runtime: fix fault during arm software floating point

The software floating point runs with m->locks++
to avoid being preempted; recognize this case in panic
and undo it so that m->locks is maintained correctly
when panicking.

Fixes #7553.

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/84030043

1 files changed, 15 insertions, 1 deletions

diff --git a/src/pkg/runtime/panic.c b/src/pkg/runtime/panic.c
index 0bf3b6a140..3af8cb67aa 100644
--- a/src/pkg/runtime/panic.c
+++ b/src/pkg/runtime/panic.c
@@ -498,7 +498,7 @@ runtime·canpanic(G *gp)
 	// and not stuck in a system call.
 	if(gp == nil || gp != m->curg)
 		return false;
-	if(m->locks != 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->gcing != 0 || m->dying != 0)
 		return false;
 	if(gp->status != Grunning || gp->syscallsp != 0)
 		return false;
@@ -526,6 +526,16 @@ runtime·panicstring(int8 *s)
 {
 	Eface err;
 
+	// m->softfloat is set during software floating point,
+	// which might cause a fault during a memory load.
+	// It increments m->locks to avoid preemption.
+	// If we're panicking, the software floating point frames
+	// will be unwound, so decrement m->locks as they would.
+	if(m->softfloat) {
+		m->locks--;
+		m->softfloat = 0;
+	}
+
 	if(m->mallocing) {
 		runtime·printf("panic: %s\n", s);
 		runtime·throw("panic during malloc");
@@ -534,6 +544,10 @@ runtime·panicstring(int8 *s)
 		runtime·printf("panic: %s\n", s);
 		runtime·throw("panic during gc");
 	}
+	if(m->locks) {
+		runtime·printf("panic: %s\n", s);
+		runtime·throw("panic holding locks");
+	}
 	runtime·newErrorCString(s, &err);
 	runtime·panic(err);
 }