From: Uros Bizjak ubizjak@gmail.com
commit 20195d011c840b01fa91a85ebcd099ca95fbf8fc upstream.
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 Signed-off-by: Catherine Hoang catherine.hoang@oracle.com Acked-by: Darrick J. Wong djwong@kernel.org --- 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 67a99d94701e..d152d0945dab 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);