Re: AS spin lock bugs

[Nick Piggin]

Jens Axboe wrote:

> 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 :)

Yeah thats was a bit silly of me I see why you got confused. I
have actually fixed this up in mm3. So it should get through to
Linus sometime after 2.6.0.


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