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

From: Jeremy Fitzhardinge
Date: Mon Jan 24 2011 - 20:42:31 EST

On 01/24/2011 05:16 PM, Nick Piggin wrote:
> On Tue, Jan 25, 2011 at 10:41 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
>> Make trylock code common regardless of ticket size.
> What's the asm for this look like?


movzwl (%rdi), %eax # lock_1(D)->slock, tmp
cmpb %ah,%al # tmp
leal 0x100(%rax), %edx # tmp, new
jne 1f
lock; cmpxchgw %dx,(%rdi) # new, lock_1(D)->slock
1: sete %dl # new
movzbl %dl,%eax # new, tmp


movw (%rdi), %dx # lock_2(D)->, old
xorl %eax, %eax # D.13954
movzbl %dh, %ecx # old, tmp70
cmpb %dl, %cl # old, tmp70
jne .L5 #,
leal 256(%rdx), %ecx #, D.13956
movl %edx, %eax # old, __ret
lock; cmpxchgw %cx,(%rdi) # D.13956,* lock
cmpw %dx, %ax # old, __ret
sete %al #, D.13954
movzbl %al, %eax # D.13954, D.13954

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.

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

How performance critical is trylock? I guess the ones in fs/dcache.c
are the ones looming large in your mind.

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