runtime: always acquire M when acquiring locks by rank

src/runtime/lockrank_off.go

@@ -27,7 +27,8 @@ func lockWithRank(l *mutex, rank lockRank) {
 // This function may be called in nosplit context and thus must be nosplit.
 //
 //go:nosplit
-func acquireLockRank(rank lockRank) {
+func acquireLockRankAndM(rank lockRank) {
+	acquirem()
 }
 
 func unlockWithRank(l *mutex) {
@@ -37,7 +38,8 @@ func unlockWithRank(l *mutex) {
 // This function may be called in nosplit context and thus must be nosplit.
 //
 //go:nosplit
-func releaseLockRank(rank lockRank) {
+func releaseLockRankAndM(rank lockRank) {
+	releasem(getg().m)
 }
 
 func lockWithRankMayAcquire(l *mutex, rank lockRank) {

src/runtime/lockrank_on.go

@@ -104,12 +104,16 @@ func printHeldLocks(gp *g) {
 	}
 }
 
-// acquireLockRank acquires a rank which is not associated with a mutex lock
+// acquireLockRankAndM acquires a rank which is not associated with a mutex
+// lock. To maintain the invariant that an M with m.locks==0 does not hold any
+// lock-like resources, it also acquires the M.
 //
 // This function may be called in nosplit context and thus must be nosplit.
 //
 //go:nosplit
-func acquireLockRank(rank lockRank) {
+func acquireLockRankAndM(rank lockRank) {
+	acquirem()
+
 	gp := getg()
 	// Log the new class. See comment on lockWithRank.
 	systemstack(func() {
@@ -189,12 +193,14 @@ func unlockWithRank(l *mutex) {
 	})
 }
 
-// releaseLockRank releases a rank which is not associated with a mutex lock
+// releaseLockRankAndM releases a rank which is not associated with a mutex
+// lock. To maintain the invariant that an M with m.locks==0 does not hold any
+// lock-like resources, it also releases the M.
 //
 // This function may be called in nosplit context and thus must be nosplit.
 //
 //go:nosplit
-func releaseLockRank(rank lockRank) {
+func releaseLockRankAndM(rank lockRank) {
 	gp := getg()
 	systemstack(func() {
 		found := false
@@ -211,6 +217,8 @@ func releaseLockRank(rank lockRank) {
 			throw("lockRank release without matching lockRank acquire")
 		}
 	})
+
+	releasem(getg().m)
 }
 
 // nosplit because it may be called from nosplit contexts.

src/runtime/proc.go

@@ -1067,7 +1067,7 @@ func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
 		dumpgstatus(gp)
 		throw("casfrom_Gscanstatus: gp->status is not in scan state")
 	}
-	releaseLockRank(lockRankGscan)
+	releaseLockRankAndM(lockRankGscan)
 }
 
 // This will return false if the gp is not in the expected status and the cas fails.
@@ -1081,7 +1081,7 @@ func castogscanstatus(gp *g, oldval, newval uint32) bool {
 		if newval == oldval|_Gscan {
 			r := gp.atomicstatus.CompareAndSwap(oldval, newval)
 			if r {
-				acquireLockRank(lockRankGscan)
+				acquireLockRankAndM(lockRankGscan)
 			}
 			return r
 
@@ -1110,8 +1110,7 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		})
 	}
 
-	acquireLockRank(lockRankGscan)
-	releaseLockRank(lockRankGscan)
+	lockWithRankMayAcquire(nil, lockRankGscan)
 
 	// See https://golang.org/cl/21503 for justification of the yield delay.
 	const yieldDelay = 5 * 1000
@@ -1245,7 +1244,7 @@ func casGToPreemptScan(gp *g, old, new uint32) {
 	if old != _Grunning || new != _Gscan|_Gpreempted {
 		throw("bad g transition")
 	}
-	acquireLockRank(lockRankGscan)
+	acquireLockRankAndM(lockRankGscan)
 	for !gp.atomicstatus.CompareAndSwap(_Grunning, _Gscan|_Gpreempted) {
 	}
 }

src/runtime/rwmutex.go

@@ -72,9 +72,7 @@ func (rw *rwmutex) rlock() {
 	// things blocking on the lock may consume all of the Ps and
 	// deadlock (issue #20903). Alternatively, we could drop the P
 	// while sleeping.
-	acquirem()
-
-	acquireLockRank(rw.readRank)
+	acquireLockRankAndM(rw.readRank)
 	lockWithRankMayAcquire(&rw.rLock, getLockRank(&rw.rLock))
 
 	if rw.readerCount.Add(1) < 0 {
@@ -116,8 +114,7 @@ func (rw *rwmutex) runlock() {
 			unlock(&rw.rLock)
 		}
 	}
-	releaseLockRank(rw.readRank)
-	releasem(getg().m)
+	releaseLockRankAndM(rw.readRank)
 }
 
 // lock locks rw for writing.