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

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Message-ID: <CA+icZUVpVPQUMuv13vonO=fed8qMxaXXTO9BYOh8ps316ePrGw@mail.gmail.com>
Date:	Fri, 15 Jun 2012 11:35:58 +0200
From:	Sedat Dilek <sedat.dilek@...glemail.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Eric B Munson <emunson@...bm.net>, 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 Tue, Jun 12, 2012 at 4:13 AM, Marcelo Tosatti <mtosatti@...hat.com> wrote:
> 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.
>

Unfortunately, I was busy this week with familiar affairs.
I have seen too late that Marcelo sent me the patch via PM.

Marcelo, could you please give credits to people next time, Thanks.

I am missing sth. like this in the commit now in Linus upstream GIT...

Reported-by: Frank Arnold <frank.arnold@...> (hidden - me reads KVM ML offline)
Reported-by: Sedat Dilek <sedat.dilek@...il.com> (lazy guy - could
have reported earlier)
Acked-by or Reviewed-by: Eric B Munson <emunson@...bm.net>

- Sedat -

[1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=e32025a56403df4386cd61a741c0a36afe79ae8a
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