runtime: ignore SIGPROF if profiling disable for thread

This avoids a deadlock on prof.signalLock between setcpuprofilerate
and cpuprof.add if a SIGPROF is delivered to the thread between the
call to setThreadCPUProfiler and acquiring prof.signalLock.

Fixes #41014

Change-Id: Ie825e8594f93a19fb1a6320ed640f4e631553596
Reviewed-on: https://go-review.googlesource.com/c/go/+/253758
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: ianlancetaylor

src/runtime/proc.go

@@ -3928,6 +3928,13 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
 		return
 	}
 
+	// If mp.profilehz is 0, then profiling is not enabled for this thread.
+	// We must check this to avoid a deadlock between setcpuprofilerate
+	// and the call to cpuprof.add, below.
+	if mp != nil && mp.profilehz == 0 {
+		return
+	}
+
 	// On mips{,le}, 64bit atomics are emulated with spinlocks, in
 	// runtime/internal/atomic. If SIGPROF arrives while the program is inside
 	// the critical section, it creates a deadlock (when writing the sample).