Subject: Re: [PATCH v4 1/2] SCSI: Asynchronous event
	notification	infrastructure
From: James Bottomley <James.Bottomley@SteelEye.com>
To: Jeff Garzik <jeff@garzik.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
       Linux-SCSI <linux-scsi@vger.kernel.org>, akpm@linux-foundation.org
In-Reply-To: <47260546.9090508@garzik.org>
References: <15624bab8dc0206e384ac8314257a900e60127c1.1193668176.git.jeff@garzik.org>
	 <20071029144208.676251F8168@havoc.gtf.org>
	 <1193673088.3383.34.camel@localhost.localdomain>
	 <47260546.9090508@garzik.org>
Content-Type: text/plain
Date: Mon, 29 Oct 2007 11:17:07 -0500
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On Mon, 2007-10-29 at 12:07 -0400, Jeff Garzik wrote:
> James Bottomley wrote:
> > This still doesn't solve the fundamental corruption problem:
> > sdev->event_work has to contain the work entry until the workqueue has
> > finished executing it (which is some unspecified time in the future).
> > As soon as you drop the sdev->list_lock, the system thinks
> > sdev->event_work is available for reuse.  If we fire another event
> > before the work queue finished processing the prior event, the queue
> > will be corrupted.
> 
> I think you're misunderstanding the workqueue code?  You can call 
> schedule_work(&sdev->event_work) from anywhere, any time you like, as 
> many times as you like.

OK, take me through it slowly then ... I think schedule_work(work)
inserts work->entry onto the workqueue list (in
workqueue.c:insert_work()).  If the event hasn't fired, it will already
be on the list, so adding the same entry to a list twice causes a list
corruption problem.

Plus, unfortunately, the CC/UA events are going to have to carry extra
sense data; they're not simply going to be triggers saying something
happened.

James


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