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runtime: remove write barrier in WaitForSigusr1

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WaitForSigusr1 registers a callback to be called on SIGUSR1 directly
from the runtime signal handler. Currently, this callback has a write
barrier in it, which can crash with a nil P if the GC is active and
the signal arrives on an M that doesn't have a P.

Fix this by recording the ID of the M that receives the signal instead
of the M itself, since that's all we needed anyway. To make sure there
are no other problems, this also lifts the callback into a package
function and marks it "go:nowritebarrierrec".

Fixes #35248.

Updates #35276, since in principle a write barrier at exactly the
wrong time while entering the scheduler could cause issues, though I
suspect that bug is unrelated.

Change-Id: I47b4bc73782efbb613785a93e381d8aaf6850826
Reviewed-on: https://go-review.googlesource.com/c/go/+/204620
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Austin Clements 2019-11-02 18:39:17 -04:00 committed by Ian Lance Taylor
parent f891b7c342
commit b82404837d
1 changed files with 19 additions and 9 deletions
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28
src/runtime/export_unix_test.go
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@ -22,7 +22,7 @@ type M = m
var waitForSigusr1 struct {
park note
mp *m
mID int64
}
// WaitForSigusr1 blocks until a SIGUSR1 is received. It calls ready
@ -38,16 +38,11 @@ func WaitForSigusr1(ready func(mp *M), timeoutNS int64) (int64, int64) {
unblocksig(_SIGUSR1)
mp := getg().m
testSigusr1 = func(gp *g) bool {
waitForSigusr1.mp = gp.m
notewakeup(&waitForSigusr1.park)
return true
}
testSigusr1 = waitForSigusr1Callback
ready(mp)
ok := notetsleepg(&waitForSigusr1.park, timeoutNS)
noteclear(&waitForSigusr1.park)
gotM := waitForSigusr1.mp
waitForSigusr1.mp = nil
gotM := waitForSigusr1.mID
testSigusr1 = nil
unlockOSThread()
@ -55,7 +50,22 @@ func WaitForSigusr1(ready func(mp *M), timeoutNS int64) (int64, int64) {
if !ok {
return -1, -1
}
return mp.id, gotM.id
return mp.id, gotM
}
// waitForSigusr1Callback is called from the signal handler during
// WaitForSigusr1. It must not have write barriers because there may
// not be a P.
//
//go:nowritebarrierrec
func waitForSigusr1Callback(gp *g) bool {
if gp == nil || gp.m == nil {
waitForSigusr1.mID = -1
} else {
waitForSigusr1.mID = gp.m.id
}
notewakeup(&waitForSigusr1.park)
return true
}
// SendSigusr1 sends SIGUSR1 to mp.