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SubjectRe: AS spin lock bugs
On Thu, Nov 13 2003, Nick Piggin wrote:
>
>
> Jens Axboe wrote:
>
> >On Thu, Nov 13 2003, Jens Axboe wrote:
> >
> >>@@ -959,12 +960,12 @@
> >> if (!aic)
> >> return;
> >>
> >>- spin_lock(&aic->lock);
> >>+ spin_lock_irqsave(&aic->lock, flags);
> >> if (arq->is_sync == REQ_SYNC) {
> >> set_bit(AS_TASK_IORUNNING, &aic->state);
> >> aic->last_end_request = jiffies;
> >> }
> >>- spin_unlock(&aic->lock);
> >>+ spin_unlock_irqrestore(&aic->lock, flags);
> >>
> >> put_io_context(arq->io_context);
> >>}
> >>
> >
> >BTW, this looks bogus. Why do you need any locking there?
> >
>
> To prevent a request completion on another queue on another CPU from
> racing with request insertion: last_end_request is undefined if the
> flag is not set. I guess you could flip the statements and put a
> smp_mb between them. Probably not worth the trouble though.

No better to make it explicit, probably doesn't matter much in
real-life. Thanks for the clarifications.

--
Jens Axboe

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