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Date: Fri, 27 Nov 2009 17:37:09 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: Jiri Slaby <jirislaby@gmail.com>, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org, mm-commits@vger.kernel.org,
       Avi Kivity <avi@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>,
       kvm@vger.kernel.org, the arch/x86 maintainers <x86@kernel.org>,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: WARNING: kernel/smp.c:292 smp_call_function_single [Was: mmotm
 2009-11-24-16-47 uploaded]
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On Fri, 27 Nov 2009, Peter Zijlstra wrote:

> On Fri, 2009-11-27 at 16:03 +0100, Jiri Slaby wrote:
> > On 11/25/2009 01:47 AM, akpm@linux-foundation.org wrote:
> > > The mm-of-the-moment snapshot 2009-11-24-16-47 has been uploaded to
> > 
> > Hi, when executing qemu-kvm I often get following warning and a hard lockup.
> > 
> > WARNING: at kernel/smp.c:292 smp_call_function_single+0xbd/0x140()
> > Hardware name: To Be Filled By O.E.M.
> > Modules linked in: kvm_intel kvm fuse ath5k ath
> > Pid: 3265, comm: qemu-kvm Not tainted 2.6.32-rc8-mm1_64 #912
> > Call Trace:
> >  [<ffffffff81039678>] warn_slowpath_common+0x78/0xb0
> >  [<ffffffffa007fd50>] ? __vcpu_clear+0x0/0xd0 [kvm_intel]
> >  [<ffffffff810396bf>] warn_slowpath_null+0xf/0x20
> >  [<ffffffff8106410d>] smp_call_function_single+0xbd/0x140
> >  [<ffffffffa0080af6>] vmx_vcpu_load+0x46/0x170 [kvm_intel]
> >  [<ffffffffa004dd94>] kvm_arch_vcpu_load+0x24/0x60 [kvm]
> >  [<ffffffffa0047a8d>] kvm_sched_in+0xd/0x10 [kvm]
> >  [<ffffffff8102de37>] finish_task_switch+0x67/0xc0
> >  [<ffffffff814699f8>] schedule+0x2f8/0x9c0
> 
> > 
> > It is a regression against 2009-11-13-19-59.
> > 
> > Any ideas?
> 
> Looks like kvm is trying to send an IPI from the preempt notifiers,
> which are called with IRQs disabled, not a sane thing to do.
> 
> If they really want that, they'll have to use a pre-allocated struct
> call_single_data and use __smp_call_function_single.

Hmm, commit 498657a moved the fire_sched_in_preempt_notifiers() call
into the irqs disabled section recently.

    sched, kvm: Fix race condition involving sched_in_preempt_notifers
    
    In finish_task_switch(), fire_sched_in_preempt_notifiers() is
    called after finish_lock_switch().
    
    However, depending on architecture, preemption can be enabled after
    finish_lock_switch() which breaks the semantics of preempt
    notifiers.
    
    So move it before finish_arch_switch(). This also makes the in-
    notifiers symmetric to out- notifiers in terms of locking - now
    both are called under rq lock.

It's not a surprise that this breaks the existing code which does the
smp function call.

Thanks,

	tglx



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