Re: WARNING: at arch/x86/kernel/kvmclock.c:127

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Jun 11, 2012 at 05:47:00PM -0400, Eric B Munson wrote:
> On Sun, 10 Jun 2012 17:47:24 +0200, Sedat Dilek wrote:
> >Hi,
> >
> >I see the same warning especially when resuming from a suspend (see
> >timestamps >=30-35sec) between Linux v3.5-rc1..v3.5-rc2.
> >
> >$ sudo grep kvmclock.c /var/log/kern.log
> >Jun  4 21:11:32 fambox kernel: [  784.037237] WARNING: at
> >arch/x86/kernel/kvmclock.c:127
> >kvm_check_and_clear_guest_paused+0x52/0x60()
> >Jun  5 20:35:57 fambox kernel: [ 1928.458060] WARNING: at
> >arch/x86/kernel/kvmclock.c:127
> >kvm_check_and_clear_guest_paused+0x52/0x60()
> >Jun  8 09:35:52 fambox kernel: [ 3290.134637] WARNING: at
> >arch/x86/kernel/kvmclock.c:127
> >kvm_check_and_clear_guest_paused+0x52/0x60()
> >Jun  8 09:35:52 fambox kernel: [ 3290.238592] WARNING: at
> >arch/x86/kernel/kvmclock.c:127
> >kvm_check_and_clear_guest_paused+0x52/0x60()
> >Jun  8 12:11:20 fambox kernel: [ 5777.023571] WARNING: at
> >arch/x86/kernel/kvmclock.c:127
> >kvm_check_and_clear_guest_paused+0x52/0x60()
> >Jun  9 13:32:50 fambox kernel: [ 2778.842695] WARNING: at
> >arch/x86/kernel/kvmclock.c:127
> >kvm_check_and_clear_guest_paused+0x52/0x60()
> >
> >From [1]:
> >
> >"...The warning itself is not required for the check_and_clear
> >function and can be removed as far as I am concerned."
> >
> >From [2] commit 3b5d56b9317fa7b5407dff1aa7b115bf6cdbd494 ("kvmclock:
> >Add functions to check if the host has stopped the vm")
> >...
> >+bool kvm_check_and_clear_guest_paused(void)
> >+{
> >+       bool ret = false;
> >+       struct pvclock_vcpu_time_info *src;
> >+
> >+       /*
> >+        * per_cpu() is safe here because this function is only
> >called from
> >+        * timer functions where preemption is already disabled.
> >+        */
> >+       WARN_ON(!in_atomic());
> >+       src = &__get_cpu_var(hv_clock);
> >+       if ((src->flags & PVCLOCK_GUEST_STOPPED) != 0) {
> >+               __this_cpu_and(hv_clock.flags,
> >~PVCLOCK_GUEST_STOPPED);
> >+               ret = true;
> >+       }
> >+
> >+       return ret;
> >+}
> >+EXPORT_SYMBOL_GPL(kvm_check_and_clear_guest_paused);
> >+
> >...
> >( The export macro was dropped in a followup commit. )
> >
> >So you mean "WARN_ON(!in_atomic());" can be deleted?

Yes.

> When I wrote the patch originally, I was under the (incorrect)
> assumption that the watch dog was only ever reset in an atomic
> context.  Given that this is not the case, the warning can be
> removed.  Though before that happens, I have a question: if this is
> called outside of an atomic context, is the use of __get_cpu_Var()
> and __this_cpu_and() invalid?

It remains valid because its called with interrupts 
disabled (see migrate_hrtimers).