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 <[email protected]> Cc: Alexander Viro <[email protected]> Cc: [email protected] Cc: [email protected] Cc: Paul E. McKenney <[email protected]> Cc: Tim Chen <[email protected]> Cc: Andi Kleen <[email protected]> --- 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); _ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/
