If I sit in a loop and do write()s to small tmpfs files,
__sb_end_write() is third-hottest kernel function due to its
smp_mb().
The stated purpose for the smp_mb() in __sb_end_write() is to
ensure "s_writers are updated before we wake up waiters". We
only wake up waiters if waitqueue_active(), but we do the
smp_mb() unconditionally.
It seems like we should be able to avoid it unless we are
actually doing the wake_up().
Cc: Jan Kara <jack@xxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
b/fs/super.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff -puN fs/super.c~selectively-do-barriers-in-__sb_end_write fs/super.c
--- a/fs/super.c~selectively-do-barriers-in-__sb_end_write 2015-06-19 15:20:37.953726659 -0700
+++ b/fs/super.c 2015-06-19 15:20:37.956726794 -0700
@@ -1147,13 +1147,14 @@ out:
void __sb_end_write(struct super_block *sb, int level)
{
percpu_counter_dec(&sb->s_writers.counter[level-1]);
- /*
- * Make sure s_writers are updated before we wake up waiters in
- * freeze_super().
- */
- smp_mb();
- if (waitqueue_active(&sb->s_writers.wait))
+ if (waitqueue_active(&sb->s_writers.wait)) {
+ /*
+ * Make sure other CPUs can see our s_writers update
+ * before we wake up waiters in freeze_super().
+ */
+ smp_mb();
wake_up(&sb->s_writers.wait);
+ }
rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_);
}
EXPORT_SYMBOL(__sb_end_write);
_
--
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