runtime: convert prof.hz to atomic type

This converts several unsynchronized reads (reads without holding
prof.signalLock) into atomic reads.

For #53821.
For #52912.

Change-Id: I421b96a22fbe26d699bcc21010c8a9e0f4efc276
Reviewed-on: https://go-review.googlesource.com/c/go/+/420196
Reviewed-by: Austin Clements <[email protected]>
Run-TryBot: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: prattmic

src/runtime/cpuprof.go

@@ -110,7 +110,7 @@ func (p *cpuProfile) add(tagPtr *unsafe.Pointer, stk []uintptr) {
 		osyield()
 	}
 
-	if prof.hz != 0 { // implies cpuprof.log != nil
+	if prof.hz.Load() != 0 { // implies cpuprof.log != nil
 		if p.numExtra > 0 || p.lostExtra > 0 || p.lostAtomic > 0 {
 			p.addExtra()
 		}

src/runtime/proc.go

@@ -4475,7 +4475,10 @@ func mcount() int32 {
 
 var prof struct {
 	signalLock atomic.Uint32
-	hz         int32
+
+	// Must hold signalLock to write. Reads may be lock-free, but
+	// signalLock should be taken to synchronize with changes.
+	hz atomic.Int32
 }
 
 func _System()                    { _System() }
@@ -4490,7 +4493,7 @@ func _VDSO()                      { _VDSO() }
 //
 //go:nowritebarrierrec
 func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
-	if prof.hz == 0 {
+	if prof.hz.Load() == 0 {
 		return
 	}
 
@@ -4587,7 +4590,7 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
 		}
 	}
 
-	if prof.hz != 0 {
+	if prof.hz.Load() != 0 {
 		// Note: it can happen on Windows that we interrupted a system thread
 		// with no g, so gp could nil. The other nil checks are done out of
 		// caution, but not expected to be nil in practice.
@@ -4631,9 +4634,9 @@ func setcpuprofilerate(hz int32) {
 	for !prof.signalLock.CompareAndSwap(0, 1) {
 		osyield()
 	}
-	if prof.hz != hz {
+	if prof.hz.Load() != hz {
 		setProcessCPUProfiler(hz)
-		prof.hz = hz
+		prof.hz.Store(hz)
 	}
 	prof.signalLock.Store(0)
 

src/runtime/signal_unix.go

@@ -502,7 +502,7 @@ var sigprofCallersUse uint32
 //go:nosplit
 //go:nowritebarrierrec
 func sigprofNonGo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
-	if prof.hz != 0 {
+	if prof.hz.Load() != 0 {
 		c := &sigctxt{info, ctx}
 		// Some platforms (Linux) have per-thread timers, which we use in
 		// combination with the process-wide timer. Avoid double-counting.
@@ -525,7 +525,7 @@ func sigprofNonGo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
 //go:nosplit
 //go:nowritebarrierrec
 func sigprofNonGoPC(pc uintptr) {
-	if prof.hz != 0 {
+	if prof.hz.Load() != 0 {
 		stk := []uintptr{
 			pc,
 			abi.FuncPCABIInternal(_ExternalCode) + sys.PCQuantum,