runtime: don't adjust timer pp field in timerWaiting status

ianlancetaylor · ianlancetaylor · commit d2d155d1ae8c · 2021-01-20T19:55:44.000Z
Before this CL, the following sequence was possible: * GC scavenger starts and sets up scavenge.timer * GC calls readyForScavenger, but sysmon is sleeping * program calls runtime.GOMAXPROCS to shrink number of processors * procresize destroys a P, the one that scavenge.timer is on * (*pp).destroy calls moveTimers, which gets to the scavenger timer * scavenger timer is timerWaiting, and moveTimers clears t.pp * sysmon wakes up and calls wakeScavenger * wakeScavengers calls stopTimer on scavenger.timer, still timerWaiting * stopTimer calls deltimer which loads t.pp, which is still nil * stopTimer tries to increment deletedTimers on nil t.pp, and crashes The point of vulnerability is the time that t.pp is set to nil by moveTimers and the time that t.pp is set to non-nil by moveTimers, which is a few instructions at most. So it's not likely and in particular is quite unlikely on x86. But with a more relaxed memory model the area of vulnerability can be somewhat larger. This appears to tbe the cause of two builder failures in a few months on linux-mips. This CL fixes the problem by making moveTimers change the status from timerWaiting to timerMoving while t.pp is clear. That will cause deltimer to wait until the status is back to timerWaiting, at which point t.pp has been set again. Fixes #43712 Change-Id: I66838319ecfbf15be66c1fac88d9bd40e2295852 Reviewed-on: https://go-review.googlesource.com/c/go/+/284775 Trust: Ian Lance Taylor <iant@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>
diff --git a/src/runtime/time.go b/src/runtime/time.go
@@ -609,8 +609,14 @@ func moveTimers(pp *p, timers []*timer) {
 		for {
 			switch s := atomic.Load(&t.status); s {
 			case timerWaiting:
+				if !atomic.Cas(&t.status, s, timerMoving) {
+					continue
+				}
 				t.pp = 0
 				doaddtimer(pp, t)
+				if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
+					badTimer()
+				}
 				break loop
 			case timerModifiedEarlier, timerModifiedLater:
 				if !atomic.Cas(&t.status, s, timerMoving) {
