[release-branch.go1.15] runtime: block signals in needm before allocating M

Otherwise, if a signal occurs just after we allocated the M,
we can deadlock if the signal handler needs to allocate an M
itself.

For #42207
Fixes #42636

Change-Id: I76f44547f419e8b1c14cbf49bf602c6e645d8c14
Reviewed-on: https://go-review.googlesource.com/c/go/+/265759
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 368c40116434532dc0b53b72fa04788ca6742898)
Reviewed-on: https://go-review.googlesource.com/c/go/+/271847

1 files changed, 4 insertions, 4 deletions

diff --git a/src/runtime/signal_unix.go b/src/runtime/signal_unix.go
index 80fd2d6604..003c7b0bc8 100644
--- a/src/runtime/signal_unix.go
+++ b/src/runtime/signal_unix.go
@@ -1032,15 +1032,15 @@ func sigfwdgo(sig uint32, info *siginfo, ctx unsafe.Pointer) bool {
 	return true
 }
 
-// msigsave saves the current thread's signal mask into mp.sigmask.
+// sigsave saves the current thread's signal mask into *p.
 // This is used to preserve the non-Go signal mask when a non-Go
 // thread calls a Go function.
 // This is nosplit and nowritebarrierrec because it is called by needm
 // which may be called on a non-Go thread with no g available.
 //go:nosplit
 //go:nowritebarrierrec
-func msigsave(mp *m) {
-	sigprocmask(_SIG_SETMASK, nil, &mp.sigmask)
+func sigsave(p *sigset) {
+	sigprocmask(_SIG_SETMASK, nil, p)
 }
 
 // msigrestore sets the current thread's signal mask to sigmask.
@@ -1112,7 +1112,7 @@ func minitSignalStack() {
 // thread's signal mask. When this is called all signals have been
 // blocked for the thread.  This starts with m.sigmask, which was set
 // either from initSigmask for a newly created thread or by calling
-// msigsave if this is a non-Go thread calling a Go function. It
+// sigsave if this is a non-Go thread calling a Go function. It
 // removes all essential signals from the mask, thus causing those
 // signals to not be blocked. Then it sets the thread's signal mask.
 // After this is called the thread can receive signals.