Re: AS spin lock bugs

[Jens Axboe <axboe@suse.de>]

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