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

[Eric B Munson]

On Mon, 11 Jun 2012 19:07:19 -0300, Marcelo Tosatti wrote:
> 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).

Thank you, that was my assumption but I wanted to confirm.  I do not 
have access to my working kernel tree and won't for 2 days.  I can't get 
to the patch until late Wednesday so if someone else wants to work that 
up I'd appreciate it.

Thanks,
Eric
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