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>

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 08159e219a33c6f8e1977e4641df61fabc8abf15..58b76bca70ec4a3475c21c6d4f9c72384c6c29be 100644 (file)
--- 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()
        }