Use !try_cmpxchg instead of cmpxchg (*ptr, old, new) != old in
xlog_cil_insert_pcp_aggregate().  x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg.

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when
cmpxchg fails. There is no need to re-read the value in the loop.

Note that the value from *ptr should be read using READ_ONCE to
prevent the compiler from merging, refetching or reordering the read.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Cc: Chandan Babu R <chandan.babu@oracle.com>
Cc: "Darrick J. Wong" <djwong@kernel.org>
---
v2: Move cilpcp variable into the loop scope. Initialize cilcpc and
    old variables at the declaration time. Use alternative form of
    the while loop.
v3: Undo reformatting of variable declarations
---
 fs/xfs/xfs_log_cil.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 391a938d690c..80da0cf87d7a 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -156,7 +156,6 @@ xlog_cil_insert_pcp_aggregate(
 	struct xfs_cil		*cil,
 	struct xfs_cil_ctx	*ctx)
 {
-	struct xlog_cil_pcp	*cilpcp;
 	int			cpu;
 	int			count = 0;
 
@@ -171,13 +170,11 @@ xlog_cil_insert_pcp_aggregate(
 	 * structures that could have a nonzero space_used.
 	 */
 	for_each_cpu(cpu, &ctx->cil_pcpmask) {
-		int	old, prev;
+		struct xlog_cil_pcp	*cilpcp = per_cpu_ptr(cil->xc_pcp, cpu);
+		int			old = READ_ONCE(cilpcp->space_used);
 
-		cilpcp = per_cpu_ptr(cil->xc_pcp, cpu);
-		do {
-			old = cilpcp->space_used;
-			prev = cmpxchg(&cilpcp->space_used, old, 0);
-		} while (old != prev);
+		while (!try_cmpxchg(&cilpcp->space_used, &old, 0))
+			;
 		count += old;
 	}
 	atomic_add(count, &ctx->space_used);
-- 
2.46.1

On Mon, 23 Sep 2024 14:22:17 +0200, Uros Bizjak wrote:
> Use !try_cmpxchg instead of cmpxchg (*ptr, old, new) != old in
> xlog_cil_insert_pcp_aggregate().  x86 CMPXCHG instruction returns
> success in ZF flag, so this change saves a compare after cmpxchg.
> 
> Also, try_cmpxchg implicitly assigns old *ptr value to "old" when
> cmpxchg fails. There is no need to re-read the value in the loop.
> 
> [...]

Applied to xfs-6.12-rc2, thanks!

[1/1] xfs: Use try_cmpxchg() in xlog_cil_insert_pcp_aggregate()
      commit: 6e19fd40d755a64f271687a21cd304ecba41a571

Best regards,
-- 
Carlos Maiolino <cem@kernel.org>

On Mon, Sep 23, 2024 at 02:22:17PM +0200, Uros Bizjak wrote:
> Use !try_cmpxchg instead of cmpxchg (*ptr, old, new) != old in
> xlog_cil_insert_pcp_aggregate().  x86 CMPXCHG instruction returns
> success in ZF flag, so this change saves a compare after cmpxchg.
> 
> Also, try_cmpxchg implicitly assigns old *ptr value to "old" when
> cmpxchg fails. There is no need to re-read the value in the loop.
> 
> Note that the value from *ptr should be read using READ_ONCE to
> prevent the compiler from merging, refetching or reordering the read.
> 
> No functional change intended.
> 
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Reviewed-by: Christoph Hellwig <hch@infradead.org>
> Cc: Chandan Babu R <chandan.babu@oracle.com>
> Cc: "Darrick J. Wong" <djwong@kernel.org>

Looks fine.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com