Re: [PATCH] sched,kvm: Fix KVM preempt_notifier usage

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/07/2015 17:42, Peter Zijlstra wrote:
> On Fri, Jul 03, 2015 at 05:38:42PM +0200, Paolo Bonzini wrote:
>> So basically this.  Can you reply with SoB and maybe Acked-by?
> 
> Ah, thanks for doing that!
> 
>> ------------- 8< ---------------
>> From: Peter Zijlstra <peterz@infradead.org>
>> Subject: [PATCH] sched, preempt_notifier: separate notifier registration from static_key inc/dec 
>>
>> Commit 1cde2930e154 ("sched/preempt: Add static_key() to preempt_notifiers")
>> had two problems.  First, the preempt-notifier API needs to sleep with the
>> addition of the static_key, we do however need to hold off preemption
>> while modifying the preempt notifier list, otherwise a preemption could
>> observe an inconsistent list state.  KVM correctly registers and
>> unregisters preempt notifiers with preemption disabled, so the sleep
>> caused dmesg splats.
>>
>> Second, KVM registers and unregisters preemption notifiers very often
>> (in vcpu_load/vcpu_put).  With a single uniprocessor guest the static key
>> would move between 0 and 1 continuously, hitting the slow path on every
>> userspace exit.
>>
>> To fix this, wrap the static_key inc/dec in a new API, and call it from
>> KVM.
>>
>> Fixes: 1cde2930e154 ("sched/preempt: Add static_key() to preempt_notifiers")
>> Reported-by: Pontus Fuchs <pontus.fuchs@gmail.com>
>> Reported-by: Takashi Iwai <tiwai@suse.de>
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Ok, I'm crossing fingers and including this in my pull request in order
to preserve bisectability.  Thanks.

Paolo