From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@qumranet.com>
Subject: Re: KVM: kvm_vcpu_block task state race
Date: Fri, 09 May 2008 10:40:47 +0300
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References: <20080508224701.GA6175@dmt>
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Cc: kvm-devel <kvm-devel@lists.sourceforge.net>
To: Marcelo Tosatti <mtosatti@redhat.com>
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Marcelo Tosatti wrote:
> There's still a race in kvm_vcpu_block(), if a wake_up_interruptible()
> call happens before the task state is set to TASK_INTERRUPTIBLE:
>
> CPU0                            CPU1
>
> kvm_vcpu_block                  
>
> add_wait_queue
>                                 
> kvm_cpu_has_interrupt = 0
>                                 set interrupt
>                                 if (waitqueue_active())
>                                         wake_up_interruptible()
>
> kvm_cpu_has_pending_timer
> kvm_arch_vcpu_runnable
> signal_pending 
>
> set_current_state(TASK_INTERRUPTIBLE)
> schedule()
>
> Can be fixed by using prepare_to_wait() which sets the task state before
> testing for the wait condition.
>
> Unfortunately it can't use wait_event_interruptible() due to
> vcpu_put/vcpu_load.
>
>   

schedule() will call vcpu_put()/vcpu_load() for us through preempt 
notifiers.  I feel a little uneasy about it, but no concreate reason why 
not to rely on it.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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