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

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Date:	Tue, 28 Feb 2012 10:57:37 -0600
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Arnd Bergmann <arnd@...db.de>
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 Tue, 2012-02-28 at 16:42 +0000, Arnd Bergmann wrote:
> 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.

So I think this is fine ... it's protected by the bdev->bd_mutex.

>  Some parts of cdrom_ioctl also
> access this variable and things like cdi->options or cdi->keeplocked.

This would be problematic because we no longer lock the ioctl.

> 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.

I'd say the latter.

James


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