| Message ID | 20221020155104.9485-1-ubizjak@gmail.com (mailing list archive) |
|---|---|
| State | Accepted, archived |
| Headers | show |
| Series | [v2] raid5-cache: use try_cmpxchg in r5l_wake_reclaim | expand |
On Thu, Oct 20, 2022 at 8:51 AM Uros Bizjak <ubizjak@gmail.com> 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 <song@kernel.org> > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> 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 >
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); }
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 <song@kernel.org> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> --- v2: Fix garbled subject line --- drivers/md/raid5-cache.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)