On 11/20/24 9:06 AM, Uros Bizjak wrote:
try_cmpxchg() loop with constant "new" value can be substituted
with just xchg() to atomically get and clear the location.
You're right. With a constant new value (0), there is no need
to loop to ensure we get a "stable" update.
Is the READ_ONCE() is still needed?
-Alex
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@xxxxxxxxx>
Cc: Chandan Babu R <chandan.babu@xxxxxxxxxx>
Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Dave Chinner <dchinner@xxxxxxxxxx>
---
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);
}
