On Wed, Nov 20, 2024 at 04:06:22PM +0100, Uros Bizjak wrote:
> try_cmpxchg() loop with constant "new" value can be substituted
> with just xchg() to atomically get and clear the location.
>
> The code on x86_64 improves from:
>
> 1e7f: 48 89 4c 24 10 mov %rcx,0x10(%rsp)
> 1e84: 48 03 14 c5 00 00 00 add 0x0(,%rax,8),%rdx
> 1e8b: 00
> 1e88: R_X86_64_32S __per_cpu_offset
> 1e8c: 8b 02 mov (%rdx),%eax
> 1e8e: 41 89 c5 mov %eax,%r13d
> 1e91: 31 c9 xor %ecx,%ecx
> 1e93: f0 0f b1 0a lock cmpxchg %ecx,(%rdx)
> 1e97: 75 f5 jne 1e8e <xlog_cil_commit+0x84e>
> 1e99: 48 8b 4c 24 10 mov 0x10(%rsp),%rcx
> 1e9e: 45 01 e9 add %r13d,%r9d
>
> to just:
>
> 1e7f: 48 03 14 cd 00 00 00 add 0x0(,%rcx,8),%rdx
> 1e86: 00
> 1e83: R_X86_64_32S __per_cpu_offset
> 1e87: 31 c9 xor %ecx,%ecx
> 1e89: 87 0a xchg %ecx,(%rdx)
> 1e8b: 41 01 cb add %ecx,%r11d
>
> No functional change intended.
>
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> Cc: Chandan Babu R <chandan.babu@xxxxxxxxxx>
> Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Cc: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/xfs_log_cil.c | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 80da0cf87d7a..9d667be1d909 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -171,11 +171,8 @@ xlog_cil_insert_pcp_aggregate(
> */
> for_each_cpu(cpu, &ctx->cil_pcpmask) {
> struct xlog_cil_pcp *cilpcp = per_cpu_ptr(cil->xc_pcp, cpu);
> - int old = READ_ONCE(cilpcp->space_used);
>
> - while (!try_cmpxchg(&cilpcp->space_used, &old, 0))
> - ;
> - count += old;
> + count += xchg(&cilpcp->space_used, 0);
> }
> atomic_add(count, &ctx->space_used);
> }
Looks fine.
Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
--
Dave Chinner
david@xxxxxxxxxxxxx
