runtime: do not split stacks in syscall status

Split stack checks (morestack) corrupt g->sched,
but g->sched must be preserved consistent for GC/traceback.
The change implements runtime.notetsleepg function,
which does entersyscall/exitsyscall and is carefully arranged
to not call any split functions in between.

R=rsc
CC=golang-dev
https://golang.org/cl/11575044

misc/cgo/test/callback.go

@@ -151,6 +151,7 @@ func testCallbackCallers(t *testing.T) {
 	n := 0
 	name := []string{
 		"test.goCallback",
+		"runtime.cgocallbackg1",
 		"runtime.cgocallbackg",
 		"runtime.cgocallback_gofunc",
 		"runtime.asmcgocall",

src/pkg/runtime/asm_amd64.s

@@ -325,41 +325,40 @@ TEXT runtime·morestack11(SB),7,$0
 // subcases of morestack01
 // with const of 8,16,...48
 TEXT runtime·morestack8(SB),7,$0
-	PUSHQ	$1
+	MOVQ	$1, R8
 	MOVQ	$morestack<>(SB), AX
 	JMP	AX
 
 TEXT runtime·morestack16(SB),7,$0
-	PUSHQ	$2
+	MOVQ	$2, R8
 	MOVQ	$morestack<>(SB), AX
 	JMP	AX
 
 TEXT runtime·morestack24(SB),7,$0
-	PUSHQ	$3
+	MOVQ	$3, R8
 	MOVQ	$morestack<>(SB), AX
 	JMP	AX
 
 TEXT runtime·morestack32(SB),7,$0
-	PUSHQ	$4
+	MOVQ	$4, R8
 	MOVQ	$morestack<>(SB), AX
 	JMP	AX
 
 TEXT runtime·morestack40(SB),7,$0
-	PUSHQ	$5
+	MOVQ	$5, R8
 	MOVQ	$morestack<>(SB), AX
 	JMP	AX
 
 TEXT runtime·morestack48(SB),7,$0
-	PUSHQ	$6
+	MOVQ	$6, R8
 	MOVQ	$morestack<>(SB), AX
 	JMP	AX
 
 TEXT morestack<>(SB),7,$0
 	get_tls(CX)
 	MOVQ	m(CX), BX
-	POPQ	AX
-	SHLQ	$35, AX
-	MOVQ	AX, m_moreframesize(BX)
+	SHLQ	$35, R8
+	MOVQ	R8, m_moreframesize(BX)
 	MOVQ	$runtime·morestack(SB), AX
 	JMP	AX
 

src/pkg/runtime/cgocall.c

@@ -255,22 +255,39 @@ struct CallbackArgs
 #define CBARGS (CallbackArgs*)((byte*)m->g0->sched.sp+4*sizeof(void*))
 #endif
 
+void runtime·cgocallbackg1(void);
+
+#pragma textflag 7
 void
 runtime·cgocallbackg(void)
 {
-	Defer d;
-	CallbackArgs *cb;
+	if(g != m->curg) {
+		runtime·prints("runtime: bad g in cgocallback");
+		runtime·exit(2);
+	}
 
 	if(m->racecall) {
-		cb = CBARGS;
-		reflect·call(cb->fn, cb->arg, cb->argsize);
-		return;
+		// We were not in syscall, so no need to call runtime·exitsyscall.
+		// However we must set m->locks for the following reason.
+		// Race detector runtime makes __tsan_symbolize cgo callback
+		// holding internal mutexes. The mutexes are not cooperative with Go scheduler.
+		// So if we deschedule a goroutine that holds race detector internal mutex
+		// (e.g. preempt it), another goroutine will deadlock trying to acquire the same mutex.
+		m->locks++;
+		runtime·cgocallbackg1();
+		m->locks--;
+	} else {
+		runtime·exitsyscall();	// coming out of cgo call
+		runtime·cgocallbackg1();
+		runtime·entersyscall();	// going back to cgo call
 	}
+}
 
-	if(g != m->curg)
-		runtime·throw("runtime: bad g in cgocallback");
-
-	runtime·exitsyscall();	// coming out of cgo call
+void
+runtime·cgocallbackg1(void)
+{
+	CallbackArgs *cb;
+	Defer d;
 
 	if(m->needextram) {
 		m->needextram = 0;
@@ -286,14 +303,14 @@ runtime·cgocallbackg(void)
 	d.free = false;
 	g->defer = &d;
 
-	if(raceenabled)
+	if(raceenabled && !m->racecall)
 		runtime·raceacquire(&cgosync);
 
 	// Invoke callback.
 	cb = CBARGS;
 	reflect·call(cb->fn, cb->arg, cb->argsize);
 
-	if(raceenabled)
+	if(raceenabled && !m->racecall)
 		runtime·racereleasemerge(&cgosync);
 
 	// Pop defer.
@@ -302,8 +319,6 @@ runtime·cgocallbackg(void)
 	if(g->defer != &d || d.fn != &unwindmf)
 		runtime·throw("runtime: bad defer entry in cgocallback");
 	g->defer = d.link;
-
-	runtime·entersyscall();	// going back to cgo call
 }
 
 static void

src/pkg/runtime/futex_test.go

@@ -2,7 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Futex is only available on Linux and FreeBSD.
+// The race detector emits calls to split stack functions so it breaks the test.
 // +build linux freebsd
+// +build !race
 
 package runtime_test
 

src/pkg/runtime/lock_futex.c

@@ -83,7 +83,11 @@ runtime·lock(Lock *l)
 		if(v == MUTEX_UNLOCKED)
 			return;
 		wait = MUTEX_SLEEPING;
+		if(m->profilehz > 0)
+			runtime·setprof(false);
 		runtime·futexsleep((uint32*)&l->key, MUTEX_SLEEPING, -1);
+		if(m->profilehz > 0)
+			runtime·setprof(true);
 	}
 }
 
@@ -122,6 +126,8 @@ runtime·notewakeup(Note *n)
 void
 runtime·notesleep(Note *n)
 {
+	if(g != m->g0)
+		runtime·throw("notesleep not on g0");
 	if(m->profilehz > 0)
 		runtime·setprof(false);
 	while(runtime·atomicload((uint32*)&n->key) == 0)
@@ -130,21 +136,21 @@ runtime·notesleep(Note *n)
 		runtime·setprof(true);
 }
 
-bool
-runtime·notetsleep(Note *n, int64 ns)
+#pragma textflag 7
+static bool
+notetsleep(Note *n, int64 ns)
 {
 	int64 deadline, now;
 
 	if(ns < 0) {
-		runtime·notesleep(n);
+		while(runtime·atomicload((uint32*)&n->key) == 0)
+			runtime·futexsleep((uint32*)&n->key, 0, -1);
 		return true;
 	}
 
 	if(runtime·atomicload((uint32*)&n->key) != 0)
 		return true;
 
-	if(m->profilehz > 0)
-		runtime·setprof(false);
 	deadline = runtime·nanotime() + ns;
 	for(;;) {
 		runtime·futexsleep((uint32*)&n->key, 0, ns);
@@ -155,11 +161,28 @@ runtime·notetsleep(Note *n, int64 ns)
 			break;
 		ns = deadline - now;
 	}
+	return runtime·atomicload((uint32*)&n->key) != 0;
+}
+
+bool
+runtime·notetsleep(Note *n, int64 ns)
+{
+	bool res;
+
+	if(g != m->g0 && !m->gcing)
+		runtime·throw("notetsleep not on g0");
+
+	if(m->profilehz > 0)
+		runtime·setprof(false);
+	res = notetsleep(n, ns);
 	if(m->profilehz > 0)
 		runtime·setprof(true);
-	return runtime·atomicload((uint32*)&n->key) != 0;
+	return res;
 }
 
+// same as runtime·notetsleep, but called on user g (not g0)
+// does not need to call runtime·setprof, because entersyscallblock does it
+// calls only nosplit functions between entersyscallblock/exitsyscall
 bool
 runtime·notetsleepg(Note *n, int64 ns)
 {
@@ -167,8 +190,9 @@ runtime·notetsleepg(Note *n, int64 ns)
 
 	if(g == m->g0)
 		runtime·throw("notetsleepg on g0");
+
 	runtime·entersyscallblock();
-	res = runtime·notetsleep(n, ns);
+	res = notetsleep(n, ns);
 	runtime·exitsyscall();
 	return res;
 }

src/pkg/runtime/lock_sema.c

@@ -81,7 +81,11 @@ unlocked:
 			}
 			if(v&LOCKED) {
 				// Queued.  Wait.
+				if(m->profilehz > 0)
+					runtime·setprof(false);
 				runtime·semasleep(-1);
+				if(m->profilehz > 0)
+					runtime·setprof(true);
 				i = 0;
 			}
 		}
@@ -149,6 +153,9 @@ runtime·notewakeup(Note *n)
 void
 runtime·notesleep(Note *n)
 {
+	if(g != m->g0)
+		runtime·throw("notesleep not on g0");
+
 	if(m->waitsema == 0)
 		m->waitsema = runtime·semacreate();
 	if(!runtime·casp((void**)&n->key, nil, m)) {  // must be LOCKED (got wakeup)
@@ -164,19 +171,13 @@ runtime·notesleep(Note *n)
 		runtime·setprof(true);
 }
 
-bool
-runtime·notetsleep(Note *n, int64 ns)
+#pragma textflag 7
+static bool
+notetsleep(Note *n, int64 ns, int64 deadline, M *mp)
 {
-	M *mp;
-	int64 deadline, now;
-
-	if(ns < 0) {
-		runtime·notesleep(n);
-		return true;
-	}
-
-	if(m->waitsema == 0)
-		m->waitsema = runtime·semacreate();
+	// Conceptually, deadline and mp are local variables.
+	// They are passed as arguments so that the space for them
+	// does not count against our nosplit stack sequence.
 
 	// Register for wakeup on n->waitm.
 	if(!runtime·casp((void**)&n->key, nil, m)) {  // must be LOCKED (got wakeup already)
@@ -185,31 +186,28 @@ runtime·notetsleep(Note *n, int64 ns)
 		return true;
 	}
 
-	if(m->profilehz > 0)
-		runtime·setprof(false);
+	if(ns < 0) {
+		// Queued.  Sleep.
+		runtime·semasleep(-1);
+		return true;
+	}
+
 	deadline = runtime·nanotime() + ns;
 	for(;;) {
 		// Registered.  Sleep.
 		if(runtime·semasleep(ns) >= 0) {
 			// Acquired semaphore, semawakeup unregistered us.
 			// Done.
-			if(m->profilehz > 0)
-				runtime·setprof(true);
 			return true;
 		}
 
 		// Interrupted or timed out.  Still registered.  Semaphore not acquired.
-		now = runtime·nanotime();
-		if(now >= deadline)
+		ns = deadline - runtime·nanotime();
+		if(ns <= 0)
 			break;
-
 		// Deadline hasn't arrived.  Keep sleeping.
-		ns = deadline - now;
 	}
 
-	if(m->profilehz > 0)
-		runtime·setprof(true);
-
 	// Deadline arrived.  Still registered.  Semaphore not acquired.
 	// Want to give up and return, but have to unregister first,
 	// so that any notewakeup racing with the return does not
@@ -226,12 +224,33 @@ runtime·notetsleep(Note *n, int64 ns)
 			if(runtime·semasleep(-1) < 0)
 				runtime·throw("runtime: unable to acquire - semaphore out of sync");
 			return true;
-		} else {
+		} else
 			runtime·throw("runtime: unexpected waitm - semaphore out of sync");
-		}
 	}
 }
 
+bool
+runtime·notetsleep(Note *n, int64 ns)
+{
+	bool res;
+
+	if(g != m->g0 && !m->gcing)
+		runtime·throw("notetsleep not on g0");
+
+	if(m->waitsema == 0)
+		m->waitsema = runtime·semacreate();
+
+	if(m->profilehz > 0)
+		runtime·setprof(false);
+	res = notetsleep(n, ns, 0, nil);
+	if(m->profilehz > 0)
+		runtime·setprof(true);
+	return res;
+}
+
+// same as runtime·notetsleep, but called on user g (not g0)
+// does not need to call runtime·setprof, because entersyscallblock does it
+// calls only nosplit functions between entersyscallblock/exitsyscall
 bool
 runtime·notetsleepg(Note *n, int64 ns)
 {
@@ -239,8 +258,12 @@ runtime·notetsleepg(Note *n, int64 ns)
 
 	if(g == m->g0)
 		runtime·throw("notetsleepg on g0");
+
+	if(m->waitsema == 0)
+		m->waitsema = runtime·semacreate();
+
 	runtime·entersyscallblock();
-	res = runtime·notetsleep(n, ns);
+	res = notetsleep(n, ns, 0, nil);
 	runtime·exitsyscall();
 	return res;
 }

src/pkg/runtime/os_darwin.c

@@ -22,19 +22,6 @@ unimplemented(int8 *name)
 	*(int32*)1231 = 1231;
 }
 
-int32
-runtime·semasleep(int64 ns)
-{
-	int32 v;
-
-	if(m->profilehz > 0)
-		runtime·setprof(false);
-	v = runtime·mach_semacquire(m->waitsema, ns);
-	if(m->profilehz > 0)
-		runtime·setprof(true);
-	return v;
-}
-
 void
 runtime·semawakeup(M *mp)
 {
@@ -155,10 +142,15 @@ runtime·unminit(void)
 // Mach IPC, to get at semaphores
 // Definitions are in /usr/include/mach on a Mac.
 
+#pragma textflag 7
 static void
 macherror(int32 r, int8 *fn)
 {
-	runtime·printf("mach error %s: %d\n", fn, r);
+	runtime·prints("mach error ");
+	runtime·prints(fn);
+	runtime·prints(": ");
+	runtime·printint(r);
+	runtime·prints("\n");
 	runtime·throw("mach error");
 }
 
@@ -405,25 +397,22 @@ int32 runtime·mach_semaphore_timedwait(uint32 sema, uint32 sec, uint32 nsec);
 int32 runtime·mach_semaphore_signal(uint32 sema);
 int32 runtime·mach_semaphore_signal_all(uint32 sema);
 
+#pragma textflag 7
 int32
-runtime·mach_semacquire(uint32 sem, int64 ns)
+runtime·semasleep(int64 ns)
 {
-	int32 r;
-	int64 secs;
+	int32 r, secs, nsecs;
 
 	if(ns >= 0) {
-		secs = ns/1000000000LL;
-		// Avoid overflow
-		if(secs > 1LL<<30)
-			secs = 1LL<<30;
-		r = runtime·mach_semaphore_timedwait(sem, secs, ns%1000000000LL);
+		secs = runtime·timediv(ns, 1000000000, &nsecs);
+		r = runtime·mach_semaphore_timedwait(m->waitsema, secs, nsecs);
 		if(r == KERN_ABORTED || r == KERN_OPERATION_TIMED_OUT)
 			return -1;
 		if(r != 0)
 			macherror(r, "semaphore_wait");
 		return 0;
 	}
-	while((r = runtime·mach_semaphore_wait(sem)) != 0) {
+	while((r = runtime·mach_semaphore_wait(m->waitsema)) != 0) {
 		if(r == KERN_ABORTED)	// interrupted
 			continue;
 		macherror(r, "semaphore_wait");

src/pkg/runtime/os_freebsd.c

@@ -41,29 +41,26 @@ getncpu(void)
 // FreeBSD's umtx_op syscall is effectively the same as Linux's futex, and
 // thus the code is largely similar. See linux/thread.c and lock_futex.c for comments.
 
+#pragma textflag 7
 void
 runtime·futexsleep(uint32 *addr, uint32 val, int64 ns)
 {
 	int32 ret;
-	Timespec ts, *tsp;
-	int64 secs;
-
-	if(ns < 0)
-		tsp = nil;
-	else {
-		secs = ns / 1000000000LL;
-		// Avoid overflow
-		if(secs > 1LL<<30)
-			secs = 1LL<<30;
-		ts.tv_sec = secs;
-		ts.tv_nsec = ns % 1000000000LL;
-		tsp = &ts;
-	}
+	Timespec ts;
 
-	ret = runtime·sys_umtx_op(addr, UMTX_OP_WAIT_UINT, val, nil, tsp);
+	if(ns < 0) {
+		ret = runtime·sys_umtx_op(addr, UMTX_OP_WAIT_UINT, val, nil, nil);
+		if(ret >= 0 || ret == -EINTR)
+			return;
+		goto fail;
+	}
+	ts.tv_nsec = 0;
+	ts.tv_sec = runtime·timediv(ns, 1000000000, (int32*)&ts.tv_nsec);
+	ret = runtime·sys_umtx_op(addr, UMTX_OP_WAIT_UINT, val, nil, &ts);
 	if(ret >= 0 || ret == -EINTR)
 		return;
 
+fail:
 	runtime·printf("umtx_wait addr=%p val=%d ret=%d\n", addr, val, ret);
 	*(int32*)0x1005 = 0x1005;
 }

src/pkg/runtime/os_linux.c

@@ -32,30 +32,25 @@ enum
 //	if(*addr == val) sleep
 // Might be woken up spuriously; that's allowed.
 // Don't sleep longer than ns; ns < 0 means forever.
+#pragma textflag 7
 void
 runtime·futexsleep(uint32 *addr, uint32 val, int64 ns)
 {
-	Timespec ts, *tsp;
-	int64 secs;
-
-	if(ns < 0)
-		tsp = nil;
-	else {
-		secs = ns/1000000000LL;
-		// Avoid overflow
-		if(secs > 1LL<<30)
-			secs = 1LL<<30;
-		ts.tv_sec = secs;
-		ts.tv_nsec = ns%1000000000LL;
-		tsp = &ts;
-	}
+	Timespec ts;
 
 	// Some Linux kernels have a bug where futex of
 	// FUTEX_WAIT returns an internal error code
 	// as an errno.  Libpthread ignores the return value
 	// here, and so can we: as it says a few lines up,
 	// spurious wakeups are allowed.
-	runtime·futex(addr, FUTEX_WAIT, val, tsp, nil, 0);
+
+	if(ns < 0) {
+		runtime·futex(addr, FUTEX_WAIT, val, nil, nil, 0);
+		return;
+	}
+	ts.tv_nsec = 0;
+	ts.tv_sec = runtime·timediv(ns, 1000000000LL, (int32*)&ts.tv_nsec);
+	runtime·futex(addr, FUTEX_WAIT, val, &ts, nil, 0);
 }
 
 // If any procs are sleeping on addr, wake up at most cnt.

src/pkg/runtime/os_netbsd.c

@@ -62,6 +62,7 @@ runtime·semacreate(void)
 	return 1;
 }
 
+#pragma textflag 7
 int32
 runtime·semasleep(int64 ns)
 {
@@ -94,8 +95,8 @@ runtime·semasleep(int64 ns)
 				runtime·lwp_park(nil, 0, &m->waitsemacount, nil);
 			} else {
 				ns += runtime·nanotime();
-				ts.tv_sec = ns/1000000000LL;
-				ts.tv_nsec = ns%1000000000LL;
+				ts.tv_nsec = 0;
+				ts.tv_sec = runtime·timediv(ns, 1000000000, (int32*)ts.tv_nsec);
 				// TODO(jsing) - potential deadlock!
 				// See above for details.
 				runtime·atomicstore(&m->waitsemalock, 0);

src/pkg/runtime/os_openbsd.c

@@ -59,11 +59,11 @@ runtime·semacreate(void)
 	return 1;
 }
 
+#pragma textflag 7
 int32
 runtime·semasleep(int64 ns)
 {
 	Timespec ts;
-	int64 secs;
 
 	// spin-mutex lock
 	while(runtime·xchg(&m->waitsemalock, 1))
@@ -78,12 +78,8 @@ runtime·semasleep(int64 ns)
 				runtime·thrsleep(&m->waitsemacount, 0, nil, &m->waitsemalock, nil);
 			else {
 				ns += runtime·nanotime();
-				secs = ns/1000000000LL;
-				// Avoid overflow
-				if(secs >= 1LL<<31)
-					secs = (1LL<<31) - 1;
-				ts.tv_sec = secs;
-				ts.tv_nsec = ns%1000000000LL;
+				ts.tv_nsec = 0;
+				ts.tv_sec = runtime·timediv(ns, 1000000000, (int32*)ts.tv_nsec);
 				runtime·thrsleep(&m->waitsemacount, CLOCK_REALTIME, &ts, &m->waitsemalock, nil);
 			}
 			// reacquire lock

src/pkg/runtime/os_plan9.c

@@ -260,6 +260,7 @@ runtime·semacreate(void)
 	return 1;
 }
 
+#pragma textflag 7
 int32
 runtime·semasleep(int64 ns)
 {
@@ -267,10 +268,7 @@ runtime·semasleep(int64 ns)
 	int32 ms;
 
 	if(ns >= 0) {
-		if(ns/1000000 > 0x7fffffffll)
-			ms = 0x7fffffff;
-		else
-			ms = ns/1000000;
+		ms = runtime·timediv(ns, 1000000, nil);
 		ret = runtime·plan9_tsemacquire(&m->waitsemacount, ms);
 		if(ret == 1)
 			return 0;  // success

src/pkg/runtime/os_windows.c

@@ -164,21 +164,19 @@ runtime·write(int32 fd, void *buf, int32 n)
 
 #define INFINITE ((uintptr)0xFFFFFFFF)
 
+#pragma textflag 7
 int32
 runtime·semasleep(int64 ns)
 {
-	uintptr ms;
-
+	// store ms in ns to save stack space
 	if(ns < 0)
-		ms = INFINITE;
-	else if(ns/1000000 > 0x7fffffffLL)
-		ms = 0x7fffffff;
+		ns = INFINITE;
 	else {
-		ms = ns/1000000;
-		if(ms == 0)
-			ms = 1;
+		ns = runtime·timediv(ns, 1000000, nil);
+		if(ns == 0)
+			ns = 1;
 	}
-	if(runtime·stdcall(runtime·WaitForSingleObject, 2, m->waitsema, ms) != 0)
+	if(runtime·stdcall(runtime·WaitForSingleObject, 2, m->waitsema, (uintptr)ns) != 0)
 		return -1;  // timeout
 	return 0;
 }
@@ -237,6 +235,7 @@ runtime·unminit(void)
 	runtime·remove_exception_handler();
 }
 
+#pragma textflag 7
 int64
 runtime·nanotime(void)
 {
@@ -266,13 +265,11 @@ time·now(int64 sec, int32 usec)
 void *
 runtime·stdcall(void *fn, int32 count, ...)
 {
-	WinCall c;
-
-	c.fn = fn;
-	c.n = count;
-	c.args = (uintptr*)&count + 1;
-	runtime·asmcgocall(runtime·asmstdcall, &c);
-	return (void*)c.r1;
+	m->wincall.fn = fn;
+	m->wincall.n = count;
+	m->wincall.args = (uintptr*)&count + 1;
+	runtime·asmcgocall(runtime·asmstdcall, &m->wincall);
+	return (void*)m->wincall.r1;
 }
 
 extern void runtime·usleep1(uint32);

src/pkg/runtime/proc.c

@@ -110,6 +110,7 @@ static void pidleput(P*);
 static void injectglist(G*);
 static void preemptall(void);
 static void preemptone(P*);
+static bool exitsyscallfast(void);
 
 // The bootstrap sequence is:
 //
@@ -1379,6 +1380,10 @@ save(void *pc, uintptr sp)
 void
 ·entersyscall(int32 dummy)
 {
+	// Disable preemption because during this function g is in Gsyscall status,
+	// but can have inconsistent g->sched, do not let GC observe it.
+	m->locks++;
+
 	if(m->profilehz > 0)
 		runtime·setprof(false);
 
@@ -1417,6 +1422,12 @@ void
 		runtime·unlock(&runtime·sched);
 		save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
 	}
+
+	// Goroutines must not split stacks in Gsyscall status (it would corrupt g->sched).
+	// We set stackguard to StackPreempt so that first split stack check calls morestack.
+	// Morestack detects this case and throws.
+	g->stackguard0 = StackPreempt;
+	m->locks--;
 }
 
 // The same as runtime·entersyscall(), but with a hint that the syscall is blocking.
@@ -1426,6 +1437,8 @@ void
 {
 	P *p;
 
+	m->locks++;  // see comment in entersyscall
+
 	if(m->profilehz > 0)
 		runtime·setprof(false);
 
@@ -1449,56 +1462,48 @@ void
 
 	// Resave for traceback during blocked call.
 	save(runtime·getcallerpc(&dummy), runtime·getcallersp(&dummy));
+
+	g->stackguard0 = StackPreempt;  // see comment in entersyscall
+	m->locks--;
 }
 
 // The goroutine g exited its system call.
 // Arrange for it to run on a cpu again.
 // This is called only from the go syscall library, not
 // from the low-level system calls used by the runtime.
+#pragma textflag 7
 void
 runtime·exitsyscall(void)
 {
-	P *p;
+	m->locks++;  // see comment in entersyscall
 
 	// Check whether the profiler needs to be turned on.
 	if(m->profilehz > 0)
 		runtime·setprof(true);
 
-	// Try to re-acquire the last P.
-	if(m->p && m->p->status == Psyscall && runtime·cas(&m->p->status, Psyscall, Prunning)) {
+	if(g->isbackground)  // do not consider blocked scavenger for deadlock detection
+		inclocked(-1);
+
+	if(exitsyscallfast()) {
 		// There's a cpu for us, so we can run.
-		m->mcache = m->p->mcache;
-		m->p->m = m;
 		m->p->tick++;
 		g->status = Grunning;
 		// Garbage collector isn't running (since we are),
 		// so okay to clear gcstack and gcsp.
 		g->gcstack = (uintptr)nil;
 		g->gcsp = (uintptr)nil;
-		if(g->preempt)  // restore the preemption request in case we've cleared it in newstack
+		m->locks--;
+		if(g->preempt) {
+			// restore the preemption request in case we've cleared it in newstack
 			g->stackguard0 = StackPreempt;
+		} else {
+			// otherwise restore the real stackguard, we've spoiled it in entersyscall/entersyscallblock
+			g->stackguard0 = g->stackguard;
+		}
 		return;
 	}
 
-	if(g->isbackground)  // do not consider blocked scavenger for deadlock detection
-		inclocked(-1);
-	// Try to get any other idle P.
-	m->p = nil;
-	if(runtime·sched.pidle) {
-		runtime·lock(&runtime·sched);
-		p = pidleget();
-		runtime·unlock(&runtime·sched);
-		if(p) {
-			acquirep(p);
-			m->p->tick++;
-			g->status = Grunning;
-			g->gcstack = (uintptr)nil;
-			g->gcsp = (uintptr)nil;
-			if(g->preempt)  // restore the preemption request in case we've cleared it in newstack
-				g->stackguard0 = StackPreempt;
-			return;
-		}
-	}
+	m->locks--;
 
 	// Call the scheduler.
 	runtime·mcall(exitsyscall0);
@@ -1513,6 +1518,33 @@ runtime·exitsyscall(void)
 	g->gcsp = (uintptr)nil;
 }
 
+#pragma textflag 7
+static bool
+exitsyscallfast(void)
+{
+	P *p;
+
+	// Try to re-acquire the last P.
+	if(m->p && m->p->status == Psyscall && runtime·cas(&m->p->status, Psyscall, Prunning)) {
+		// There's a cpu for us, so we can run.
+		m->mcache = m->p->mcache;
+		m->p->m = m;
+		return true;
+	}
+	// Try to get any other idle P.
+	m->p = nil;
+	if(runtime·sched.pidle) {
+		runtime·lock(&runtime·sched);
+		p = pidleget();
+		runtime·unlock(&runtime·sched);
+		if(p) {
+			acquirep(p);
+			return true;
+		}
+	}
+	return false;
+}
+
 // runtime·exitsyscall slow path on g0.
 // Failed to acquire P, enqueue gp as runnable.
 static void

src/pkg/runtime/runtime.c

@@ -220,6 +220,9 @@ runtime·check(void)
 	if(offsetof(struct y1, y) != 1) runtime·throw("bad offsetof y1.y");
 	if(sizeof(struct y1) != 2) runtime·throw("bad sizeof y1");
 
+	if(runtime·timediv(12345LL*1000000000+54321, 1000000000, &e) != 12345 || e != 54321)
+		runtime·throw("bad timediv");
+
 	uint32 z;
 	z = 1;
 	if(!runtime·cas(&z, 1, 2))
@@ -407,3 +410,30 @@ runtime·parsedebugvars(void)
 		p++;
 	}
 }
+
+// Poor mans 64-bit division.
+// This is a very special function, do not use it if you are not sure what you are doing.
+// int64 division is lowered into _divv() call on 386, which does not fit into nosplit functions.
+// Handles overflow in a time-specific manner.
+#pragma textflag 7
+int32
+runtime·timediv(int64 v, int32 div, int32 *rem)
+{
+	int32 res, bit;
+
+	if(v >= div*0x7fffffffLL) {
+		if(rem != nil)
+			*rem = 0;
+		return 0x7fffffff;
+	}
+	res = 0;
+	for(bit = 0x40000000; bit != 0; bit >>= 1) {
+		if(v >= (int64)bit*div) {
+			v -= (int64)bit*div;
+			res += bit;
+		}
+	}
+	if(rem != nil)
+		*rem = v;
+	return res;
+}

src/pkg/runtime/runtime.h

@@ -222,6 +222,29 @@ struct	GCStats
 	uint64	nosyield;
 	uint64	nsleep;
 };
+
+struct	WinCall
+{
+	void	(*fn)(void*);
+	uintptr	n;	// number of parameters
+	void*	args;	// parameters
+	uintptr	r1;	// return values
+	uintptr	r2;
+	uintptr	err;	// error number
+};
+struct	SEH
+{
+	void*	prev;
+	void*	handler;
+};
+// describes how to handle callback
+struct	WinCallbackContext
+{
+	void*	gobody;		// Go function to call
+	uintptr	argsize;	// callback arguments size (in bytes)
+	uintptr	restorestack;	// adjust stack on return by (in bytes) (386 only)
+};
+
 struct	G
 {
 	// stackguard0 can be set to StackPreempt as opposed to stackguard
@@ -327,6 +350,7 @@ struct	M
 
 #ifdef GOOS_windows
 	void*	thread;		// thread handle
+	WinCall	wincall;
 #endif
 #ifdef GOOS_plan9
 	int8*		notesig;
@@ -432,28 +456,6 @@ struct	Itab
 	void	(*fun[])(void);
 };
 
-struct	WinCall
-{
-	void	(*fn)(void*);
-	uintptr	n;	// number of parameters
-	void*	args;	// parameters
-	uintptr	r1;	// return values
-	uintptr	r2;
-	uintptr	err;	// error number
-};
-struct	SEH
-{
-	void*	prev;
-	void*	handler;
-};
-// describes how to handle callback
-struct	WinCallbackContext
-{
-	void*	gobody;		// Go function to call
-	uintptr	argsize;	// callback arguments size (in bytes)
-	uintptr	restorestack;	// adjust stack on return by (in bytes) (386 only)
-};
-
 #ifdef GOOS_windows
 enum {
    Windows = 1
@@ -816,6 +818,7 @@ int32	runtime·gcount(void);
 void	runtime·mcall(void(*)(G*));
 uint32	runtime·fastrand1(void);
 void	runtime·rewindmorestack(Gobuf*);
+int32	runtime·timediv(int64, int32, int32*);
 
 void runtime·setmg(M*, G*);
 void runtime·newextram(void);

src/pkg/runtime/stack.c

@@ -246,6 +246,8 @@ runtime·newstack(void)
 			runtime·throw("runtime: preempt g0");
 		if(oldstatus == Grunning && m->p == nil)
 			runtime·throw("runtime: g is running but p is not");
+		if(oldstatus == Gsyscall && m->locks == 0)
+			runtime·throw("runtime: stack split during syscall");
 		// Be conservative about where we preempt.
 		// We are interested in preempting user Go code, not runtime code.
 		if(oldstatus != Grunning || m->locks || m->mallocing || m->gcing || m->p->status != Prunning) {