runtime: execLock might deadlock on darwin/ios

[rsc]

If you run

GOEXPERIMENT=staticlockranking go test strings

on a Mac, it fails badly, because the preemptM can be called while holding various locks that conflict with execLock.rlock, specifically this part:

// On Darwin, don't try to preempt threads during exec.
// Issue #41702.
if GOOS == "darwin" || GOOS == "ios" {
	execLock.rlock()
}

This is indicative of a real potential deadlock on macOS and iOS, though not one that seems to have been reported yet. So probably it's very rare if it can happen at all. Unclear how to fix that.

/cc @aclements

[ianlancetaylor]

The lock is only held while calling syscall.Exec, which is very rare. And the amount of time for a potential deadlock is very very short.

[ianlancetaylor]

We could probably change the code so that instead of acquiring the lock in preemptM we just check whether we are currently in syscall.Exec. If we are, we could just skip the preemption signal. Most likely the process is going away anyhow. If it doesn't go away, we will presumably get another chance at preempting the thread.

[prattmic]

I just modeled rwmutex more thoroughly in https://go.dev/cl/555055, which may be why this is showing up just now. Plus we don't have a darwin staticlockrank builder.

[gopherbot]

Change https://go.dev/cl/612416 mentions this issue: runtime: avoid lock order violation during preemption on Darwin