runtime: don't use maps in js note implementation

notes are used in sensitive locations in the runtime, such as those with
write barriers forbidden. Maps aren't designed for this sort of internal
use.

Notably, newm -> notewakeup doesn't allow write barriers, but mapaccess1
-> panic contains write barriers. The js runtime only builds right now
because the map access is optimized to mapaccess1_fast64, which happens
to not have a panic call.

The initial swisstable map implementation doesn't have a fast64 variant.
While we could add one, it is a bad idea in general to use a map in such
a fragile location. Simplify the implementation by storing the metadata
directly in the note, and using a linked list for checkTimeouts.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-js-wasm
Change-Id: Ib9d39f064ae4ad32dcc873f799428717eb6c2d5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/595558
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: prattmic

src/runtime/lock_js.go

@@ -63,29 +63,21 @@ func unlock2(l *mutex) {
 
 // One-time notifications.
 
-type noteWithTimeout struct {
-	gp       *g
-	deadline int64
-}
-
-var (
-	notes            = make(map[*note]*g)
-	notesWithTimeout = make(map[*note]noteWithTimeout)
-)
+// Linked list of notes with a deadline.
+var allDeadlineNotes *note
 
 func noteclear(n *note) {
-	n.key = note_cleared
+	n.status = note_cleared
 }
 
 func notewakeup(n *note) {
-	// gp := getg()
-	if n.key == note_woken {
+	if n.status == note_woken {
 		throw("notewakeup - double wakeup")
 	}
-	cleared := n.key == note_cleared
-	n.key = note_woken
+	cleared := n.status == note_cleared
+	n.status = note_woken
 	if cleared {
-		goready(notes[n], 1)
+		goready(n.gp, 1)
 	}
 }
 
@@ -113,48 +105,50 @@ func notetsleepg(n *note, ns int64) bool {
 		}
 
 		id := scheduleTimeoutEvent(delay)
-		mp := acquirem()
-		notes[n] = gp
-		notesWithTimeout[n] = noteWithTimeout{gp: gp, deadline: deadline}
-		releasem(mp)
+
+		n.gp = gp
+		n.deadline = deadline
+		if allDeadlineNotes != nil {
+			allDeadlineNotes.allprev = n
+		}
+		n.allnext = allDeadlineNotes
+		allDeadlineNotes = n
 
 		gopark(nil, nil, waitReasonSleep, traceBlockSleep, 1)
 
 		clearTimeoutEvent(id) // note might have woken early, clear timeout
 
-		mp = acquirem()
-		delete(notes, n)
-		delete(notesWithTimeout, n)
-		releasem(mp)
+		n.gp = nil
+		n.deadline = 0
+		if n.allprev != nil {
+			n.allprev.allnext = n.allnext
+		}
+		if allDeadlineNotes == n {
+			allDeadlineNotes = n.allnext
+		}
+		n.allprev = nil
+		n.allnext = nil
 
-		return n.key == note_woken
+		return n.status == note_woken
 	}
 
-	for n.key != note_woken {
-		mp := acquirem()
-		notes[n] = gp
-		releasem(mp)
+	for n.status != note_woken {
+		n.gp = gp
 
 		gopark(nil, nil, waitReasonZero, traceBlockGeneric, 1)
 
-		mp = acquirem()
-		delete(notes, n)
-		releasem(mp)
+		n.gp = nil
 	}
 	return true
 }
 
 // checkTimeouts resumes goroutines that are waiting on a note which has reached its deadline.
-// TODO(drchase): need to understand if write barriers are really okay in this context.
-//
-//go:yeswritebarrierrec
 func checkTimeouts() {
 	now := nanotime()
-	// TODO: map iteration has the write barriers in it; is that okay?
-	for n, nt := range notesWithTimeout {
-		if n.key == note_cleared && now >= nt.deadline {
-			n.key = note_timeout
-			goready(nt.gp, 1)
+	for n := allDeadlineNotes; n != nil; n = n.allnext {
+		if n.status == note_cleared && n.deadline != 0 && now >= n.deadline {
+			n.status = note_timeout
+			goready(n.gp, 1)
 		}
 	}
 }

src/runtime/note_js.go

@@ -0,0 +1,40 @@
+// Copyright 2024 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.
+
+package runtime
+
+// sleep and wakeup on one-time events.
+// before any calls to notesleep or notewakeup,
+// must call noteclear to initialize the Note.
+// then, exactly one thread can call notesleep
+// and exactly one thread can call notewakeup (once).
+// once notewakeup has been called, the notesleep
+// will return.  future notesleep will return immediately.
+// subsequent noteclear must be called only after
+// previous notesleep has returned, e.g. it's disallowed
+// to call noteclear straight after notewakeup.
+//
+// notetsleep is like notesleep but wakes up after
+// a given number of nanoseconds even if the event
+// has not yet happened.  if a goroutine uses notetsleep to
+// wake up early, it must wait to call noteclear until it
+// can be sure that no other goroutine is calling
+// notewakeup.
+//
+// notesleep/notetsleep are generally called on g0,
+// notetsleepg is similar to notetsleep but is called on user g.
+type note struct {
+	status int32
+
+	// The G waiting on this note.
+	gp *g
+
+	// Deadline, if any. 0 indicates no timeout.
+	deadline int64
+
+	// allprev and allnext are used to form the allDeadlineNotes linked
+	// list. These are unused if there is no deadline.
+	allprev *note
+	allnext *note
+}

src/runtime/note_other.go

@@ -0,0 +1,33 @@
+// Copyright 2024 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.
+
+//go:build !js
+
+package runtime
+
+// sleep and wakeup on one-time events.
+// before any calls to notesleep or notewakeup,
+// must call noteclear to initialize the Note.
+// then, exactly one thread can call notesleep
+// and exactly one thread can call notewakeup (once).
+// once notewakeup has been called, the notesleep
+// will return.  future notesleep will return immediately.
+// subsequent noteclear must be called only after
+// previous notesleep has returned, e.g. it's disallowed
+// to call noteclear straight after notewakeup.
+//
+// notetsleep is like notesleep but wakes up after
+// a given number of nanoseconds even if the event
+// has not yet happened.  if a goroutine uses notetsleep to
+// wake up early, it must wait to call noteclear until it
+// can be sure that no other goroutine is calling
+// notewakeup.
+//
+// notesleep/notetsleep are generally called on g0,
+// notetsleepg is similar to notetsleep but is called on user g.
+type note struct {
+	// Futex-based impl treats it as uint32 key,
+	// while sema-based impl as M* waitm.
+	key uintptr
+}

src/runtime/runtime2.go

@@ -170,33 +170,6 @@ type mutex struct {
 	key uintptr
 }
 
-// sleep and wakeup on one-time events.
-// before any calls to notesleep or notewakeup,
-// must call noteclear to initialize the Note.
-// then, exactly one thread can call notesleep
-// and exactly one thread can call notewakeup (once).
-// once notewakeup has been called, the notesleep
-// will return.  future notesleep will return immediately.
-// subsequent noteclear must be called only after
-// previous notesleep has returned, e.g. it's disallowed
-// to call noteclear straight after notewakeup.
-//
-// notetsleep is like notesleep but wakes up after
-// a given number of nanoseconds even if the event
-// has not yet happened.  if a goroutine uses notetsleep to
-// wake up early, it must wait to call noteclear until it
-// can be sure that no other goroutine is calling
-// notewakeup.
-//
-// notesleep/notetsleep are generally called on g0,
-// notetsleepg is similar to notetsleep but is called on user g.
-type note struct {
-	// Futex-based impl treats it as uint32 key,
-	// while sema-based impl as M* waitm.
-	// Used to be a union, but unions break precise GC.
-	key uintptr
-}
-
 type funcval struct {
 	fn uintptr
 	// variable-size, fn-specific data here