Re: [PATCH 6/6] x86/ticketlock: make __ticket_spin_trylock common

From: Jeremy Fitzhardinge
Date: Thu Jan 27 2011 - 18:54:07 EST

On 01/24/2011 05:58 PM, Nick Piggin wrote:
>> The C version can't take advantage of the fact that the cmpxchg directly
>> sets the flags, so it ends up re-comparing the old and swapped-out
>> values to set the return. And it doesn't re-use the same sete to set
>> the return value in the quick failed-to-acquire path.
> Hm.

Adding a "cmpxchg_flag" which does its own sete and returns a boolean
"success" flag, the whole thing goes to:

movw (%rdi), %ax # lock_2(D)->, old
xorl %edx, %edx # D.13949
movzbl %ah, %ecx # old, tmp69
cmpb %al, %cl # old, tmp69
jne .L5 #,
leal 256(%rax), %edx #, D.13951
lock; cmpxchgw %dx,(%rdi); sete %al # D.13951,* lock, __ret
movzbl %al, %edx # __ret, D.13949
movl %edx, %eax # D.13949,

The eax/edx shuffle is a bit unfortunate I can't see it hurting very much.

>> It might be worth having a generic cmpxchg() variant which returns a
>> succeed/fail flag rather than the fetched value, to avoid comparison in
>> this case - since many (most?) cmpxchg() callers end up doing that
>> comparison.
>> How performance critical is trylock? I guess the ones in fs/dcache.c
>> are the ones looming large in your mind.
> Well they are on on the reclaim/free path rather than the _hottest_
> paths, but yes they are performance critical.

I think that code looks pretty reasonable.


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