Commit 20195d01 authored by Uros Bizjak's avatar Uros Bizjak Committed by Carlos Maiolino
Browse files

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: default avatarUros Bizjak <ubizjak@gmail.com>
Reviewed-by: default avatarChristoph Hellwig <hch@infradead.org>
Cc: Chandan Babu R <chandan.babu@oracle.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: default avatarDave Chinner <dchinner@redhat.com>
Signed-off-by: default avatarCarlos Maiolino <cem@kernel.org>
parent 6148b779

fs/xfs/xfs_log_cil.c

+4 −7
Original line number Diff line number Diff line
@@ -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);