Re: [PATCH 2/2] futex: Use atomic64_try_cmpxchg_relaxed() in get_inode_sequence_number()

From: André Almeida
Date: Thu Oct 10 2024 - 14:09:22 EST


Em 10/10/2024 04:10, Uros Bizjak escreveu:
Optimize get_inode_sequence_number() to use simpler and faster:

!atomic64_try_cmpxchg_relaxed(*ptr, &old, new)

instead of:

atomic64_cmpxchg relaxed(*ptr, old, new) != old

The x86 CMPXCHG instruction returns success in ZF flag, so
this change saves a compare after cmpxchg. The generated
code improves from:

3da: 31 c0 xor %eax,%eax
3dc: f0 48 0f b1 8a 38 01 lock cmpxchg %rcx,0x138(%rdx)
3e3: 00 00
3e5: 48 85 c0 test %rax,%rax
3e8: 48 0f 44 c1 cmove %rcx,%rax

to:

3da: 31 c0 xor %eax,%eax
3dc: f0 48 0f b1 8a 38 01 lock cmpxchg %rcx,0x138(%rdx)
3e3: 00 00
3e5: 48 0f 44 c1 cmove %rcx,%rax

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: "André Almeida" <andrealmeid@xxxxxxxxxx>

Reviewed-by: André Almeida <andrealmeid@xxxxxxxxxx>