Messages in this thread | | | Date | Thu, 17 Sep 2015 19:07:52 +0200 | From | Dominik Dingel <> | Subject | Re: single_task_running() vs. preemption warnings (was Re: [PATCH] kvm: fix preemption warnings in kvm_vcpu_block) |
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On Thu, 17 Sep 2015 18:45:00 +0200 Paolo Bonzini <pbonzini@redhat.com> 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 >
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