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

    > 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).
    Stefan Richter
    -=====-===-- --=- ===--

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