RE: [PATCH] atomics: Use atomic_try_cmpxchg_release in rcuref_put_slowpath()

[David Laight]

From: Uros Bizjak
> Sent: 09 May 2023 16:03
> 
> Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
> in rcuref_put_slowpath(). 86 CMPXCHG instruction returns success in
> ZF flag, so this change saves a compare after cmpxchg.  Additionaly,
> the compiler reorders some code blocks to follow likely/unlikely
> annotations in the atomic_try_cmpxchg macro, improving the code from
> 
>   9a:	f0 0f b1 0b          	lock cmpxchg %ecx,(%rbx)
>   9e:	83 f8 ff             	cmp    $0xffffffff,%eax
>   a1:	74 04                	je     a7 <rcuref_put_slowpath+0x27>
>   a3:	31 c0                	xor    %eax,%eax
> 
> to
> 
>   9a:	f0 0f b1 0b          	lock cmpxchg %ecx,(%rbx)
>   9e:	75 4c                	jne    ec <rcuref_put_slowpath+0x6c>
>   a0:	b0 01                	mov    $0x1,%al
> 
> No functional change intended.

While I'm not against the change I bet you can't detect
any actual difference. IIRC:
- The 'cmp+je' get merged into a single u-op.
- The 'lock cmpxchg' will take long enough that the instruction
  decoder won't be a bottleneck.
- Whether the je/jne is predicted taken is pretty much random.
  So you'll speculatively execute somewhere (could be anywhere)
  while the locked cycle completes.
So the only change is three less bytes of object code.
That will change the cache line alignment of later code.

	David

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