2022-10-20 16:05:33

by Uros Bizjak

Subject: [PATCH v2] raid5-cache: use try_cmpxchg in r5l_wake_reclaim

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
r5l_wake_reclaim. 86 CMPXCHG instruction returns success in ZF flag, so
this change saves a compare after cmpxchg (and related move instruction in
front of 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.

Cc: Song Liu <[email protected]>
Signed-off-by: Uros Bizjak <[email protected]>
---
v2: Fix garbled subject line
---
drivers/md/raid5-cache.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 832d8566e165..a63023aae21e 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -1565,11 +1565,12 @@ void r5l_wake_reclaim(struct r5l_log *log, sector_t space)

if (!log)
return;
+
+ target = READ_ONCE(log->reclaim_target);
do {
- target = log->reclaim_target;
if (new < target)
return;
- } while (cmpxchg(&log->reclaim_target, target, new) != target);
+ } while (!try_cmpxchg(&log->reclaim_target, &target, new));
md_wakeup_thread(log->reclaim_thread);
}

--
2.37.3

2022-10-20 19:37:34

by Song Liu

Subject: Re: [PATCH v2] raid5-cache: use try_cmpxchg in r5l_wake_reclaim

On Thu, Oct 20, 2022 at 8:51 AM Uros Bizjak <[email protected]> wrote:
>
> Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
> r5l_wake_reclaim. 86 CMPXCHG instruction returns success in ZF flag, so
> this change saves a compare after cmpxchg (and related move instruction in
> front of 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.
>
> Cc: Song Liu <[email protected]>
> Signed-off-by: Uros Bizjak <[email protected]>

Applied to md-next. Thanks!

Song

> ---
> v2: Fix garbled subject line
> ---
> drivers/md/raid5-cache.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
> index 832d8566e165..a63023aae21e 100644
> --- a/drivers/md/raid5-cache.c
> +++ b/drivers/md/raid5-cache.c
> @@ -1565,11 +1565,12 @@ void r5l_wake_reclaim(struct r5l_log *log, sector_t space)
>
> if (!log)
> return;
> +
> + target = READ_ONCE(log->reclaim_target);
> do {
> - target = log->reclaim_target;
> if (new < target)
> return;
> - } while (cmpxchg(&log->reclaim_target, target, new) != target);
> + } while (!try_cmpxchg(&log->reclaim_target, &target, new));
> md_wakeup_thread(log->reclaim_thread);
> }
>
> --
> 2.37.3
>