linux-kernel - [PATCH] xfs: Use xchg() in xlog_cil_insert_pcp

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20241120150725.3378-1-ubizjak@gmail.com>
Date: Wed, 20 Nov 2024 16:06:22 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: linux-xfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Chandan Babu R <chandan.babu@...cle.com>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <dchinner@...hat.com>
Subject: [PATCH] xfs: Use xchg() in xlog_cil_insert_pcp_aggregate()

try_cmpxchg() loop with constant "new" value can be substituted
with just xchg() to atomically get and clear the location.

The code on x86_64 improves from:

    1e7f:	48 89 4c 24 10       	mov    %rcx,0x10(%rsp)
    1e84:	48 03 14 c5 00 00 00 	add    0x0(,%rax,8),%rdx
    1e8b:	00
			1e88: R_X86_64_32S	__per_cpu_offset
    1e8c:	8b 02                	mov    (%rdx),%eax
    1e8e:	41 89 c5             	mov    %eax,%r13d
    1e91:	31 c9                	xor    %ecx,%ecx
    1e93:	f0 0f b1 0a          	lock cmpxchg %ecx,(%rdx)
    1e97:	75 f5                	jne    1e8e <xlog_cil_commit+0x84e>
    1e99:	48 8b 4c 24 10       	mov    0x10(%rsp),%rcx
    1e9e:	45 01 e9             	add    %r13d,%r9d

to just:

    1e7f:	48 03 14 cd 00 00 00 	add    0x0(,%rcx,8),%rdx
    1e86:	00
			1e83: R_X86_64_32S	__per_cpu_offset
    1e87:	31 c9                	xor    %ecx,%ecx
    1e89:	87 0a                	xchg   %ecx,(%rdx)
    1e8b:	41 01 cb             	add    %ecx,%r11d

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Chandan Babu R <chandan.babu@...cle.com>
Cc: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>
Cc: Dave Chinner <dchinner@...hat.com>
---
 fs/xfs/xfs_log_cil.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index 80da0cf87d7a..9d667be1d909 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -171,11 +171,8 @@ xlog_cil_insert_pcp_aggregate(
 	 */
 	for_each_cpu(cpu, &ctx->cil_pcpmask) {
 		struct xlog_cil_pcp	*cilpcp = per_cpu_ptr(cil->xc_pcp, cpu);
-		int			old = READ_ONCE(cilpcp->space_used);
 
-		while (!try_cmpxchg(&cilpcp->space_used, &old, 0))
-			;
-		count += old;
+		count += xchg(&cilpcp->space_used, 0);
 	}
 	atomic_add(count, &ctx->space_used);
 }
-- 
2.42.0