linux-kernel - Re: Strange block/scsi/workqueue issue

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Date:	Tue, 12 Apr 2011 11:51:45 +0900
From:	Tejun Heo <tj@...nel.org>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Steven Whitehouse <swhiteho@...hat.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <jaxboe@...ionio.com>
Subject: Re: Strange block/scsi/workqueue issue

Hello, James.

On Mon, Apr 11, 2011 at 07:47:56PM -0500, James Bottomley wrote:
> Actually, I don't think it's anything to do with the user process stuff.
> The problem seems to be that the block delay function ends up being the
> last user of the SCSI device, so it does the final put of the sdev when
> it's finished processing.  This will trigger queue destruction
> (blk_cleanup_queue) and so on with your analysis.

Hmm... this I can understand.

> The problem seems to be that with the new workqueue changes, the queue
> itself may no longer be the last holder of a reference on the sdev
> because the queue destruction is in the sdev release function and a
> queue cannot now be destroyed from its own delayed work.  This is a bit
> contrary to the principles SCSI was using, which was that we drive queue
> lifetime from the sdev, not vice versa.

But confused here.  Why does it make any difference whether the
release operation is in the request_fn context or not?  What makes
SCSI refcounting different from others?

Thanks.

-- 
tejun
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