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@xxxxxxxxx>
Cc: Chandan Babu R <chandan.babu@xxxxxxxxxx>
Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
---
v2: Move cilpcp variable into the loop scope. Initialize cilcpc and
old variables at the declaration time. Use alternative form of
the while loop.
---
fs/xfs/xfs_log_cil.c | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)
diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 391a938d690c..d4e06e6f050f 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -156,9 +156,8 @@ xlog_cil_insert_pcp_aggregate(
struct xfs_cil *cil,
struct xfs_cil_ctx *ctx)
{
- struct xlog_cil_pcp *cilpcp;
- int cpu;
- int count = 0;
+ int cpu;
+ int count = 0;
/* Trigger atomic updates then aggregate only for the first caller */
if (!test_and_clear_bit(XLOG_CIL_PCP_SPACE, &cil->xc_flags))
@@ -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);
--
2.42.0
