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Date: Wed, 24 Jul 2002 15:35:38 +0200 (MET DST)
From: Bartlomiej Zolnierkiewicz <B.Zolnierkiewicz@elka.pw.edu.pl>
To: Jens Axboe <axboe@suse.de>
cc: <martin@dalecki.de>, <linux-kernel@vger.kernel.org>
Subject: Re: please DON'T run 2.5.27 with IDE!
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On Wed, 24 Jul 2002, Jens Axboe wrote:

> On Wed, Jul 24 2002, Marcin Dalecki wrote:
> > Jens Axboe wrote:
> >
> > >>>>2.5.27:drivers/block/ll_rw_blk.c
> > >>>>void blk_start_queue(request_queue_t *q)
> > >>>>{
> > >>>>       if (test_and_clear_bit(QUEUE_FLAG_STOPPED, &q->queue_flags)) {
> > >>>>               unsigned long flags;
> > >>>>
> > >>>>               spin_lock_irqsave(q->queue_lock, flags);
> > >>>>               if (!elv_queue_empty(q))
> > >>>>                       q->request_fn(q);
> > >>>>               spin_unlock_irqrestore(q->queue_lock, flags);
> > >>>>       }
> > >>>>}
> >
> > >There were buggy versions at one point, however they may not have made it
> > >into a full release. In that case it was just bk version of 2.5.19-pre
> > >effectively. I forget the details :-)
> >
> > Naj - it's far more trivial I just looked at wrong tree at hand...
> > But anyway. What happens if somone does set QUEUE_FLAG_STOPPED
> > between the test_and_claer_bit and taking the spin_lock? Setting
> > the QUEUE_FLAG_STOPPED isn't maintaining the spin_lock protection!
>
> It doesn't matter. If QUEUE_FLAG_STOPPED was set when entering
> blk_start_queue(), it will call into the request_fn. If blk_stop_queue()
> is called between clearing QUEUE_FLAG_STOPPED in blk_start_queue() and
> grabbing the spin_lock, the worst that can happen is a spurios extra
> request_fn call.
>
> > My goal is to make sure that the QUEUE_FLAG_STOPPED has a valid value
> > *inside* the q->request_fn call.
>
> So you want the queue_lock to protect the flags as well... I don't
> really see the point of this.

If driver corectly uses blk_start/stop_queue() it is not needed.
Whole point of introducing this flag was not to take lock to check
status of queue.

--
Bartlomiej

> --
> Jens Axboe



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