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

[Josef Bacik <jbacik@fb.com>]

On 10/08/2015 05:35 PM, Kosuke Tatsukawa wrote:
> btrfs_bio_counter_sub() seems to be missing a memory barrier which might
> cause the waker to not notice the waiter and miss sending a wake_up as
> in the following figure.
> 
> 	btrfs_bio_counter_sub			btrfs_rm_dev_replace_blocked
> ------------------------------------------------------------------------
> if (waitqueue_active(&fs_info->replace_wait))
> /* The CPU might reorder the test for
>     the waitqueue up here, before
>     prior writes complete */
> 					/* wait_event */
> 					 /* __wait_event */
> 					  /* ___wait_event */
> 					  long __int = prepare_to_wait_event(&wq,
> 					    &__wait, state);
> 					  if (!percpu_counter_sum(&fs_info->bio_counter))
> percpu_counter_sub(&fs_info->bio_counter,
>    amount);
> 					  schedule()

percpu_counter_sub can't be reordered, in its most basic form it does
preempt_disable/enable which in its most basic form does barrier().  Thanks,

Josef