next == 0 || (nextAdj != 0 && nextAdj < next) {
   744  		next = nextAdj
   745  	}
   746  	return next
   747  }
   748  
   749  // runtimer examines the first timer in timers. If it is ready based on now,
   750  // it runs the timer and removes or updates it.
   751  // Returns 0 if it ran a timer, -1 if there are no more timers, or the time
   752  // when the first timer should run.
   753  // The caller must have locked the timers for pp.
   754  // If a timer is run, this will temporarily unlock the timers.
   755  //go:systemstack
   756  func runtimer(pp *p, now int64) int64 {
   757  	for {
   758  		t := pp.timers[0]
   759  		if t.pp.ptr() != pp {
   760  			throw("runtimer: bad p")
   761  		}
   762  		switch s := atomic.Load(&t.status); s {
   763  		case timerWaiting:
   764  			if t.when > now {
   765  				// Not ready to run.
   766  				return t.when
   767  			}
   768  
   769  			if !atomic.Cas(&t.status, s, timerRunning) {
   770  				continue
   771  			}
   772  			// Note that runOneTimer may temporarily unlock
   773  			// pp.timersLock.
   774  			runOneTimer(pp, t, now)
   775  			return 0
   776  
   777  		case timerDeleted:
   778  			if !atomic.Cas(&t.status, s, timerRemoving) {
   779  				continue
   780  			}
   781  			dodeltimer0(pp)
   782  			if !atomic.Cas(&t.status, timerRemoving, timerRemoved) {
   783  				badTimer()
   784  			}
   785  			atomic.Xadd(&pp.deletedTimers, -1)
   786  			if len(pp.timers) == 0 {
   787  				return -1
   788  			}
   789  
   790  		case timerModifiedEarlier, timerModifiedLater:
   791  			if !atomic.Cas(&t.status, s, timerMoving) {
   792  				continue
   793  			}
   794  			t.when = t.nextwhen
   795  			dodeltimer0(pp)
   796  			doaddtimer(pp, t)
   797  			if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
   798  				badTimer()
   799  			}
   800  
   801  		case timerModifying:
   802  			// Wait for modification to complete.
   803  			osyield()
   804  
   805  		case timerNoStatus, timerRemoved:
   806  			// Should not see a new or inactive timer on the heap.
   807  			badTimer()
   808  		case timerRunning, timerRemoving, timerMoving:
   809  			// These should only be set when timers are locked,
   810  			// and we didn't do it.
   811  			badTimer()
   812  		default:
   813  			badTimer()
   814  		}
   815  	}
   816  }
   817  
   818  // runOneTimer runs a single timer.
   819  // The caller must have locked the timers for pp.
   820  // This will temporarily unlock the timers while running the timer function.
   821  //go:systemstack
   822  func runOneTimer(pp *p, t *timer, now int64) {
   823  	if raceenabled {
   824  		ppcur := getg().m.p.ptr()
   825  		if ppcur.timerRaceCtx == 0 {
   826  			ppcur.timerRaceCtx = racegostart(funcPC(runtimer) + sys.PCQuantum)
   827  		}
   828  		raceacquirectx(ppcur.timerRaceCtx, unsafe.Pointer(t))
   829  	}
   830  
   831  	f := t.f
   832  	arg := t.arg
   833  	seq := t.seq
   834  
   835  	if t.period > 0 {
   836  		// Leave in heap but adjust next time to fire.
   837  		delta := t.when - now
   838  		t.when += t.period * (1 + -delta/t.period)
   839  		if t.when < 0 { // check for overflow.
   840  			t.when = maxWhen
   841  		}
   842  		siftdownTimer(pp.timers, 0)
   843  		if !atomic.Cas(&t.status, timerRunning, timerWaiting) {
   844  			badTimer()
   845  		}
   846  		updateTimer0When(pp)
   847  	} else {
   848  		// Remove from heap.
   849  		dodeltimer0(pp)
   850  		if !atomic.Cas(&t.status, timerRunning, timerNoStatus) {
   851  			badTimer()
   852  		}
   853  	}
   854  
   855  	if raceenabled {
   856  		// Temporarily use the current P's racectx for g0.
   857  		gp := getg()
   858  		if gp.racectx != 0 {
   859  			throw("runOneTimer: unexpected racectx")
   860  		}
   861  		gp.racectx = gp.m.p.ptr().timerRaceCtx
   862  	}
   863  
   864  	unlock(&pp.timersLock)
   865  
   866  	f(arg, seq)
   867  
   868  	lock(&pp.timersLock)
   869  
   870  	if raceenabled {
   871  		gp := getg()
   872  		gp.racectx = 0
   873  	}
   874  }
   875  
   876  // clearDeletedTimers removes all deleted timers from the P's timer heap.
   877  // This is used to avoid clogging up the heap if the program
   878  // starts a lot of long-running timers and then stops them.
   879  // For example, this can happen via context.WithTimeout.
   880  //
   881  // This is the only function that walks through the entire timer heap,
   882  // other than moveTimers which only runs when the world is stopped.
   883  //
   884  // The caller must have locked the timers for pp.
   885  func clearDeletedTimers(pp *p) {
   886  	// We are going to clear all timerModifiedEarlier timers.
   887  	// Do this now in case new ones show up while we are looping.
   888  	atomic.Store64(&pp.timerModifiedEarliest, 0)
   889  
   890  	cdel := int32(0)
   891  	to := 0
   892  	changedHeap := false
   893  	timers := pp.timers
   894  nextTimer:
   895  	for _, t := range timers {
   896  		for {
   897  			switch s := atomic.Load(&t.status); s {
   898  			case timerWaiting:
   899  				if changedHeap {
   900  					timers[to] = t
   901  					siftupTimer(timers, to)
   902  				}
   903  				to++
   904  				continue nextTimer
   905  			case timerModifiedEarlier, timerModifiedLater:
   906  				if atomic.Cas(&t.status, s, timerMoving) {
   907  					t.when = t.nextwhen
   908  					timers[to] = t
   909  					siftupTimer(timers, to)
   910  					to++
   911  					changedHeap = true
   912  					if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
   913  						badTimer()
   914  					}
   915  					continue nextTimer
   916  				}
   917  			case timerDeleted:
   918  				if atomic.Cas(&t.status, s, timerRemoving) {
   919  					t.pp = 0
   920  					cdel++
   921  					if !atomic.Cas(&t.status, timerRemoving, timerRemoved) {
   922  						badTimer()
   923  					}
   924  					changedHeap = true
   925  					continue nextTimer
   926  				}
   927  			case timerModifying:
   928  				// Loop until modification complete.
   929  				osyield()
   930  			case timerNoStatus, timerRemoved:
   931  				// We should not see these status values in a timer heap.
   932  				badTimer()
   933  			case timerRunning, timerRemoving, timerMoving:
   934  				// Some other P thinks it owns this timer,
   935  				// which should not happen.
   936  				badTimer()
   937  			default:
   938  				badTimer()
   939  			}
   940  		}
   941  	}
   942  
   943  	// Set remaining slots in timers slice to nil,
   944  	// so that the timer values can be garbage collected.
   945  	for i := to; i < len(timers); i++ {
   946  		timers[i] = nil
   947  	}
   948  
   949  	atomic.Xadd(&pp.deletedTimers, -cdel)
   950  	atomic.Xadd(&pp.numTimers, -cdel)
   951  
   952  	timers = timers[:to]
   953  	pp.timers = timers
   954  	updateTimer0When(pp)
   955  
   956  	if verifyTimers {
   957  		verifyTimerHeap(pp)
   958  	}
   959  }
   960  
   961  // verifyTimerHeap verifies that the timer heap is in a valid state.
   962  // This is only for debugging, and is only called if verifyTimers is true.
   963  // The caller must have locked the timers.
   964  func verifyTimerHeap(pp *p) {
   965  	for i, t := range pp.timers {
   966  		if i == 0 {
   967  			// First timer has no parent.
   968  			continue
   969  		}
   970  
   971  		// The heap is 4-ary. See siftupTimer and siftdownTimer.
   972  		p := (i - 1) / 4
   973  		if t.when < pp.timers[p].when {
   974  			print("bad timer heap at ", i, ": ", p, ": ", pp.timers[p].when, ", ", i, ": ", t.when, "\n")
   975  			throw("bad timer heap")
   976  		}
   977  	}
   978  	if numTimers := int(atomic.Load(&pp.numTimers)); len(pp.timers) != numTimers {
   979  		println("timer heap len", len(pp.timers), "!= numTimers", numTimers)
   980  		throw("bad timer heap len")
   981  	}
   982  }
   983  
   984  // updateTimer0When sets the P's timer0When field.
   985  // The caller must have locked the timers for pp.
   986  func updateTimer0When(pp *p) {
   987  	if len(pp.timers) == 0 {
   988  		atomic.Store64(&pp.timer0When, 0)
   989  	} else {
   990  		atomic.Store64(&pp.timer0When, uint64(pp.timers[0].when))
   991  	}
   992  }
   993  
   994  // updateTimerModifiedEarliest updates the recorded nextwhen field of the
   995  // earlier timerModifiedEarier value.
   996  // The timers for pp will not be locked.
   997  func updateTimerModifiedEarliest(pp *p, nextwhen int64) {
   998  	for {
   999  		old := atomic.Load64(&pp.timerModifiedEarliest)
  1000  		if old != 0 && int64(old) < nextwhen {
  1001  			return
  1002  		}
  1003  		if atomic.Cas64(&pp.timerModifiedEarliest, old, uint64(nextwhen)) {
  1004  			return
  1005  		}
  1006  	}
  1007  }
  1008  
  1009  // timeSleepUntil returns the time when the next timer should fire,
  1010  // and the P that holds the timer heap that that timer is on.
  1011  // This is only called by sysmon and checkdead.
  1012  func timeSleepUntil() (int64, *p) {
  1013  	next := int64(maxWhen)
  1014  	var pret *p
  1015  
  1016  	// Prevent allp slice changes. This is like retake.
  1017  	lock(&allpLock)
  1018  	for _, pp := range allp {
  1019  		if pp == nil {
  1020  			// This can happen if procresize has grown
  1021  			// allp but not yet created new Ps.
  1022  			continue
  1023  		}
  1024  
  1025  		w := int64(atomic.Load64(&pp.timer0When))
  1026  		if w != 0 && w < next {
  1027  			next = w
  1028  			pret = pp
  1029  		}
  1030  
  1031  		w = int64(atomic.Load64(&pp.timerModifiedEarliest))
  1032  		if w != 0 && w < next {
  1033  			next = w
  1034  			pret = pp
  1035  		}
  1036  	}
  1037  	unlock(&allpLock)
  1038  
  1039  	return next, pret
  1040  }
  1041  
  1042  // Heap maintenance algorithms.
  1043  // These algorithms check for slice index errors manually.
  1044  // Slice index error can happen if the program is using racy
  1045  // access to timers. We don't want to panic here, because
  1046  // it will cause the program to crash with a mysterious
  1047  // "panic holding locks" message. Instead, we panic while not
  1048  // holding a lock.
  1049  
  1050  // siftupTimer puts the timer at position i in the right place
  1051  // in the heap by moving it up toward the top of the heap.
  1052  // It returns the smallest changed index.
  1053  func siftupTimer(t []*timer, i int) int {
  1054  	if i >= len(t) {
  1055  		badTimer()
  1056  	}
  1057  	when := t[i].when
  1058  	if when <= 0 {
  1059  		badTimer()
  1060  	}
  1061  	tmp := t[i]
  1062  	for i > 0 {
  1063  		p := (i - 1) / 4 // parent
  1064  		if when >= t[p].when {
  1065  			break
  1066  		}
  1067  		t[i] = t[p]
  1068  		i = p
  1069  	}
  1070  	if tmp != t[i] {
  1071  		t[i] = tmp
  1072  	}
  1073  	return i
  1074  }
  1075  
  1076  // siftdownTimer puts the timer at position i in the right place
  1077  // in the heap by moving it down toward the bottom of the heap.
  1078  func siftdownTimer(t []*timer, i int) {
  1079  	n := len(t)
  1080  	if i >= n {
  1081  		badTimer()
  1082  	}
  1083  	when := t[i].when
  1084  	if when <= 0 {
  1085  		badTimer()
  1086  	}
  1087  	tmp := t[i]
  1088  	for {
  1089  		c := i*4 + 1 // left child
  1090  		c3 := c + 2  // mid child
  1091  		if c >= n {
  1092  			break
  1093  		}
  1094  		w := t[c].when
  1095  		if c+1 < n && t[c+1].when < w {
  1096  			w = t[c+1].when
  1097  			c++
  1098  		}
  1099  		if c3 < n {
  1100  			w3 := t[c3].when
  1101  			if c3+1 < n && t[c3+1].when < w3 {
  1102  				w3 = t[c3+1].when
  1103  				c3++
  1104  			}
  1105  			if w3 < w {
  1106  				w = w3
  1107  				c = c3
  1108  			}
  1109  		}
  1110  		if w >= when {
  1111  			break
  1112  		}
  1113  		t[i] = t[c]
  1114  		i = c
  1115  	}
  1116  	if tmp != t[i] {
  1117  		t[i] = tmp
  1118  	}
  1119  }
  1120  
  1121  // badTimer is called if the timer data structures have been corrupted,
  1122  // presumably due to racy use by the program. We panic here rather than
  1123  // panicing due to invalid slice access while holding locks.
  1124  // See issue #25686.
  1125  func badTimer() {
  1126  	throw("timer data corruption")
  1127  }
  1128  

View as plain text