On Wed 24-06-15 17:16:06, Dave Hansen wrote:
>
> From: Dave Hansen <dave.han...@linux.intel.com>
This has nothing to do with fsnotify so just remove it from the subject
line please. Thanks!
> 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().
>
> __sb_end_write() uses the barrier to avoid races with freeze_super()
> and its calls to sb_wait_write(). But, now that freeze_super() is
> calling synchronize_rcu() before each sb_wait_write() call, we can
> use that to our advantage.
>
> The synchronize_rcu() ensures that all __sb_end_write() will see
> freeze_super()'s updates to s_writers.counter. That, in turn,
> guarantees that __sb_end_write() will try to wake up any subsequent
> call by freeze_super() to sb_wait_write().
What gains does this patch bring?
Otherwise the patch looks good to me. You can add:
Reviewed-by: Jan Kara <j...@suse.cz>
Honza
> Cc: Jan Kara <j...@suse.cz>
> Cc: Alexander Viro <v...@zeniv.linux.org.uk>
> Cc: linux-fsde...@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Cc: Tim Chen <tim.c.c...@linux.intel.com>
> Cc: Andi Kleen <a...@linux.intel.com>
> Signed-off-by: Dave Hansen <dave.han...@linux.intel.com>
> ---
>
> b/fs/super.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 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 2015-06-24
> 17:14:35.315142611 -0700
> +++ b/fs/super.c 2015-06-24 17:14:35.318142745 -0700
> @@ -1146,14 +1146,23 @@ out:
> */
> void __sb_end_write(struct super_block *sb, int level)
> {
> + rcu_read_lock();
> percpu_counter_dec(&sb->s_writers.counter[level-1]);
> /*
> - * Make sure s_writers are updated before we wake up waiters in
> - * freeze_super().
> + * We are racing here with freeze_super()'s calls to
> + * sb_wait_write(). We want to ensure that we call
> + * wake_up() whenever one of those calls _might_ be
> + * in sb_wait_write().
> + *
> + * Since freeze_super() does a synchronize_rcu() before
> + * each of its sb_wait_write() calls, it can guarantee
> + * that it sees our update to s_writers.counter as well
> + * as that we see its update to s_writers.frozen.
> */
> - smp_mb();
> - if (waitqueue_active(&sb->s_writers.wait))
> + if (unlikely(sb->s_writers.frozen >= level))
> wake_up(&sb->s_writers.wait);
> + rcu_read_unlock();
> +
> rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_);
> }
> EXPORT_SYMBOL(__sb_end_write);
> _
--
Jan Kara <j...@suse.cz>
SUSE Labs, CR
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