runtime: allow cgo callbacks on non-Go threads

Fixes #4435.

R=golang-dev, iant, alex.brainman, minux.ma, dvyukov
CC=golang-dev
https://golang.org/cl/7304104

16 files changed, 448 insertions, 34 deletions

diff --git a/src/pkg/runtime/asm_386.s b/src/pkg/runtime/asm_386.s
index 901d9f3241..f09ddd028d 100644
--- a/src/pkg/runtime/asm_386.s
+++ b/src/pkg/runtime/asm_386.s
@@ -476,21 +476,33 @@ TEXT runtime·asmcgocall(SB),7,$0
 // cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
 // See cgocall.c for more details.
 TEXT runtime·cgocallback(SB),7,$12
-	MOVL	fn+0(FP), AX
-	MOVL	frame+4(FP), BX
-	MOVL	framesize+8(FP), DX
-
-	// Save current m->g0->sched.sp on stack and then set it to SP.
+	// If m is nil, Go did not create the current thread.
+	// Call needm to obtain one for temporary use.
+	// In this case, we're running on the thread stack, so there's
+	// lots of space, but the linker doesn't know. Hide the call from
+	// the linker analysis by using an indirect call through AX.
 	get_tls(CX)
+#ifdef GOOS_windows
+	CMPL	CX, $0
+	JNE	3(PC)
+	PUSHL	$0
+	JMP needm
+#endif
 	MOVL	m(CX), BP
-
-	// If m is nil, it is almost certainly because we have been called
-	// on a thread that Go did not create.  We're going to crash as
-	// soon as we try to use m; instead, try to print a nice error and exit.
+	PUSHL	BP
 	CMPL	BP, $0
-	JNE 2(PC)
-	CALL	runtime·badcallback(SB)
+	JNE	havem
+needm:
+	MOVL	$runtime·needm(SB), AX
+	CALL	AX
+	get_tls(CX)
+	MOVL	m(CX), BP
 
+havem:
+	// Now there's a valid m, and we're running on its m->g0.
+	// Save current m->g0->sched.sp on stack and then set it to SP.
+	// Save current sp in m->g0->sched.sp in preparation for
+	// switch back to m->curg stack.
 	MOVL	m_g0(BP), SI
 	PUSHL	(g_sched+gobuf_sp)(SI)
 	MOVL	SP, (g_sched+gobuf_sp)(SI)
@@ -509,6 +521,10 @@ TEXT runtime·cgocallback(SB),7,$12
 	// a frame size of 12, the same amount that we use below),
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
+	MOVL	fn+0(FP), AX
+	MOVL	frame+4(FP), BX
+	MOVL	framesize+8(FP), DX
+
 	MOVL	m_curg(BP), SI
 	MOVL	SI, g(CX)
 	MOVL	(g_sched+gobuf_sp)(SI), DI  // prepare stack as DI
@@ -546,10 +562,38 @@ TEXT runtime·cgocallback(SB),7,$12
 	MOVL	SI, g(CX)
 	MOVL	(g_sched+gobuf_sp)(SI), SP
 	POPL	(g_sched+gobuf_sp)(SI)
+	
+	// If the m on entry was nil, we called needm above to borrow an m
+	// for the duration of the call. Since the call is over, return it with dropm.
+	POPL	BP
+	CMPL	BP, $0
+	JNE 3(PC)
+	MOVL	$runtime·dropm(SB), AX
+	CALL	AX
 
 	// Done!
 	RET
 
+// void setmg(M*, G*); set m and g. for use by needm.
+TEXT runtime·setmg(SB), 7, $0
+#ifdef GOOS_windows
+	MOVL	mm+0(FP), AX
+	CMPL	AX, $0
+	JNE	settls
+	MOVL	$0, 0x14(FS)
+	RET
+settls:
+	LEAL	m_tls(AX), AX
+	MOVL	AX, 0x14(FS)
+#endif
+	MOVL	mm+0(FP), AX
+	get_tls(CX)
+	MOVL	mm+0(FP), AX
+	MOVL	AX, m(CX)
+	MOVL	gg+4(FP), BX
+	MOVL	BX, g(CX)
+	RET
+
 // check that SP is in range [g->stackbase, g->stackguard)
 TEXT runtime·stackcheck(SB), 7, $0
 	get_tls(CX)
diff --git a/src/pkg/runtime/asm_amd64.s b/src/pkg/runtime/asm_amd64.s
index 216c89c29f..159b7639be 100644
--- a/src/pkg/runtime/asm_amd64.s
+++ b/src/pkg/runtime/asm_amd64.s
@@ -509,21 +509,33 @@ TEXT runtime·asmcgocall(SB),7,$0
 // cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
 // See cgocall.c for more details.
 TEXT runtime·cgocallback(SB),7,$24
-	MOVQ	fn+0(FP), AX
-	MOVQ	frame+8(FP), BX
-	MOVQ	framesize+16(FP), DX
-
-	// Save current m->g0->sched.sp on stack and then set it to SP.
+	// If m is nil, Go did not create the current thread.
+	// Call needm to obtain one for temporary use.
+	// In this case, we're running on the thread stack, so there's
+	// lots of space, but the linker doesn't know. Hide the call from
+	// the linker analysis by using an indirect call through AX.
 	get_tls(CX)
+#ifdef GOOS_windows
+	CMPQ	CX, $0
+	JNE	3(PC)
+	PUSHQ	$0
+	JMP	needm
+#endif
 	MOVQ	m(CX), BP
-	
-	// If m is nil, it is almost certainly because we have been called
-	// on a thread that Go did not create.  We're going to crash as
-	// soon as we try to use m; instead, try to print a nice error and exit.
+	PUSHQ	BP
 	CMPQ	BP, $0
-	JNE 2(PC)
-	CALL	runtime·badcallback(SB)
+	JNE	havem
+needm:
+	MOVQ	$runtime·needm(SB), AX
+	CALL	AX
+	get_tls(CX)
+	MOVQ	m(CX), BP
 
+havem:
+	// Now there's a valid m, and we're running on its m->g0.
+	// Save current m->g0->sched.sp on stack and then set it to SP.
+	// Save current sp in m->g0->sched.sp in preparation for
+	// switch back to m->curg stack.
 	MOVQ	m_g0(BP), SI
 	PUSHQ	(g_sched+gobuf_sp)(SI)
 	MOVQ	SP, (g_sched+gobuf_sp)(SI)
@@ -542,6 +554,10 @@ TEXT runtime·cgocallback(SB),7,$24
 	// a frame size of 24, the same amount that we use below),
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
+	MOVQ	fn+0(FP), AX
+	MOVQ	frame+8(FP), BX
+	MOVQ	framesize+16(FP), DX
+
 	MOVQ	m_curg(BP), SI
 	MOVQ	SI, g(CX)
 	MOVQ	(g_sched+gobuf_sp)(SI), DI  // prepare stack as DI
@@ -579,10 +595,37 @@ TEXT runtime·cgocallback(SB),7,$24
 	MOVQ	SI, g(CX)
 	MOVQ	(g_sched+gobuf_sp)(SI), SP
 	POPQ	(g_sched+gobuf_sp)(SI)
+	
+	// If the m on entry was nil, we called needm above to borrow an m
+	// for the duration of the call. Since the call is over, return it with dropm.
+	POPQ	BP
+	CMPQ	BP, $0
+	JNE 3(PC)
+	MOVQ	$runtime·dropm(SB), AX
+	CALL	AX
 
 	// Done!
 	RET
 
+// void setmg(M*, G*); set m and g. for use by needm.
+TEXT runtime·setmg(SB), 7, $0
+	MOVQ	mm+0(FP), AX
+#ifdef GOOS_windows
+	CMPQ	AX, $0
+	JNE	settls
+	MOVQ	$0, 0x28(GS)
+	RET
+settls:
+	LEAQ	m_tls(AX), AX
+	MOVQ	AX, 0x28(GS)
+#endif
+	get_tls(CX)
+	MOVQ	mm+0(FP), AX
+	MOVQ	AX, m(CX)
+	MOVQ	gg+8(FP), BX
+	MOVQ	BX, g(CX)
+	RET
+
 // check that SP is in range [g->stackbase, g->stackguard)
 TEXT runtime·stackcheck(SB), 7, $0
 	get_tls(CX)
diff --git a/src/pkg/runtime/asm_arm.s b/src/pkg/runtime/asm_arm.s
index 9af5a8a0df..b0678bcd0b 100644
--- a/src/pkg/runtime/asm_arm.s
+++ b/src/pkg/runtime/asm_arm.s
@@ -290,11 +290,31 @@ TEXT	runtime·asmcgocall(SB),7,$0
 // cgocallback(void (*fn)(void*), void *frame, uintptr framesize)
 // See cgocall.c for more details.
 TEXT	runtime·cgocallback(SB),7,$16
+	// Load m and g from thread-local storage.
+	MOVW	cgo_load_gm(SB), R0
+	CMP	$0, R0
+	BL.NE	(R0)
+
+	// If m is nil, Go did not create the current thread.
+	// Call needm to obtain one for temporary use.
+	// In this case, we're running on the thread stack, so there's
+	// lots of space, but the linker doesn't know. Hide the call from
+	// the linker analysis by using an indirect call.
+	MOVW	m, savedm-16(SP)
+	CMP	$0, m
+	B.NE havem
+	MOVW	$runtime·needm(SB), R0
+	BL	(R0)
+
+havem:
+	// Now there's a valid m, and we're running on its m->g0.
+	// Save current m->g0->sched.sp on stack and then set it to SP.
+	// Save current sp in m->g0->sched.sp in preparation for
+	// switch back to m->curg stack.
 	MOVW	fn+0(FP), R0
 	MOVW	frame+4(FP), R1
 	MOVW	framesize+8(FP), R2
 
-	// Save current m->g0->sched.sp on stack and then set it to SP.
 	MOVW	m_g0(m), R3
 	MOVW	(g_sched+gobuf_sp)(R3), R4
 	MOVW.W	R4, -4(R13)
@@ -314,6 +334,8 @@ TEXT	runtime·cgocallback(SB),7,$16
 	// a frame size of 16, the same amount that we use below),
 	// so that the traceback will seamlessly trace back into
 	// the earlier calls.
+
+	// Save current m->g0->sched.sp on stack and then set it to SP.
 	MOVW	m_curg(m), g
 	MOVW	(g_sched+gobuf_sp)(g), R4 // prepare stack as R4
 
@@ -350,9 +372,29 @@ TEXT	runtime·cgocallback(SB),7,$16
 	ADD	$4, R13
 	MOVW	R6, (g_sched+gobuf_sp)(g)
 
+	// If the m on entry was nil, we called needm above to borrow an m
+	// for the duration of the call. Since the call is over, return it with dropm.
+	MOVW	savedm-16(SP), R6
+	CMP	$0, R6
+	B.NE	3(PC)
+	MOVW	$runtime·dropm(SB), R0
+	BL	(R0)
+
 	// Done!
 	RET
 
+// void setmg(M*, G*); set m and g. for use by needm.
+TEXT runtime·setmg(SB), 7, $-4
+	MOVW	mm+0(FP), m
+	MOVW	gg+4(FP), g
+
+	// Save m and g to thread-local storage.
+	MOVW	cgo_save_gm(SB), R0
+	CMP	$0, R0
+	BL.NE	(R0)
+
+	RET
+
 TEXT runtime·getcallerpc(SB),7,$-4
 	MOVW	0(SP), R0
 	RET
diff --git a/src/pkg/runtime/cgocall.c b/src/pkg/runtime/cgocall.c
index 519a5386b9..4f68b466fe 100644
--- a/src/pkg/runtime/cgocall.c
+++ b/src/pkg/runtime/cgocall.c
@@ -216,6 +216,11 @@ runtime·cgocallbackg(void (*fn)(void), void *arg, uintptr argsize)
 
 	runtime·exitsyscall();	// coming out of cgo call
 
+	if(m->needextram) {
+		m->needextram = 0;
+		runtime·newextram();
+	}
+
 	// Add entry to defer stack in case of panic.
 	d.fn = (byte*)unwindm;
 	d.siz = 0;
diff --git a/src/pkg/runtime/os_windows.h b/src/pkg/runtime/os_windows.h
index e8962265d5..cf0ecb68ee 100644
--- a/src/pkg/runtime/os_windows.h
+++ b/src/pkg/runtime/os_windows.h
@@ -29,6 +29,7 @@ byte *runtime·compilecallback(Eface fn, bool cleanstack);
 void *runtime·callbackasm(void);
 
 void runtime·install_exception_handler(void);
+void runtime·remove_exception_handler(void);
 
 // TODO(brainman): should not need those
 #define	NSIG	65
diff --git a/src/pkg/runtime/proc.c b/src/pkg/runtime/proc.c
index b83bd9066f..67d6dad488 100644
--- a/src/pkg/runtime/proc.c
+++ b/src/pkg/runtime/proc.c
@@ -25,6 +25,7 @@ int32	runtime·gcwaiting;
 G*	runtime·allg;
 G*	runtime·lastg;
 M*	runtime·allm;
+M*	runtime·extram;
 
 int8*	runtime·goos;
 int32	runtime·ncpu;
@@ -792,8 +793,11 @@ runtime·mstart(void)
 
 	// Install signal handlers; after minit so that minit can
 	// prepare the thread to be able to handle the signals.
-	if(m == &runtime·m0)
+	if(m == &runtime·m0) {
 		runtime·initsig();
+		if(runtime·iscgo)
+			runtime·newextram();
+	}
 
 	schedule(nil);
 
@@ -838,9 +842,9 @@ matchmg(void)
 	}
 }
 
-// Create a new m.  It will start off with a call to runtime·mstart.
+// Allocate a new m unassociated with any thread.
 M*
-runtime·newm(void)
+runtime·allocm(void)
 {
 	M *mp;
 	static Type *mtype;  // The Go type M
@@ -854,23 +858,228 @@ runtime·newm(void)
 	mp = runtime·cnew(mtype);
 	mcommoninit(mp);
 
+	if(runtime·iscgo || Windows)
+		mp->g0 = runtime·malg(-1);
+	else
+		mp->g0 = runtime·malg(8192);
+	
+	return mp;
+}
+
+static M* lockextra(bool nilokay);
+static void unlockextra(M*);
+
+// needm is called when a cgo callback happens on a
+// thread without an m (a thread not created by Go).
+// In this case, needm is expected to find an m to use
+// and return with m, g initialized correctly.
+// Since m and g are not set now (likely nil, but see below)
+// needm is limited in what routines it can call. In particular
+// it can only call nosplit functions (textflag 7) and cannot
+// do any scheduling that requires an m.
+//
+// In order to avoid needing heavy lifting here, we adopt
+// the following strategy: there is a stack of available m's
+// that can be stolen. Using compare-and-swap
+// to pop from the stack has ABA races, so we simulate
+// a lock by doing an exchange (via casp) to steal the stack
+// head and replace the top pointer with MLOCKED (1).
+// This serves as a simple spin lock that we can use even
+// without an m. The thread that locks the stack in this way
+// unlocks the stack by storing a valid stack head pointer.
+//
+// In order to make sure that there is always an m structure
+// available to be stolen, we maintain the invariant that there
+// is always one more than needed. At the beginning of the
+// program (if cgo is in use) the list is seeded with a single m.
+// If needm finds that it has taken the last m off the list, its job
+// is - once it has installed its own m so that it can do things like
+// allocate memory - to create a spare m and put it on the list.
+//
+// Each of these extra m's also has a g0 and a curg that are
+// pressed into service as the scheduling stack and current
+// goroutine for the duration of the cgo callback.
+//
+// When the callback is done with the m, it calls dropm to
+// put the m back on the list.
+#pragma textflag 7
+void
+runtime·needm(byte x)
+{
+	M *mp;
+
+	// Lock extra list, take head, unlock popped list.
+	// nilokay=false is safe here because of the invariant above,
+	// that the extra list always contains or will soon contain
+	// at least one m.
+	mp = lockextra(false);
+
+	// Set needextram when we've just emptied the list,
+	// so that the eventual call into cgocallbackg will
+	// allocate a new m for the extra list. We delay the
+	// allocation until then so that it can be done 
+	// after exitsyscall makes sure it is okay to be
+	// running at all (that is, there's no garbage collection
+	// running right now).	
+	mp->needextram = mp->schedlink == nil;
+	unlockextra(mp->schedlink);
+	
+	// Install m and g (= m->g0) and set the stack bounds
+	// to match the current stack. We don't actually know
+	// how big the stack is, like we don't know how big any
+	// scheduling stack is, but we assume there's at least 32 kB,
+	// which is more than enough for us.
+	runtime·setmg(mp, mp->g0);
+	g->stackbase = (uintptr)(&x + 1024);
+	g->stackguard = (uintptr)(&x - 32*1024);
+
+	// On windows/386, we need to put an SEH frame (two words)
+	// somewhere on the current stack. We are called
+	// from needm, and we know there is some available
+	// space one word into the argument frame. Use that.
+	m->seh = (SEH*)((uintptr*)&x + 1);
+
+	// Initialize this thread to use the m.
+	runtime·asminit();
+	runtime·minit();
+}
+
+// newextram allocates an m and puts it on the extra list.
+// It is called with a working local m, so that it can do things
+// like call schedlock and allocate.
+void
+runtime·newextram(void)
+{
+	M *mp, *mnext;
+	G *gp;
+
+	// Scheduler protects allocation of new m's and g's.
+	// Create extra goroutine locked to extra m.
+	// The goroutine is the context in which the cgo callback will run.
+	// The sched.pc will never be returned to, but setting it to
+	// runtime.goexit makes clear to the traceback routines where
+	// the goroutine stack ends.
+	schedlock();
+	mp = runtime·allocm();
+	gp = runtime·malg(4096);
+	gp->sched.pc = (void*)runtime·goexit;
+	gp->sched.sp = gp->stackbase;
+	gp->sched.g = gp;
+	gp->status = Gsyscall;
+	mp->curg = gp;
+	mp->locked = LockInternal;
+	mp->lockedg = gp;
+	gp->lockedm = mp;
+	schedunlock();
+
+	// Add m to the extra list.
+	mnext = lockextra(true);
+	mp->schedlink = mnext;
+	unlockextra(mp);
+}
+
+// dropm is called when a cgo callback has called needm but is now
+// done with the callback and returning back into the non-Go thread.
+// It puts the current m back onto the extra list.
+//
+// The main expense here is the call to signalstack to release the
+// m's signal stack, and then the call to needm on the next callback
+// from this thread. It is tempting to try to save the m for next time,
+// which would eliminate both these costs, but there might not be 
+// a next time: the current thread (which Go does not control) might exit.
+// If we saved the m for that thread, there would be an m leak each time
+// such a thread exited. Instead, we acquire and release an m on each
+// call. These should typically not be scheduling operations, just a few
+// atomics, so the cost should be small.
+//
+// TODO(rsc): An alternative would be to allocate a dummy pthread per-thread
+// variable using pthread_key_create. Unlike the pthread keys we already use
+// on OS X, this dummy key would never be read by Go code. It would exist
+// only so that we could register at thread-exit-time destructor.
+// That destructor would put the m back onto the extra list.
+// This is purely a performance optimization. The current version,
+// in which dropm happens on each cgo call, is still correct too.
+// We may have to keep the current version on systems with cgo
+// but without pthreads, like Windows.
+void
+runtime·dropm(void)
+{
+	M *mp, *mnext;
+
+	// Undo whatever initialization minit did during needm.
+	runtime·unminit();
+
+	// Clear m and g, and return m to the extra list.
+	// After the call to setmg we can only call nosplit functions.
+	mp = m;
+	runtime·setmg(nil, nil);
+
+	mnext = lockextra(true);
+	mp->schedlink = mnext;
+	unlockextra(mp);
+}
+
+#define MLOCKED ((M*)1)
+
+// lockextra locks the extra list and returns the list head.
+// The caller must unlock the list by storing a new list head
+// to runtime.extram. If nilokay is true, then lockextra will
+// return a nil list head if that's what it finds. If nilokay is false,
+// lockextra will keep waiting until the list head is no longer nil.
+#pragma textflag 7
+static M*
+lockextra(bool nilokay)
+{
+	M *mp;
+	void (*yield)(void);
+	
+	for(;;) {
+		mp = runtime·atomicloadp(&runtime·extram);
+		if(mp == MLOCKED) {
+			yield = runtime·osyield;
+			yield();
+			continue;
+		}
+		if(mp == nil && !nilokay) {
+			runtime·usleep(1);
+			continue;
+		}
+		if(!runtime·casp(&runtime·extram, mp, MLOCKED)) {
+			yield = runtime·osyield;
+			yield();
+			continue;
+		}
+		break;
+	}
+	return mp;
+}
+
+#pragma textflag 7
+static void
+unlockextra(M *mp)
+{
+	runtime·atomicstorep(&runtime·extram, mp);
+}
+
+
+// Create a new m.  It will start off with a call to runtime·mstart.
+M*
+runtime·newm(void)
+{
+	M *mp;
+	
+	mp = runtime·allocm();
+
 	if(runtime·iscgo) {
 		CgoThreadStart ts;
 
 		if(libcgo_thread_start == nil)
 			runtime·throw("libcgo_thread_start missing");
-		// pthread_create will make us a stack.
-		mp->g0 = runtime·malg(-1);
 		ts.m = mp;
 		ts.g = mp->g0;
 		ts.fn = runtime·mstart;
 		runtime·asmcgocall(libcgo_thread_start, &ts);
 	} else {
-		if(Windows)
-			// windows will layout sched stack on os stack
-			mp->g0 = runtime·malg(-1);
-		else
-			mp->g0 = runtime·malg(8192);
 		runtime·newosproc(mp, mp->g0, (byte*)mp->g0->stackbase, runtime·mstart);
 	}
 
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index e63877e681..8162874bbe 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -289,6 +289,7 @@ struct	M
 	uint32	waitsemalock;
 	GCStats	gcstats;
 	bool	racecall;
+	bool	needextram;
 	void*	racepc;
 	uint32	moreframesize_minalloc;
 
@@ -657,10 +658,11 @@ void	runtime·ready(G*);
 byte*	runtime·getenv(int8*);
 int32	runtime·atoi(byte*);
 void	runtime·newosproc(M *mp, G *gp, void *stk, void (*fn)(void));
-void	runtime·signalstack(byte*, int32);
 G*	runtime·malg(int32);
 void	runtime·asminit(void);
 void	runtime·minit(void);
+void	runtime·unminit(void);
+void	runtime·signalstack(byte*, int32);
 Func*	runtime·findfunc(uintptr);
 int32	runtime·funcline(Func*, uintptr);
 void*	runtime·stackalloc(uint32);
@@ -683,6 +685,8 @@ int32	runtime·gcount(void);
 void	runtime·mcall(void(*)(G*));
 uint32	runtime·fastrand1(void);
 
+void runtime·setmg(M*, G*);
+void runtime·newextram(void);
 void	runtime·exit(int32);
 void	runtime·breakpoint(void);
 void	runtime·gosched(void);
diff --git a/src/pkg/runtime/sys_windows_386.s b/src/pkg/runtime/sys_windows_386.s
index ab6d7f2209..dbc4352e2e 100644
--- a/src/pkg/runtime/sys_windows_386.s
+++ b/src/pkg/runtime/sys_windows_386.s
@@ -303,3 +303,15 @@ TEXT runtime·install_exception_handler(SB),7,$0
 	MOVL	DX, 0(FS)
 
 	RET
+
+// void remove_exception_handler()
+TEXT runtime·remove_exception_handler(SB),7,$0
+	get_tls(CX)
+	MOVL	m(CX), CX		// m
+
+	// Remove SEH frame
+	MOVL	m_seh(CX), DX
+	MOVL	seh_prev(DX), AX
+	MOVL	AX, 0(FS)
+
+	RET
diff --git a/src/pkg/runtime/sys_windows_amd64.s b/src/pkg/runtime/sys_windows_amd64.s
index b2b8de5025..33ec33640b 100644
--- a/src/pkg/runtime/sys_windows_amd64.s
+++ b/src/pkg/runtime/sys_windows_amd64.s
@@ -343,3 +343,6 @@ TEXT runtime·settls(SB),7,$0
 TEXT runtime·install_exception_handler(SB),7,$0
 	CALL	runtime·setstacklimits(SB)
 	RET
+
+TEXT runtime·remove_exception_handler(SB),7,$0
+	RET
diff --git a/src/pkg/runtime/thread_darwin.c b/src/pkg/runtime/thread_darwin.c
index 83c638067d..1d6037b48b 100644
--- a/src/pkg/runtime/thread_darwin.c
+++ b/src/pkg/runtime/thread_darwin.c
@@ -121,6 +121,13 @@ runtime·minit(void)
 	runtime·setprof(m->profilehz > 0);
 }
 
+// Called from dropm to undo the effect of an minit.
+void
+runtime·unminit(void)
+{
+	runtime·signalstack(nil, 0);
+}
+
 // Mach IPC, to get at semaphores
 // Definitions are in /usr/include/mach on a Mac.
 
diff --git a/src/pkg/runtime/thread_freebsd.c b/src/pkg/runtime/thread_freebsd.c
index 861e6b0f70..4d5f69a9f3 100644
--- a/src/pkg/runtime/thread_freebsd.c
+++ b/src/pkg/runtime/thread_freebsd.c
@@ -130,6 +130,13 @@ runtime·minit(void)
 	runtime·sigprocmask(&sigset_none, nil);
 }
 
+// Called from dropm to undo the effect of an minit.
+void
+runtime·unminit(void)
+{
+	runtime·signalstack(nil, 0);
+}
+
 void
 runtime·sigpanic(void)
 {
diff --git a/src/pkg/runtime/thread_linux.c b/src/pkg/runtime/thread_linux.c
index fdd40c223e..02a5eaee2f 100644
--- a/src/pkg/runtime/thread_linux.c
+++ b/src/pkg/runtime/thread_linux.c
@@ -181,6 +181,13 @@ runtime·minit(void)
 	runtime·rtsigprocmask(SIG_SETMASK, &sigset_none, nil, sizeof(Sigset));
 }
 
+// Called from dropm to undo the effect of an minit.
+void
+runtime·unminit(void)
+{
+	runtime·signalstack(nil, 0);
+}
+
 void
 runtime·sigpanic(void)
 {
diff --git a/src/pkg/runtime/thread_netbsd.c b/src/pkg/runtime/thread_netbsd.c
index bbe7df6e98..ebef45e757 100644
--- a/src/pkg/runtime/thread_netbsd.c
+++ b/src/pkg/runtime/thread_netbsd.c
@@ -199,6 +199,13 @@ runtime·minit(void)
 	runtime·sigprocmask(SIG_SETMASK, &sigset_none, nil);
 }
 
+// Called from dropm to undo the effect of an minit.
+void
+runtime·unminit(void)
+{
+	runtime·signalstack(nil, 0);
+}
+
 void
 runtime·sigpanic(void)
 {
diff --git a/src/pkg/runtime/thread_openbsd.c b/src/pkg/runtime/thread_openbsd.c
index ce8043f016..8433e8bae5 100644
--- a/src/pkg/runtime/thread_openbsd.c
+++ b/src/pkg/runtime/thread_openbsd.c
@@ -176,6 +176,13 @@ runtime·minit(void)
 	runtime·sigprocmask(SIG_SETMASK, sigset_none);
 }
 
+// Called from dropm to undo the effect of an minit.
+void
+runtime·unminit(void)
+{
+	runtime·signalstack(nil, 0);
+}
+
 void
 runtime·sigpanic(void)
 {
diff --git a/src/pkg/runtime/thread_plan9.c b/src/pkg/runtime/thread_plan9.c
index 932135dca8..bca0deac62 100644
--- a/src/pkg/runtime/thread_plan9.c
+++ b/src/pkg/runtime/thread_plan9.c
@@ -23,6 +23,13 @@ runtime·minit(void)
 	runtime·setfpmasks();
 }
 
+// Called from dropm to undo the effect of an minit.
+void
+runtime·unminit(void)
+{
+}
+
+
 static int32
 getproccount(void)
 {
@@ -82,6 +89,7 @@ runtime·initsig(void)
 {
 }
 
+#pragma textflag 7
 void
 runtime·osyield(void)
 {
diff --git a/src/pkg/runtime/thread_windows.c b/src/pkg/runtime/thread_windows.c
index 600a48ab62..7110c6efe4 100644
--- a/src/pkg/runtime/thread_windows.c
+++ b/src/pkg/runtime/thread_windows.c
@@ -135,6 +135,7 @@ runtime·write(int32 fd, void *buf, int32 n)
 	return written;
 }
 
+#pragma textflag 7
 void
 runtime·osyield(void)
 {
@@ -211,6 +212,13 @@ runtime·minit(void)
 	runtime·install_exception_handler();
 }
 
+// Called from dropm to undo the effect of an minit.
+void
+runtime·unminit(void)
+{
+	runtime·remove_exception_handler();
+}
+
 int64
 runtime·nanotime(void)
 {