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    Subject[PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate code
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    From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

    Hi all,

    I'm proposing this series for 3[.0].1.

    This is a repost of a series to clean up the x86 ticket lock
    code by converting it to a mostly C implementation and removing
    lots of duplicate code relating to the ticket size.

    The last time I posted this series, the only significant comments
    were from Nick Piggin, specifically relating to:

    1. A wrongly placed barrier on unlock (which may have allowed the
    compiler to move things out of the locked region. I went
    belt-and-suspenders by having two barriers to prevent motion
    into or out of the locked region.

    2. With NR_CPUS < 256 the ticket size is 8 bits. The compiler doesn't
    use the same trick as the hand-coded asm to directly compare the high
    and low bytes in the word, but does a bit of extra shuffling around.
    However, the Intel optimisation guide and several x86 experts have
    opined that its best to avoid the high-byte operations anyway, since
    they will cause a partial word stall, and the gcc-generated code should
    be better.

    Overall the compiler-generated code is very similar to the hand-coded
    versions, with the partial byte operations being the only significant
    difference. (Curiously, gcc does generate a high-byte compare for me
    in trylock, so it can if it wants to.)

    I've been running with this code in place for several months on 4 core
    systems without any problems.

    I couldn't measure a consistent performance difference between the two
    implemenations; there seemed to be +/- ~1% +/-, which is the level of
    variation I see from simply recompiling the kernel with slightly
    different code alignment.

    Overall, I think the large reduction in code size is a big win.

    Thanks,
    J

    Jeremy Fitzhardinge (7):
    x86/ticketlock: clean up types and accessors
    x86/ticketlock: convert spin loop to C
    x86/ticketlock: Use C for __ticket_spin_unlock
    x86/ticketlock: make large and small ticket versions of spin_lock the
    same
    x86/ticketlock: make __ticket_spin_lock common
    x86/ticketlock: make __ticket_spin_trylock common
    x86/ticketlock: prevent memory accesses from reordered out of lock
    region

    arch/x86/include/asm/spinlock.h | 147 ++++++++++++---------------------
    arch/x86/include/asm/spinlock_types.h | 22 +++++-
    2 files changed, 74 insertions(+), 95 deletions(-)

    --
    1.7.5.4



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