linux-kernel - Re: [PATCH] [SCSI] sr: fix multi-drive performance, remove BKL replacement

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Date:	Tue, 28 Feb 2012 16:42:16 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Jens Axboe <axboe@...nel.dk>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [SCSI] sr: fix multi-drive performance, remove BKL replacement

On Tuesday 28 February 2012, James Bottomley wrote:
> On Tue, 2012-02-28 at 17:09 +0100, Stefan Richter wrote:
> > On Feb 28 James Bottomley wrote:
> > 
> > While I do remove sr_mutex aroud scsi_cd_get/put() calls, these ones
> > internally use another lock: sr_ref_mutex.  Always did, still do, since
> > neither Arnd's mechanical BKL pushdown and BKL-to-mutex conversions
> > patches nor my patch changed that.  This sr_ref_mutex also protects sr's
> > reference counting outside of the three block_device_operations methods
> > which I changed.
> > 
> > I suppose I could have mentioned right away in the changelog that the
> > sr driver's own reference counting serialization remains in place, via that
> > other mutex.
> 
> OK, agreed ... the thing that caught my eye was the get/open and the
> release/put, but I think that's completely safe.

I took another look and I believe the cdi->use_count in
cdrom_open/cdrom_release still requires some protection that is
currently provided by sr_mutex. Some parts of cdrom_ioctl also
access this variable and things like cdi->options or cdi->keeplocked.

I could imagine that you can get rid of the mutex if you turn those
into atomics and bitops, but there may be other parts of cdrom_device_info
that need locking. A safer option to solve the performance problems
could be to replace sr_mutex with a per-device mutex inside of
cdrom_device_info.

	Arnd
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