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Wong" , Christoph Hellwig Subject: [PATCH v2] xfs: Use try_cmpxchg() in xlog_cil_insert_pcp_aggregate() Date: Sun, 22 Sep 2024 19:07:16 +0200 Message-ID: <20240922170746.11361-1-ubizjak@gmail.com> X-Mailer: git-send-email 2.42.0 Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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 Cc: Chandan Babu R Cc: "Darrick J. Wong" Cc: Christoph Hellwig Reviewed-by: Christoph Hellwig --- 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);