xfs: Use try_cmpxchg() in xlog_cil_insert_pcp_aggregate()

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>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 391a938..80da0cf 100644 (file)
--- 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);