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Date: Wed, 24 Jul 2002 15:25:29 +0200
From: Jens Axboe <axboe@suse.de>
To: Marcin Dalecki <dalecki@evision.ag>
Cc: Bartlomiej Zolnierkiewicz <B.Zolnierkiewicz@elka.pw.edu.pl>,
       martin@dalecki.de, linux-kernel@vger.kernel.org
Subject: Re: please DON'T run 2.5.27 with IDE!
Message-ID: <20020724132529.GD15201@suse.de>
References: <20020724124124.GA15201@suse.de> <Pine.SOL.4.30.0207241446130.15605-100000@mion.elka.pw.edu.pl> <20020724125037.GB15201@suse.de> <3D3EA6E9.7000601@evision.ag>
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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.

-- 
Jens Axboe

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