runtime: use acquirem/releasem more widely

We've copy-pasted the pattern of releasem in many places. This CL
replaces almost everywhere that manipulates g.m.locks and g.preempt
with calls to acquirem/releasem. There are a few where we do something
more complicated, like where exitsyscall has to restore the stack
bound differently depending on the preempt flag, which this CL leaves
alone.

Change-Id: Ia7a46c261daea6e7802b80e7eb9227499f460433
Reviewed-on: https://go-review.googlesource.com/c/go/+/170064
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/proc.go

@@ -645,7 +645,7 @@ func ready(gp *g, traceskip int, next bool) {
 
 	// Mark runnable.
 	_g_ := getg()
-	_g_.m.locks++ // disable preemption because it can be holding p in a local var
+	mp := acquirem() // disable preemption because it can be holding p in a local var
 	if status&^_Gscan != _Gwaiting {
 		dumpgstatus(gp)
 		throw("bad g->status in ready")
@@ -657,10 +657,7 @@ func ready(gp *g, traceskip int, next bool) {
 	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
 		wakep()
 	}
-	_g_.m.locks--
-	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in Case we've cleared it in newstack
-		_g_.stackguard0 = stackPreempt
-	}
+	releasem(mp)
 }
 
 // freezeStopWait is a large value that freezetheworld sets
@@ -1080,9 +1077,7 @@ func stopTheWorldWithSema() {
 }
 
 func startTheWorldWithSema(emitTraceEvent bool) int64 {
-	_g_ := getg()
-
-	_g_.m.locks++ // disable preemption because it can be holding p in a local var
+	mp := acquirem() // disable preemption because it can be holding p in a local var
 	if netpollinited() {
 		list := netpoll(false) // non-blocking
 		injectglist(&list)
@@ -1132,10 +1127,7 @@ func startTheWorldWithSema(emitTraceEvent bool) int64 {
 		wakep()
 	}
 
-	_g_.m.locks--
-	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
-		_g_.stackguard0 = stackPreempt
-	}
+	releasem(mp)
 
 	return startTime
 }
@@ -1464,7 +1456,7 @@ type cgothreadstart struct {
 //go:yeswritebarrierrec
 func allocm(_p_ *p, fn func()) *m {
 	_g_ := getg()
-	_g_.m.locks++ // disable GC because it can be called from sysmon
+	acquirem() // disable GC because it can be called from sysmon
 	if _g_.m.p == 0 {
 		acquirep(_p_) // temporarily borrow p for mallocs in this function
 	}
@@ -1505,10 +1497,7 @@ func allocm(_p_ *p, fn func()) *m {
 	if _p_ == _g_.m.p.ptr() {
 		releasep()
 	}
-	_g_.m.locks--
-	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
-		_g_.stackguard0 = stackPreempt
-	}
+	releasem(_g_.m)
 
 	return mp
 }
@@ -3255,7 +3244,7 @@ func newproc1(fn *funcval, argp *uint8, narg int32, callergp *g, callerpc uintpt
 		_g_.m.throwing = -1 // do not dump full stacks
 		throw("go of nil func value")
 	}
-	_g_.m.locks++ // disable preemption because it can be holding p in a local var
+	acquirem() // disable preemption because it can be holding p in a local var
 	siz := narg
 	siz = (siz + 7) &^ 7
 
@@ -3350,10 +3339,7 @@ func newproc1(fn *funcval, argp *uint8, narg int32, callergp *g, callerpc uintpt
 	if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 && mainStarted {
 		wakep()
 	}
-	_g_.m.locks--
-	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
-		_g_.stackguard0 = stackPreempt
-	}
+	releasem(_g_.m)
 }
 
 // saveAncestors copies previous ancestors of the given caller g and