runtime: skip sysmon workaround on NetBSD >= 9.2

Detect the NetBSD version in osinit and only enable the workaround for
the kernel bug identified in #42515 for NetBSD versions older than 9.2.

For #42515
For #46495

Change-Id: I808846c7f8e47e5f7cc0a2f869246f4bd90d8e22
Reviewed-on: https://go-review.googlesource.com/c/go/+/324472
Trust: Tobias Klauser <[email protected]>
Trust: Benny Siegert <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: tklauser

src/runtime/os_netbsd.go

@@ -101,6 +101,9 @@ var sigset_all = sigset{[4]uint32{^uint32(0), ^uint32(0), ^uint32(0), ^uint32(0)
 
 // From NetBSD's <sys/sysctl.h>
 const (
+	_CTL_KERN   = 1
+	_KERN_OSREV = 3
+
 	_CTL_HW        = 6
 	_HW_NCPU       = 3
 	_HW_PAGESIZE   = 7
@@ -138,6 +141,13 @@ func getPageSize() uintptr {
 	return 0
 }
 
+func getOSRev() int {
+	if osrev, ok := sysctlInt([]uint32{_CTL_KERN, _KERN_OSREV}); ok {
+		return int(osrev)
+	}
+	return 0
+}
+
 //go:nosplit
 func semacreate(mp *m) {
 }
@@ -252,6 +262,7 @@ func osinit() {
 	if physPageSize == 0 {
 		physPageSize = getPageSize()
 	}
+	needSysmonWorkaround = getOSRev() < 902000000 // NetBSD 9.2
 }
 
 var urandom_dev = []byte("/dev/urandom\x00")

src/runtime/proc.go

@@ -5229,6 +5229,10 @@ func checkdead() {
 // This is a variable for testing purposes. It normally doesn't change.
 var forcegcperiod int64 = 2 * 60 * 1e9
 
+// needSysmonWorkaround is true if the workaround for
+// golang.org/issue/42515 is needed on NetBSD.
+var needSysmonWorkaround bool = false
+
 // Always runs without a P, so write barriers are not allowed.
 //
 //go:nowritebarrierrec
@@ -5337,7 +5341,7 @@ func sysmon() {
 			}
 		}
 		mDoFixup()
-		if GOOS == "netbsd" {
+		if GOOS == "netbsd" && needSysmonWorkaround {
 			// netpoll is responsible for waiting for timer
 			// expiration, so we typically don't have to worry
 			// about starting an M to service timers. (Note that