runtime: resolve checkdead panic by refining startm lock handling in caller context

This change addresses a `checkdead` panic caused by a race condition between
`sysmon->startm` and `checkdead` callers, due to prematurely releasing the
scheduler lock. The solution involves allowing a `startm` caller to acquire the
scheduler lock and call `startm` in this context. A new `lockheld` bool
argument is added to `startm`, which manages all lock and unlock calls within
the function. The`startIdle` function variable in `injectglist` is updated to
call `startm` with the lock held, ensuring proper lock handling in this
specific case. This refined lock handling resolves the observed race condition
issue.

Fixes #59600

Change-Id: I11663a15536c10c773fc2fde291d959099aa71be
Reviewed-on: https://go-review.googlesource.com/c/go/+/487316
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Michael Pratt <[email protected]>

Author: lucoffe

src/runtime/proc.go

@@ -2435,10 +2435,15 @@ func mspinning() {
 // Callers passing a non-nil P must call from a non-preemptible context. See
 // comment on acquirem below.
 //
+// Argument lockheld indicates whether the caller already acquired the
+// scheduler lock. Callers holding the lock when making the call must pass
+// true. The lock might be temporarily dropped, but will be reacquired before
+// returning.
+//
 // Must not have write barriers because this may be called without a P.
 //
 //go:nowritebarrierrec
-func startm(pp *p, spinning bool) {
+func startm(pp *p, spinning, lockheld bool) {
 	// Disable preemption.
 	//
 	// Every owned P must have an owner that will eventually stop it in the
@@ -2456,7 +2461,9 @@ func startm(pp *p, spinning bool) {
 	// startm. Callers passing a nil P may be preemptible, so we must
 	// disable preemption before acquiring a P from pidleget below.
 	mp := acquirem()
-	lock(&sched.lock)
+	if !lockheld {
+		lock(&sched.lock)
+	}
 	if pp == nil {
 		if spinning {
 			// TODO(prattmic): All remaining calls to this function
@@ -2466,7 +2473,9 @@ func startm(pp *p, spinning bool) {
 		}
 		pp, _ = pidleget(0)
 		if pp == nil {
-			unlock(&sched.lock)
+			if !lockheld {
+				unlock(&sched.lock)
+			}
 			releasem(mp)
 			return
 		}
@@ -2480,6 +2489,8 @@ func startm(pp *p, spinning bool) {
 		// could find no idle P while checkdead finds a runnable G but
 		// no running M's because this new M hasn't started yet, thus
 		// throwing in an apparent deadlock.
+		// This apparent deadlock is possible when startm is called
+		// from sysmon, which doesn't count as a running M.
 		//
 		// Avoid this situation by pre-allocating the ID for the new M,
 		// thus marking it as 'running' before we drop sched.lock. This
@@ -2494,12 +2505,18 @@ func startm(pp *p, spinning bool) {
 			fn = mspinning
 		}
 		newm(fn, pp, id)
+
+		if lockheld {
+			lock(&sched.lock)
+		}
 		// Ownership transfer of pp committed by start in newm.
 		// Preemption is now safe.
 		releasem(mp)
 		return
 	}
-	unlock(&sched.lock)
+	if !lockheld {
+		unlock(&sched.lock)
+	}
 	if nmp.spinning {
 		throw("startm: m is spinning")
 	}
@@ -2528,24 +2545,24 @@ func handoffp(pp *p) {
 
 	// if it has local work, start it straight away
 	if !runqempty(pp) || sched.runqsize != 0 {
-		startm(pp, false)
+		startm(pp, false, false)
 		return
 	}
 	// if there's trace work to do, start it straight away
 	if (trace.enabled || trace.shutdown) && traceReaderAvailable() != nil {
-		startm(pp, false)
+		startm(pp, false, false)
 		return
 	}
 	// if it has GC work, start it straight away
 	if gcBlackenEnabled != 0 && gcMarkWorkAvailable(pp) {
-		startm(pp, false)
+		startm(pp, false, false)
 		return
 	}
 	// no local work, check that there are no spinning/idle M's,
 	// otherwise our help is not required
 	if sched.nmspinning.Load()+sched.npidle.Load() == 0 && sched.nmspinning.CompareAndSwap(0, 1) { // TODO: fast atomic
 		sched.needspinning.Store(0)
-		startm(pp, true)
+		startm(pp, true, false)
 		return
 	}
 	lock(&sched.lock)
@@ -2567,14 +2584,14 @@ func handoffp(pp *p) {
 	}
 	if sched.runqsize != 0 {
 		unlock(&sched.lock)
-		startm(pp, false)
+		startm(pp, false, false)
 		return
 	}
 	// If this is the last running P and nobody is polling network,
 	// need to wakeup another M to poll network.
 	if sched.npidle.Load() == gomaxprocs-1 && sched.lastpoll.Load() != 0 {
 		unlock(&sched.lock)
-		startm(pp, false)
+		startm(pp, false, false)
 		return
 	}
 
@@ -2623,7 +2640,7 @@ func wakep() {
 	// see at least one running M (ours).
 	unlock(&sched.lock)
 
-	startm(pp, true)
+	startm(pp, true, false)
 
 	releasem(mp)
 }
@@ -3376,8 +3393,8 @@ func injectglist(glist *gList) {
 				break
 			}
 
+			startm(pp, false, true)
 			unlock(&sched.lock)
-			startm(pp, false)
 			releasem(mp)
 		}
 	}
@@ -5484,7 +5501,7 @@ func sysmon() {
 			// See issue 42515 and
 			// https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=50094.
 			if next := timeSleepUntil(); next < now {
-				startm(nil, false)
+				startm(nil, false, false)
 			}
 		}
 		if scavenger.sysmonWake.Load() != 0 {
@@ -5756,7 +5773,7 @@ func schedEnableUser(enable bool) {
 		globrunqputbatch(&sched.disable.runnable, n)
 		unlock(&sched.lock)
 		for ; n != 0 && sched.npidle.Load() != 0; n-- {
-			startm(nil, false)
+			startm(nil, false, false)
 		}
 	} else {
 		unlock(&sched.lock)