Re: dm: fix AB-BA deadlock in __dm_destroy()

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Oct 01 2015 at  4:31am -0400,
Junichi Nomura <j-nomura@ce.jp.nec.com> wrote:

> __dm_destroy() takes io_barrier SRCU lock (dm_get_live_table) and
> suspend_lock in reverse order. That can cause AB-BA deadlock:
> 
> Example:
> 
>   __dm_destroy                    dm_swap_table
>   ---------------------------------------------------
>                                   mutex_lock(suspend_lock)
>   dm_get_live_table()
>     srcu_read_lock(io_barrier)
>                                   dm_sync_table()
>                                     synchronize_srcu(io_barrier)
>                                       .. waiting for dm_put_live_table()
>   mutex_lock(suspend_lock)
>     .. waiting for suspend_lock
> 
> This patch fixes the lock ordering.
> 
> Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
> Fixes: ab7c7bb6f4ab ("dm: hold suspend_lock while suspending device during device deletion")
> Cc: Mikulas Patocka <mpatocka@redhat.com>

> ---
> The problem could be reproduced with this script but it might take long.
> (In my environment, it took more than 10 minutes)

Hi,

Thanks for fixing this.  What prompted you to chase this down?  Was it
the work you were doing to reproduce Bart's blk-mq mpath failure that
exposed this issue?

FYI, interestingly, your fix looks to be applicable to this issue too:
https://bugzilla.redhat.com/show_bug.cgi?id=1267650

Thanks again,
Mike