runtime: drop NetBSD kernel bug sysmon workaround fixed in NetBSD 9.2

[prattmic]

#42515 identified a NetBSD kernel bug (kern/50094) affecting the Go runtime. http://golang.org/cl/277332 introduced a (rather subpar) workaround for this issue.

@jdolecek-zz notes that the fix for this bug is included in NetBSD 9.2, making the workaround unnecessary.

Ideally we just drop http://golang.org/cl/277332, but this depends on our support policy for NetBSD. https://github.com/golang/go/wiki/NetBSD states that we currently support NetBSD 7+, but not what the official policy is. (OTOH, e.g., OpenBSD is explicitly supported only for the two most recent releases: https://github.com/golang/go/wiki/OpenBSD#longterm-support).

Based on my reading of https://www.netbsd.org/releases/formal.html, my interpretation would be that Go should support NetBSD 8 and 9 (and presumably drop 8 when 10 is released?). Do others agree? If so, we should probably add a version check to the workaround and keep it until NetBSD 9 is dropped.

cc @bsiegert @dmitshur @mknyszek

[dmitshur]

I agree we should find a support policy that automatically drops support for old NetBSD releases when new ones are made, similar to how OpenBSD has one.

CC @golang/release.

[tklauser]

Another option to already disable the workaround for NetBSD 9.2 while still retaining compatibility with older NetBSD versions would be to check sysctlInt([]uint32{_CTL_KERN, _KERN_OSREV}) in runtime.osinit and disable the fix if that value is >= 902000000.

I've sent https://golang.org/cl/324472 implementing that.

[gopherbot]

Change https://golang.org/cl/324472 mentions this issue: runtime: skip sysmon workaround on NetBSD >= 9.2

[hzzb]

Another option to already disable the workaround for NetBSD 9.2 while still retaining compatibility with older NetBSD versions would be to check sysctlInt([]uint32{_CTL_KERN, _KERN_OSREV}) in runtime.osinit and disable the fix if that value is >= 902000000.

I've sent https://golang.org/cl/324472 implementing that.

The CL you sent does os version checking in os_netbsd.go. It seems to me that no GOOS == "netbsd" rechecking needed in sysmon. Because needSysmonWorkaround would be true only on netbsd platform. right?

CC @tklauser

[tklauser]

Another option to already disable the workaround for NetBSD 9.2 while still retaining compatibility with older NetBSD versions would be to check sysctlInt([]uint32{_CTL_KERN, _KERN_OSREV}) in runtime.osinit and disable the fix if that value is >= 902000000.
I've sent https://golang.org/cl/324472 implementing that.

The CL you sent does os version checking in os_netbsd.go. It seems to me that no GOOS == "netbsd" rechecking needed in sysmon. Because needSysmonWorkaround would be true only on netbsd platform. right?

CC @tklauser

Yes, needSysmonWorkaround will only be true on netbsd. However, the additional GOOS == "netbsd" check will lead the compiler to omit the entire branch on non-netbsd platforms, i.e. needSysmonWorkaround will not need to be checked at runtime.

[hzzb]

Another option to already disable the workaround for NetBSD 9.2 while still retaining compatibility with older NetBSD versions would be to check sysctlInt([]uint32{_CTL_KERN, _KERN_OSREV}) in runtime.osinit and disable the fix if that value is >= 902000000.
I've sent https://golang.org/cl/324472 implementing that.

The CL you sent does os version checking in os_netbsd.go. It seems to me that no GOOS == "netbsd" rechecking needed in sysmon. Because needSysmonWorkaround would be true only on netbsd platform. right?
CC @tklauser

Yes, needSysmonWorkaround will only be true on netbsd. However, the additional GOOS == "netbsd" check will lead the compiler to omit the entire branch on non-netbsd platforms, i.e. needSysmonWorkaround will not need to be checked at runtime.

Got it. Thanks for your explanation.