Re: single_task_running() vs. preemption warnings (was Re: [PATCH] kvm: fix preemption warnings in kvm_vcpu_block)

[Tim Chen]

On Thu, 2015-09-17 at 19:07 +0200, Dominik Dingel wrote:
> On Thu, 17 Sep 2015 18:45:00 +0200
> Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> > 
> > 
> > On 17/09/2015 18:27, Dominik Dingel wrote:
> > > +			preempt_disable();
> > > +			solo = single_task_running();
> > > +			preempt_enable();
> > > +
> > >  			cur = ktime_get();
> > > -		} while (single_task_running() && ktime_before(cur, stop));
> > 
> > That's the obvious way to fix it, but the TOCTTOU problem (which was in
> > the buggy code too) is obvious too. :)  And the only other user of
> > single_task_running() in drivers/crypto/mcryptd.c has the same issue.
> 
> Right, worst thing we fly another round.
> 
> I am not sure about the case for mcryptd.c. I think it might be that the worker
> there is bounded to one cpu and will not be migrated.
> 
> I really need to look more in the details what is happening with that worker.
> 
> > In fact, because of the way the function is used ("maybe I can do a
> > little bit of work before going to sleep") it will likely be called many
> > times in a loop.  This in turn means that:
> > 
> > - any wrong result due to a concurrent process migration would be
> > rectified very soon
> > 
> > - preempt_disable()/preempt_enable() can actually be just as expensive
> > or more expensive than single_task_running() itself.
> > 
> > Therefore, I wonder if single_task_running() should just use
> > raw_smp_processor_id().  At least the TOCTTOU issue can be clearly
> > documented in the function comment, instead of being hidden behind each
> > of the callers.
> 
> Yes to be useful it should probably call raw_smp_processor_id,
> and as a lot of code actually already does just does that I do not really see much
> down sides.
> 
> @Tim, would it be okay if I change single_task_running and add a specific comment on top?

I have no objection to change single_task_running to use
raw_smp_processor_id.  The worker in mcryptd is bound to
the cpu so it has no migration/preemption issue.  So it shouldn't care
which smp_processor_id version is being used.  Yes, please add a comment
to alert the user of this caveat should you change single_task_running.

Thanks.

Tim


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