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

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Date:	Tue, 28 Feb 2012 17:22:28 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Jens Axboe <axboe@...nel.dk>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] [SCSI] sr: fix multi-drive performance, remove BKL
 replacement

On Feb 28 Stefan Richter wrote:
> On Feb 28 James Bottomley wrote:
> > On Tue, 2012-02-28 at 15:32 +0100, Stefan Richter wrote:
> > > Commit 2a48fc0ab242 "block: autoconvert trivial BKL users to private
> > > mutex" and other commits at the time mechanically swapped BKL for
> > > per-driver global mutexes.  If the sr driver is any indication, these
> > > replacements have still not been checked by anybody for their
> > > necessessity, removed where possible, or the sections they serialize
> > > reduced to a necessary minimum.
> > > 
> > > The sr_mutex in particular very noticably degraded performance of
> > > CD-DA ripping with multiple drives in parallel.  When several
> > > instances of "grip" are used with two or more drives, their GUIs
> > > became laggier, as did the KDE file manager GUI, and drive utilization
> > > was reduced.  (During ripping, drive lights flicker instead of staying
> > > on most of the time.)  IOW time to rip a stack of CDs was increased.
> > > I didn't measure this but it is highly noticeable.
> > > 
> > > On the other hand, I don't see what state sr_mutex would protect.
> > > So I removed it entirely and that works fine for me.
> > > 
> > I'm afraid you can't do that:  The problem is that we have an entangled
> > set of reference counts that need to be taken and released atomically.
> > If we don't surround them with a mutex you get undefined results from
> > racing last release with new acquire.  You can see this usage in sd.c.
> 
> 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.

PS:
Entirely independently of this, it is important to remember that sr has
been and still is broken WRT hot removal/ hot unbinding of devices.
http://marc.info/?t=132484687700001
I haven't tried the last proposed patches from Jun'ichi Nomura or from Bart
Van Assche yet.  While I tested my BKL-replacement-removal patch, I made
sure to quit all processes that accessed /dev/sr*, including udisks-daemon,
every time before I unplugged a CD-ROM drive.  Or actually /almost/ every
time; sometimes I missed to kill udisks-daemon but was lucky enough not to
hit that bug in these sessions.
-- 
Stefan Richter
-=====-===-- --=- ===--
http://arcgraph.de/sr/
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