runtime: don't test sig.inuse in sigsend

Signals can be delivered on a different thread. There is no necessary
happens-before relationship between setting sig.inuse in signal_enable
and checking it in sigsend. It is theoretically possible, if unlikely,
that sig.inuse is set by thread 1, thread 2 receives a signal, does not
see that sig.inuse is set, and discards the signal. This could happen
if the signal is received immediately after the first call to signal_enable.

For #33174

Change-Id: Idb0f1c77847b7d4d418bd139e801c0c4460531d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/312131
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: ianlancetaylor

src/runtime/sigqueue.go

@@ -72,7 +72,7 @@ const (
 // It runs from the signal handler, so it's limited in what it can do.
 func sigsend(s uint32) bool {
 	bit := uint32(1) << uint(s&31)
-	if !sig.inuse || s >= uint32(32*len(sig.wanted)) {
+	if s >= uint32(32*len(sig.wanted)) {
 		return false
 	}