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.
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.
Thanks,
Paolo
On Thu, 17 Sep 2015 18:45:00 +0200
Paolo Bonzini <[email protected]> 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
>
On Thu, 2015-09-17 at 19:07 +0200, Dominik Dingel wrote:
> On Thu, 17 Sep 2015 18:45:00 +0200
> Paolo Bonzini <[email protected]> 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
On Thu, Sep 17, 2015 at 01:32:55PM -0700, Tim Chen wrote:
> 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.
We actually have raw_rq() for that, and the whole if thing looks rather
superfluous. So something like the below, except with a suitable comment
on and tested etc.. ;-)
---
kernel/sched/core.c | 5 +----
1 file changed, 1 insertion(+), 4 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6ab415aa15c4..f39c0498e284 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2666,10 +2666,7 @@ unsigned long nr_running(void)
*/
bool single_task_running(void)
{
- if (cpu_rq(smp_processor_id())->nr_running == 1)
- return true;
- else
- return false;
+ return raw_rq()->nr_running == 1;
}
EXPORT_SYMBOL(single_task_running);