sync: throw, not panic, for unlock of unlocked mutex

The panic leaves the lock in an unusable state.
Trying to panic with a usable state makes the lock significantly
less efficient and scalable (see early CL patch sets and discussion).

Instead, use runtime.throw, which will crash the program directly.

In general throw is reserved for when the runtime detects truly
serious, unrecoverable problems. This problem is certainly serious,
and, without a significant performance hit, is unrecoverable.

Fixes #13879.

Change-Id: I41920d9e2317270c6f909957d195bd8b68177f8d
Reviewed-on: https://go-review.googlesource.com/31359
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index 9e108cc437..f78e67f9bb 100644
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -576,6 +576,11 @@ func dopanic(unused int) {
 	*(*int)(nil) = 0
 }
 
+//go:linkname sync_throw sync.throw
+func sync_throw(s string) {
+	throw(s)
+}
+
 //go:nosplit
 func throw(s string) {
 	print("fatal error: ", s, "\n")