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Subject[PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate code
Date
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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