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

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Message-ID: <0669740d6e2f3648f1c5c9a50e72f6eb@mgebm.net>
Date:	Mon, 11 Jun 2012 17:47:00 -0400
From:	Eric B Munson <emunson@...bm.net>
To:	<sedat.dilek@...il.com>
Cc:	<kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...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 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?

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?

Thanks,
Eric
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