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?
> Thanks,
>
> Paolo
>
--
To unsubscribe from this list: send the line "unsubscribe linux-crypto" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
