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SubjectRe: [PATCH 2/2] irq_work: Fix racy IRQ_WORK_BUSY flag setting
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On Tue, 2012-10-30 at 14:45 -0400, Steven Rostedt wrote:
> On Wed, 2012-10-31 at 03:33 +0900, anish kumar wrote:
>
> > > CPU 0 CPU 1
> > >
> > > data = something flags = IRQ_WORK_BUSY
> > > smp_mb() (implicit with cmpxchg smp_mb()
> > > on flags in claim) execute_work (sees data from CPU
> > > 0)
> > > try to claim
> > >
> > As I understand without the memory barrier proposed by you the situation
> > would be as below:
> > CPU 0 CPU 1
> >
> > data = something flags = IRQ_WORK_BUSY
> > smp_mb() (implicit with cmpxchg execute_work (sees data from CPU 0)
> > on flags in claim)
> > _success_ in claiming and goes
>
> Correct, because it would see the stale value of flags.
>
> > ahead and execute the work(wrong?)
> > cmpxchg cause flag to IRQ_WORK_BUSY
> >
> > Now knows the flag==IRQ_WORK_BUSY
> >
> > Am I right?
>
> right.
>
> >
> > Probably a stupid question.Why do we return the bool from irq_work_queue
> > when no one bothers to check the return value?Wouldn't it be better if
> > this function is void as used by the users of this function or am I
> > looking at the wrong code.
>
> Not a stupid question, as I was asking that to myself just earlier
> today. But forgot to mention it as well. Especially, because it makes it
> look like there's a bug in the code. Maybe someday someone will care if
> their work was finished by itself, or some other CPU.
>
> Probably should just nix the return value.
Can I send a patch to fix this?
>
> -- Steve
>
>




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