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SubjectRe: AS spin lock bugs
On Thu, Nov 13 2003, Jens Axboe wrote:
> 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.

Ah, it would be clearer as:

if (arq->is_sync == REQ_SYNC) {
spin_lock(&aic->lock);
set_bit(AS_TASK_IORUNNING, &aic->state);
aic->last_end_request = jiffies;
spin_unlock(&aic->lock);
}

Then it doesn't need comments :)

--
Jens Axboe

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