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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2012-02-28 at 17:09 +0100, 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.

OK, agreed ... the thing that caught my eye was the get/open and the
release/put, but I think that's completely safe.

> > The sr.c use case looks like bd_mutex would mediate ... but that's
> > because it doesn't use driver shutdown and has no power management
> > functions ... I think I have vague memories that someone is working on
> > pm for cdroms?
> > 
> > I don't think the mutex needs to be on the ioctls, though, which is
> > what's causing your performance problems, right?
> 
> I guess sr_block_open/release are less of an issue; after all they are
> still partly serialized across all sr devices (the sections which are
> under the mentioned sr_ref_mutex protection).

They're also per bdev serialised by bd_mutex, so yes.

James