lock_js.go

     1  // Copyright 2018 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  //go:build js && wasm
     6  
     7  package runtime
     8  
     9  import _ "unsafe" // for go:linkname
    10  
    11  // js/wasm has no support for threads yet. There is no preemption.
    12  
    13  const (
    14  	mutex_unlocked = 0
    15  	mutex_locked   = 1
    16  
    17  	note_cleared = 0
    18  	note_woken   = 1
    19  	note_timeout = 2
    20  
    21  	active_spin     = 4
    22  	active_spin_cnt = 30
    23  	passive_spin    = 1
    24  )
    25  
    26  func lock(l *mutex) {
    27  	lockWithRank(l, getLockRank(l))
    28  }
    29  
    30  func lock2(l *mutex) {
    31  	if l.key == mutex_locked {
    32  		// js/wasm is single-threaded so we should never
    33  		// observe this.
    34  		throw("self deadlock")
    35  	}
    36  	gp := getg()
    37  	if gp.m.locks < 0 {
    38  		throw("lock count")
    39  	}
    40  	gp.m.locks++
    41  	l.key = mutex_locked
    42  }
    43  
    44  func unlock(l *mutex) {
    45  	unlockWithRank(l)
    46  }
    47  
    48  func unlock2(l *mutex) {
    49  	if l.key == mutex_unlocked {
    50  		throw("unlock of unlocked lock")
    51  	}
    52  	gp := getg()
    53  	gp.m.locks--
    54  	if gp.m.locks < 0 {
    55  		throw("lock count")
    56  	}
    57  	l.key = mutex_unlocked
    58  }
    59  
    60  // One-time notifications.
    61  
    62  type noteWithTimeout struct {
    63  	gp       *g
    64  	deadline int64
    65  }
    66  
    67  var (
    68  	notes            = make(map[*note]*g)
    69  	notesWithTimeout = make(map[*note]noteWithTimeout)
    70  )
    71  
    72  func noteclear(n *note) {
    73  	n.key = note_cleared
    74  }
    75  
    76  func notewakeup(n *note) {
    77  	// gp := getg()
    78  	if n.key == note_woken {
    79  		throw("notewakeup - double wakeup")
    80  	}
    81  	cleared := n.key == note_cleared
    82  	n.key = note_woken
    83  	if cleared {
    84  		goready(notes[n], 1)
    85  	}
    86  }
    87  
    88  func notesleep(n *note) {
    89  	throw("notesleep not supported by js")
    90  }
    91  
    92  func notetsleep(n *note, ns int64) bool {
    93  	throw("notetsleep not supported by js")
    94  	return false
    95  }
    96  
    97  // same as runtime·notetsleep, but called on user g (not g0)
    98  func notetsleepg(n *note, ns int64) bool {
    99  	gp := getg()
   100  	if gp == gp.m.g0 {
   101  		throw("notetsleepg on g0")
   102  	}
   103  
   104  	if ns >= 0 {
   105  		deadline := nanotime() + ns
   106  		delay := ns/1000000 + 1 // round up
   107  		if delay > 1<<31-1 {
   108  			delay = 1<<31 - 1 // cap to max int32
   109  		}
   110  
   111  		id := scheduleTimeoutEvent(delay)
   112  		mp := acquirem()
   113  		notes[n] = gp
   114  		notesWithTimeout[n] = noteWithTimeout{gp: gp, deadline: deadline}
   115  		releasem(mp)
   116  
   117  		gopark(nil, nil, waitReasonSleep, traceBlockSleep, 1)
   118  
   119  		clearTimeoutEvent(id) // note might have woken early, clear timeout
   120  
   121  		mp = acquirem()
   122  		delete(notes, n)
   123  		delete(notesWithTimeout, n)
   124  		releasem(mp)
   125  
   126  		return n.key == note_woken
   127  	}
   128  
   129  	for n.key != note_woken {
   130  		mp := acquirem()
   131  		notes[n] = gp
   132  		releasem(mp)
   133  
   134  		gopark(nil, nil, waitReasonZero, traceBlockGeneric, 1)
   135  
   136  		mp = acquirem()
   137  		delete(notes, n)
   138  		releasem(mp)
   139  	}
   140  	return true
   141  }
   142  
   143  // checkTimeouts resumes goroutines that are waiting on a note which has reached its deadline.
   144  // TODO(drchase): need to understand if write barriers are really okay in this context.
   145  //
   146  //go:yeswritebarrierrec
   147  func checkTimeouts() {
   148  	now := nanotime()
   149  	// TODO: map iteration has the write barriers in it; is that okay?
   150  	for n, nt := range notesWithTimeout {
   151  		if n.key == note_cleared && now >= nt.deadline {
   152  			n.key = note_timeout
   153  			goready(nt.gp, 1)
   154  		}
   155  	}
   156  }
   157  
   158  // events is a stack of calls from JavaScript into Go.
   159  var events []*event
   160  
   161  type event struct {
   162  	// g was the active goroutine when the call from JavaScript occurred.
   163  	// It needs to be active when returning to JavaScript.
   164  	gp *g
   165  	// returned reports whether the event handler has returned.
   166  	// When all goroutines are idle and the event handler has returned,
   167  	// then g gets resumed and returns the execution to JavaScript.
   168  	returned bool
   169  }
   170  
   171  type timeoutEvent struct {
   172  	id int32
   173  	// The time when this timeout will be triggered.
   174  	time int64
   175  }
   176  
   177  // diff calculates the difference of the event's trigger time and x.
   178  func (e *timeoutEvent) diff(x int64) int64 {
   179  	if e == nil {
   180  		return 0
   181  	}
   182  
   183  	diff := x - idleTimeout.time
   184  	if diff < 0 {
   185  		diff = -diff
   186  	}
   187  	return diff
   188  }
   189  
   190  // clear cancels this timeout event.
   191  func (e *timeoutEvent) clear() {
   192  	if e == nil {
   193  		return
   194  	}
   195  
   196  	clearTimeoutEvent(e.id)
   197  }
   198  
   199  // The timeout event started by beforeIdle.
   200  var idleTimeout *timeoutEvent
   201  
   202  // beforeIdle gets called by the scheduler if no goroutine is awake.
   203  // If we are not already handling an event, then we pause for an async event.
   204  // If an event handler returned, we resume it and it will pause the execution.
   205  // beforeIdle either returns the specific goroutine to schedule next or
   206  // indicates with otherReady that some goroutine became ready.
   207  // TODO(drchase): need to understand if write barriers are really okay in this context.
   208  //
   209  //go:yeswritebarrierrec
   210  func beforeIdle(now, pollUntil int64) (gp *g, otherReady bool) {
   211  	delay := int64(-1)
   212  	if pollUntil != 0 {
   213  		// round up to prevent setTimeout being called early
   214  		delay = (pollUntil-now-1)/1e6 + 1
   215  		if delay > 1e9 {
   216  			// An arbitrary cap on how long to wait for a timer.
   217  			// 1e9 ms == ~11.5 days.
   218  			delay = 1e9
   219  		}
   220  	}
   221  
   222  	if delay > 0 && (idleTimeout == nil || idleTimeout.diff(pollUntil) > 1e6) {
   223  		// If the difference is larger than 1 ms, we should reschedule the timeout.
   224  		idleTimeout.clear()
   225  
   226  		idleTimeout = &timeoutEvent{
   227  			id:   scheduleTimeoutEvent(delay),
   228  			time: pollUntil,
   229  		}
   230  	}
   231  
   232  	if len(events) == 0 {
   233  		// TODO: this is the line that requires the yeswritebarrierrec
   234  		go handleAsyncEvent()
   235  		return nil, true
   236  	}
   237  
   238  	e := events[len(events)-1]
   239  	if e.returned {
   240  		return e.gp, false
   241  	}
   242  	return nil, false
   243  }
   244  
   245  var idleStart int64
   246  
   247  func handleAsyncEvent() {
   248  	idleStart = nanotime()
   249  	pause(getcallersp() - 16)
   250  }
   251  
   252  // clearIdleTimeout clears our record of the timeout started by beforeIdle.
   253  func clearIdleTimeout() {
   254  	idleTimeout.clear()
   255  	idleTimeout = nil
   256  }
   257  
   258  // pause sets SP to newsp and pauses the execution of Go's WebAssembly code until an event is triggered.
   259  func pause(newsp uintptr)
   260  
   261  // scheduleTimeoutEvent tells the WebAssembly environment to trigger an event after ms milliseconds.
   262  // It returns a timer id that can be used with clearTimeoutEvent.
   263  //
   264  //go:wasmimport gojs runtime.scheduleTimeoutEvent
   265  func scheduleTimeoutEvent(ms int64) int32
   266  
   267  // clearTimeoutEvent clears a timeout event scheduled by scheduleTimeoutEvent.
   268  //
   269  //go:wasmimport gojs runtime.clearTimeoutEvent
   270  func clearTimeoutEvent(id int32)
   271  
   272  // handleEvent gets invoked on a call from JavaScript into Go. It calls the event handler of the syscall/js package
   273  // and then parks the handler goroutine to allow other goroutines to run before giving execution back to JavaScript.
   274  // When no other goroutine is awake any more, beforeIdle resumes the handler goroutine. Now that the same goroutine
   275  // is running as was running when the call came in from JavaScript, execution can be safely passed back to JavaScript.
   276  func handleEvent() {
   277  	sched.idleTime.Add(nanotime() - idleStart)
   278  
   279  	e := &event{
   280  		gp:       getg(),
   281  		returned: false,
   282  	}
   283  	events = append(events, e)
   284  
   285  	if !eventHandler() {
   286  		// If we did not handle a window event, the idle timeout was triggered, so we can clear it.
   287  		clearIdleTimeout()
   288  	}
   289  
   290  	// wait until all goroutines are idle
   291  	e.returned = true
   292  	gopark(nil, nil, waitReasonZero, traceBlockGeneric, 1)
   293  
   294  	events[len(events)-1] = nil
   295  	events = events[:len(events)-1]
   296  
   297  	// return execution to JavaScript
   298  	idleStart = nanotime()
   299  	pause(getcallersp() - 16)
   300  }
   301  
   302  // eventHandler retrieves and executes handlers for pending JavaScript events.
   303  // It returns true if an event was handled.
   304  var eventHandler func() bool
   305  
   306  //go:linkname setEventHandler syscall/js.setEventHandler
   307  func setEventHandler(fn func() bool) {
   308  	eventHandler = fn
   309  }
   310
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