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

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Date:	Mon, 11 Jun 2012 23:13:52 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Eric B Munson <emunson@...bm.net>
Cc:	sedat.dilek@...il.com, kvm@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, x86@...nel.org
Subject: Re: WARNING: at arch/x86/kernel/kvmclock.c:127

On Mon, Jun 11, 2012 at 06:10:34PM -0400, Eric B Munson wrote:
> 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

Removed the warning in kvm.git master.

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