runtime: rename m.gcing to m.preemptoff and make it a string

m.gcing has become overloaded to mean "don't preempt this g" in
general.  Once the garbage collector is preemptible, the one thing it
*won't* mean is that we're in the garbage collector.

So, rename gcing to "preemptoff" and make it a string giving a reason
that preemption is disabled.  gcing was never set to anything but 0 or
1, so we don't have to worry about there being a stack of reasons.

Change-Id: I4337c29e8e942e7aa4f106fc29597e1b5de4ef46
Reviewed-on: https://go-review.googlesource.com/3660Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/debug.go

@@ -24,13 +24,13 @@ func GOMAXPROCS(n int) int {
 
 	semacquire(&worldsema, false)
 	gp := getg()
-	gp.m.gcing = 1
+	gp.m.preemptoff = "GOMAXPROCS"
 	systemstack(stoptheworld)
 
 	// newprocs will be processed by starttheworld
 	newprocs = int32(n)
 
-	gp.m.gcing = 0
+	gp.m.preemptoff = ""
 	semrelease(&worldsema)
 	systemstack(starttheworld)
 	return ret

src/runtime/lock_futex.go

@@ -180,7 +180,7 @@ func notetsleep_internal(n *note, ns int64) bool {
 
 func notetsleep(n *note, ns int64) bool {
 	gp := getg()
-	if gp != gp.m.g0 && gp.m.gcing == 0 {
+	if gp != gp.m.g0 && gp.m.preemptoff != "" {
 		throw("notetsleep not on g0")
 	}
 

src/runtime/lock_sema.go

@@ -240,7 +240,7 @@ func notetsleep_internal(n *note, ns int64, gp *g, deadline int64) bool {
 
 func notetsleep(n *note, ns int64) bool {
 	gp := getg()
-	if gp != gp.m.g0 && gp.m.gcing == 0 {
+	if gp != gp.m.g0 && gp.m.preemptoff != "" {
 		throw("notetsleep not on g0")
 	}
 	if gp.m.waitsema == 0 {

src/runtime/malloc.go

@@ -362,7 +362,7 @@ func gcwork(force int32) {
 	// Ok, we're doing it!  Stop everybody else
 
 	mp := acquirem()
-	mp.gcing = 1
+	mp.preemptoff = "gcing"
 	releasem(mp)
 	gctimer.count++
 	if force == 0 {
@@ -443,7 +443,7 @@ func gcwork(force int32) {
 	}
 
 	// all done
-	mp.gcing = 0
+	mp.preemptoff = ""
 
 	if force == 0 {
 		gctimer.cycle.sweep = nanotime()

src/runtime/mem.go

@@ -81,14 +81,14 @@ func ReadMemStats(m *MemStats) {
 	// a pending garbage collection already calling it.
 	semacquire(&worldsema, false)
 	gp := getg()
-	gp.m.gcing = 1
+	gp.m.preemptoff = "read mem stats"
 	systemstack(stoptheworld)
 
 	systemstack(func() {
 		readmemstats_m(m)
 	})
 
-	gp.m.gcing = 0
+	gp.m.preemptoff = ""
 	gp.m.locks++
 	semrelease(&worldsema)
 	systemstack(starttheworld)
@@ -99,14 +99,14 @@ func ReadMemStats(m *MemStats) {
 func runtime_debug_WriteHeapDump(fd uintptr) {
 	semacquire(&worldsema, false)
 	gp := getg()
-	gp.m.gcing = 1
+	gp.m.preemptoff = "write heap dump"
 	systemstack(stoptheworld)
 
 	systemstack(func() {
 		writeheapdump_m(fd)
 	})
 
-	gp.m.gcing = 0
+	gp.m.preemptoff = ""
 	gp.m.locks++
 	semrelease(&worldsema)
 	systemstack(starttheworld)

src/runtime/mgc.go

@@ -143,12 +143,12 @@ var gcpercent int32
 // The procedure is:
 //
 //	semacquire(&worldsema);
-//	m.gcing = 1;
+//	m.preemptoff = "reason";
 //	stoptheworld();
 //
 //	... do stuff ...
 //
-//	m.gcing = 0;
+//	m.preemptoff = "";
 //	semrelease(&worldsema);
 //	starttheworld();
 //

src/runtime/mprof.go

@@ -522,7 +522,7 @@ func GoroutineProfile(p []StackRecord) (n int, ok bool) {
 	if n <= len(p) {
 		gp := getg()
 		semacquire(&worldsema, false)
-		gp.m.gcing = 1
+		gp.m.preemptoff = "profile"
 		systemstack(stoptheworld)
 
 		n = NumGoroutine()
@@ -544,7 +544,7 @@ func GoroutineProfile(p []StackRecord) (n int, ok bool) {
 			}
 		}
 
-		gp.m.gcing = 0
+		gp.m.preemptoff = ""
 		semrelease(&worldsema)
 		systemstack(starttheworld)
 	}
@@ -567,7 +567,7 @@ func Stack(buf []byte, all bool) int {
 	if all {
 		semacquire(&worldsema, false)
 		gp := getg()
-		gp.m.gcing = 1
+		gp.m.preemptoff = "stack trace"
 		systemstack(stoptheworld)
 	}
 
@@ -591,7 +591,7 @@ func Stack(buf []byte, all bool) int {
 
 	if all {
 		gp := getg()
-		gp.m.gcing = 0
+		gp.m.preemptoff = ""
 		semrelease(&worldsema)
 		systemstack(starttheworld)
 	}

src/runtime/panic.go

@@ -353,11 +353,14 @@ func gopanic(e interface{}) {
 		print("\n")
 		throw("panic during malloc")
 	}
-	if gp.m.gcing != 0 {
+	if gp.m.preemptoff != "" {
 		print("panic: ")
 		printany(e)
 		print("\n")
-		throw("panic during gc")
+		print("preempt off reason: ")
+		print(gp.m.preemptoff)
+		print("\n")
+		throw("panic during preemptoff")
 	}
 	if gp.m.locks != 0 {
 		print("panic: ")

src/runtime/panic1.go

@@ -135,7 +135,7 @@ func canpanic(gp *g) bool {
 	if gp == nil || gp != _m_.curg {
 		return false
 	}
-	if _m_.locks-_m_.softfloat != 0 || _m_.mallocing != 0 || _m_.throwing != 0 || _m_.gcing != 0 || _m_.dying != 0 {
+	if _m_.locks-_m_.softfloat != 0 || _m_.mallocing != 0 || _m_.throwing != 0 || _m_.preemptoff != "" || _m_.dying != 0 {
 		return false
 	}
 	status := readgstatus(gp)

src/runtime/proc1.go

@@ -2462,7 +2462,7 @@ func sigprof(pc *uint8, sp *uint8, lr *uint8, gp *g, mp *m) {
 				pc = (*uint8)(unsafe.Pointer(uintptr(funcPC(_ExternalCode) + _PCQuantum)))
 			}
 			stk[0] = uintptr(unsafe.Pointer(pc))
-			if mp.gcing != 0 || mp.helpgc != 0 {
+			if mp.preemptoff != "" || mp.helpgc != 0 {
 				stk[1] = funcPC(_GC) + _PCQuantum
 			} else {
 				stk[1] = funcPC(_System) + _PCQuantum
@@ -3021,7 +3021,7 @@ func schedtrace(detailed bool) {
 		if lockedg != nil {
 			id3 = lockedg.goid
 		}
-		print("  M", mp.id, ": p=", id1, " curg=", id2, " mallocing=", mp.mallocing, " throwing=", mp.throwing, " gcing=", mp.gcing, ""+" locks=", mp.locks, " dying=", mp.dying, " helpgc=", mp.helpgc, " spinning=", mp.spinning, " blocked=", getg().m.blocked, " lockedg=", id3, "\n")
+		print("  M", mp.id, ": p=", id1, " curg=", id2, " mallocing=", mp.mallocing, " throwing=", mp.throwing, " preemptoff=", mp.preemptoff, ""+" locks=", mp.locks, " dying=", mp.dying, " helpgc=", mp.helpgc, " spinning=", mp.spinning, " blocked=", getg().m.blocked, " lockedg=", id3, "\n")
 	}
 
 	lock(&allglock)

src/runtime/runtime2.go

@@ -244,7 +244,7 @@ type m struct {
 	id            int32
 	mallocing     int32
 	throwing      int32
-	gcing         int32
+	preemptoff    string // if != "", keep curg running on this m
 	locks         int32
 	softfloat     int32
 	dying         int32

src/runtime/stack1.go

@@ -208,7 +208,7 @@ func stackalloc(n uint32) stack {
 		}
 		var x gclinkptr
 		c := thisg.m.mcache
-		if c == nil || thisg.m.gcing != 0 || thisg.m.helpgc != 0 {
+		if c == nil || thisg.m.preemptoff != "" || thisg.m.helpgc != 0 {
 			// c == nil can happen in the guts of exitsyscall or
 			// procresize. Just get a stack from the global pool.
 			// Also don't touch stackcache during gc
@@ -271,7 +271,7 @@ func stackfree(stk stack) {
 		}
 		x := gclinkptr(v)
 		c := gp.m.mcache
-		if c == nil || gp.m.gcing != 0 || gp.m.helpgc != 0 {
+		if c == nil || gp.m.preemptoff != "" || gp.m.helpgc != 0 {
 			lock(&stackpoolmu)
 			stackpoolfree(x, order)
 			unlock(&stackpoolmu)
@@ -648,7 +648,8 @@ func newstack() {
 
 	// Be conservative about where we preempt.
 	// We are interested in preempting user Go code, not runtime code.
-	// If we're holding locks, mallocing, or GCing, don't preempt.
+	// If we're holding locks, mallocing, or preemption is disabled, don't
+	// preempt.
 	// This check is very early in newstack so that even the status change
 	// from Grunning to Gwaiting and back doesn't happen in this case.
 	// That status change by itself can be viewed as a small preemption,
@@ -658,7 +659,7 @@ func newstack() {
 	// it needs a lock held by the goroutine), that small preemption turns
 	// into a real deadlock.
 	if preempt {
-		if thisg.m.locks != 0 || thisg.m.mallocing != 0 || thisg.m.gcing != 0 || thisg.m.p.status != _Prunning {
+		if thisg.m.locks != 0 || thisg.m.mallocing != 0 || thisg.m.preemptoff != "" || thisg.m.p.status != _Prunning {
 			// Let the goroutine keep running for now.
 			// gp->preempt is set, so it will be preempted next time.
 			gp.stackguard0 = gp.stack.lo + _StackGuard

src/runtime/trace.go

@@ -121,7 +121,7 @@ func StartTrace() error {
 	// of all goroutines at the beginning of the trace.
 	semacquire(&worldsema, false)
 	_g_ := getg()
-	_g_.m.gcing = 1
+	_g_.m.preemptoff = "start tracing"
 	systemstack(stoptheworld)
 
 	// We are in stop-the-world, but syscalls can finish and write to trace concurrently.
@@ -133,7 +133,7 @@ func StartTrace() error {
 
 	if trace.enabled || trace.shutdown {
 		unlock(&trace.bufLock)
-		_g_.m.gcing = 0
+		_g_.m.preemptoff = ""
 		semrelease(&worldsema)
 		systemstack(starttheworld)
 		return errorString("tracing is already enabled")
@@ -162,7 +162,7 @@ func StartTrace() error {
 
 	unlock(&trace.bufLock)
 
-	_g_.m.gcing = 0
+	_g_.m.preemptoff = ""
 	semrelease(&worldsema)
 	systemstack(starttheworld)
 	return nil
@@ -175,7 +175,7 @@ func StopTrace() {
 	// and also to avoid races with traceEvent.
 	semacquire(&worldsema, false)
 	_g_ := getg()
-	_g_.m.gcing = 1
+	_g_.m.preemptoff = "stop tracing"
 	systemstack(stoptheworld)
 
 	// See the comment in StartTrace.
@@ -183,7 +183,7 @@ func StopTrace() {
 
 	if !trace.enabled {
 		unlock(&trace.bufLock)
-		_g_.m.gcing = 0
+		_g_.m.preemptoff = ""
 		semrelease(&worldsema)
 		systemstack(starttheworld)
 		return
@@ -224,7 +224,7 @@ func StopTrace() {
 
 	unlock(&trace.bufLock)
 
-	_g_.m.gcing = 0
+	_g_.m.preemptoff = ""
 	semrelease(&worldsema)
 	systemstack(starttheworld)