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

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Message-ID: <ff5b99576bc8f0cbe81a432ddae5f9b5@mgebm.net>
Date:	Mon, 11 Jun 2012 18:10:34 -0400
From:	Eric B Munson <emunson@...bm.net>
To:	Marcelo Tosatti <mtosatti@...hat.com>
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, 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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