[release-branch.go1.8] runtime: do not call wakep from enlistWorker, to avoid possible deadlock

We have seen one instance of a production job suddenly spinning to
100% CPU and becoming unresponsive. In that one instance, a SIGQUIT
was sent after 328 minutes of spinning, and the stacks showed a single
goroutine in "IO wait (scan)" state.

Looking for things that might get stuck if a goroutine got stuck in
scanning a stack, we found that injectglist does:

	lock(&sched.lock)
	var n int
	for n = 0; glist != nil; n++ {
		gp := glist
		glist = gp.schedlink.ptr()
		casgstatus(gp, _Gwaiting, _Grunnable)
		globrunqput(gp)
	}
	unlock(&sched.lock)

and that casgstatus spins on gp.atomicstatus until the _Gscan bit goes
away. Essentially, this code locks sched.lock and then while holding
sched.lock, waits to lock gp.atomicstatus.

The code that is doing the scan is:

	if castogscanstatus(gp, s, s|_Gscan) {
		if !gp.gcscandone {
			scanstack(gp, gcw)
			gp.gcscandone = true
		}
		restartg(gp)
		break loop
	}

More analysis showed that scanstack can, in a rare case, end up
calling back into code that acquires sched.lock. For example:

	runtime.scanstack at proc.go:866
	calls runtime.gentraceback at mgcmark.go:842
	calls runtime.scanstack$1 at traceback.go:378
	calls runtime.scanframeworker at mgcmark.go:819
	calls runtime.scanblock at mgcmark.go:904
	calls runtime.greyobject at mgcmark.go:1221
	calls (*runtime.gcWork).put at mgcmark.go:1412
	calls (*runtime.gcControllerState).enlistWorker at mgcwork.go:127
	calls runtime.wakep at mgc.go:632
	calls runtime.startm at proc.go:1779
	acquires runtime.sched.lock at proc.go:1675

This path was found with an automated deadlock-detecting tool.
There are many such paths but they all go through enlistWorker -> wakep.

The evidence strongly suggests that one of these paths is what caused
the deadlock we observed. We're running those jobs with
GOTRACEBACK=crash now to try to get more information if it happens
again.

Further refinement and analysis shows that if we drop the wakep call
from enlistWorker, the remaining few deadlock cycles found by the tool
are all false positives caused by not understanding the effect of calls
to func variables.

The enlistWorker -> wakep call was intended only as a performance
optimization, it rarely executes, and if it does execute at just the
wrong time it can (and plausibly did) cause the deadlock we saw.

Comment it out, to avoid the potential deadlock.

Fixes #19112.
Unfixes #14179.

Change-Id: I6f7e10b890b991c11e79fab7aeefaf70b5d5a07b
Reviewed-on: https://go-review.googlesource.com/37093
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-on: https://go-review.googlesource.com/37022
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mgc.go

@@ -628,10 +628,12 @@ func (c *gcControllerState) endCycle() {
 //go:nowritebarrier
 func (c *gcControllerState) enlistWorker() {
 	// If there are idle Ps, wake one so it will run an idle worker.
-	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
-		wakep()
-		return
-	}
+	// NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112.
+	//
+	//	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
+	//		wakep()
+	//		return
+	//	}
 
 	// There are no idle Ps. If we need more dedicated workers,
 	// try to preempt a running P so it will switch to a worker.