runtime: prevent preemption while releasing worldsema in gcStart

Currently, as a result of us releasing worldsema now to allow STW events
during a mark phase, we release worldsema between starting the world and
having the goroutine block in STW mode. This inserts preemption points
which, if followed through, could lead to a deadlock. Specifically,
because user goroutine scheduling is disabled in STW mode, the goroutine
will block before properly releasing worldsema.

The fix here is to prevent preemption while releasing the worldsema.

Fixes #38404.
Updates #19812.

Change-Id: I8ed5b3aa108ab2e4680c38e77b0584fb75690e3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/228337
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 08159e219a..58b76bca70 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -1373,6 +1373,10 @@ func gcStart(trigger gcTrigger) {
 	// the world.
 	gcController.markStartTime = now
 
+	// In STW mode, we could block the instant systemstack
+	// returns, so make sure we're not preemptible.
+	mp = acquirem()
+
 	// Concurrent mark.
 	systemstack(func() {
 		now = startTheWorldWithSema(trace.enabled)
@@ -1385,10 +1389,10 @@ func gcStart(trigger gcTrigger) {
 	// this goroutine becomes runnable again, and we could
 	// self-deadlock otherwise.
 	semrelease(&worldsema)
+	releasem(mp)
 
-	// In STW mode, we could block the instant systemstack
-	// returns, so don't do anything important here. Make sure we
-	// block rather than returning to user code.
+	// Make sure we block instead of returning to user code
+	// in STW mode.
 	if mode != gcBackgroundMode {
 		Gosched()
 	}