linux-kernel - Re: [RFC PATCH 1/8] rqbased-dm: allow blk_get_request() to be called from interrupt context

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Date:	Fri, 22 Dec 2006 18:32:22 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>,
	Kiyoshi Ueda <k-ueda@...jp.nec.com>, agk@...hat.com,
	mchristi@...hat.com, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, j-nomura@...jp.nec.com
Subject: Re: [RFC PATCH 1/8] rqbased-dm: allow blk_get_request() to be called from interrupt context

On Fri, Dec 22 2006, Christoph Hellwig wrote:
> On Wed, Dec 20, 2006 at 12:50:02PM -0500, Kiyoshi Ueda wrote:
> > Because I'd like to use blk_get_request() in q->request_fn()
> > which can be called from interrupt context like below:
> >   scsi_io_completion -> scsi_end_request -> scsi_next_command
> >   -> scsi_run_queue -> blk_run_queue -> q->request_fn
> 
> > Or request should not be allocated in q->request_fn() anyway?
> > Do you have any other ideas?
> 
> The right long-term fix is to make sure request_fn is only ever called
> from process context.  This means moving retry handling to a thread instead
> of a softirq, but this will need careful performance testing and tweaking.

It's a lot more than just retry handling. Most driver reinvoke queueing
from the completion handling, so basically most of the time request_fn
is run from either softirq (like SCSI) or interrupt context (like IDE).

-- 
Jens Axboe

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