Re: [PATCH] btrfs: fix waitqueue_active without memory barrier in btrfs

[David Sterba <dsterba@suse.cz>]

On Fri, Oct 09, 2015 at 12:35:48AM +0000, Kosuke Tatsukawa wrote:
> This patch removes the call to waitqueue_active() leaving just wake_up()
> behind.  This fixes the problem because the call to spin_lock_irqsave()
> in wake_up() will be an ACQUIRE operation.

Either we can switch it to wake_up or put the barrier before the check.
Not all instances of waitqueue_active need the barrier though.

> I found this issue when I was looking through the linux source code
> for places calling waitqueue_active() before wake_up*(), but without
> preceding memory barriers, after sending a patch to fix a similar
> issue in drivers/tty/n_tty.c  (Details about the original issue can be
> found here: https://lkml.org/lkml/2015/9/28/849).

There are more in btrfs:

https://www.mail-archive.com/linux-btrfs%40vger.kernel.org/msg41914.html

> @@ -918,9 +918,7 @@ void btrfs_bio_counter_inc_noblocked(struct btrfs_fs_info *fs_info)
>  void btrfs_bio_counter_sub(struct btrfs_fs_info *fs_info, s64 amount)
>  {
>  	percpu_counter_sub(&fs_info->bio_counter, amount);
> -
> -	if (waitqueue_active(&fs_info->replace_wait))
> -		wake_up(&fs_info->replace_wait);
> +	wake_up(&fs_info->replace_wait);

Chris had a comment on that one in
https://www.mail-archive.com/linux-btrfs%40vger.kernel.org/msg42551.html
it's in performance critial context and the explicit wake_up is even
worse than the barrier.