Re: [PATCH]dm-mpath: fix for race condition between multipath_dtr and pg_init_done.

[Mike Snitzer <snitzer@redhat.com>]

Thanks for reporting this.  Much appreciated.  More comments below.

On Thu, Oct 17 2013 at  1:31pm -0400,
Merla, ShivaKrishna <ShivaKrishna.Merla@netapp.com> wrote:

> Whenever multipath_dtr is happening, we should prevent queueing any further path
> activation work. There was a kernel panic where after pg_init_done() decrements
> pg_init_in_progress to 0, wait_for_pg_init_completion call assumes there are no
> more pending path management commands. But if pg_init_required is set by
> pg_init_done call due to retriable mode_select errors , then process_queued_ios()
> will again queue the path activation work. If free_multipath call has been
> completed by the time activate_path work is called, kernel panic was seen on
> accessing multipath members.

Your locking looks suspect to me, see comment inlined below multipath_dtr

But shouldn't we just train multipath_wait_for_pg_init_completion() to
look at m->pg_init_required?  Have it wait for both pg_init_required and
pg_init_in_progress to be zero?  We'd also have to audit that
pg_init_required cannot be set while pg_init_in_progress.

> BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
> RIP: 0010:[<ffffffffa003db1b>]  [<ffffffffa003db1b>] activate_path+0x1b/0x30 [dm_multipath]
> [<ffffffff81090ac0>] worker_thread+0x170/0x2a0
> [<ffffffff81096c80>] ? autoremove_wake_function+0x0/0x40
> 
> This patch will fix the issue by preventing any further path activations when
> multipath structures are being freed during table suspend/reload.
> 
> Signed-off-by: Shiva Krishna Merla<shivakrishna.merla@netapp.com>
> Reviewed-by: Krishnasamy Somasundaram<somasundaram.krishnasamy@netapp.com>
> Tested-by: Speagle Andy<Andy.Speagle@netapp.com>
>
> ---
> --- a/drivers/md/dm-mpath.c	2013-01-29 10:12:10.000000000 -0600
> +++ b/drivers/md/dm-mpath.c	2013-10-17 09:23:21.896062928 -0500
> @@ -73,6 +73,7 @@ struct multipath {
>  
>  	wait_queue_head_t pg_init_wait;	/* Wait for pg_init completion */
>  
> +	unsigned dtr_in_progress;	/* multipath destroy in progress */
>  	unsigned pg_init_required;	/* pg_init needs calling? */
>  	unsigned pg_init_in_progress;	/* Only one pg_init allowed at once */
>  	unsigned pg_init_delay_retry;	/* Delay pg_init retry? */
> @@ -498,7 +499,8 @@ static void process_queued_ios(struct wo
>  	    (!pgpath && !m->queue_if_no_path))
>  		must_queue = 0;
>  
> -	if (m->pg_init_required && !m->pg_init_in_progress && pgpath)
> +	if (m->pg_init_required && !m->pg_init_in_progress && pgpath &&
> +	    !m->dtr_in_progress)
>  		__pg_init_all_paths(m);
>  
>  	spin_unlock_irqrestore(&m->lock, flags);
> @@ -952,6 +954,7 @@ static void multipath_dtr(struct dm_targ
>  {
>  	struct multipath *m = ti->private;
>  
> +	m->dtr_in_progress = 1;
>  	flush_multipath_work(m);
>  	free_multipath(m);
>  }

Don't we need synchronization in multipath_dtr?  Otherwise isn't there
still potential for a narrow race when checking m->dtr_in_progress from
process_queued_ios() or pg_init_limit_reached()?

Anyway, my concerns should be moot if multipath_wait_for_pg_init_completion()
is updated to look at pg_init_required.  But I could be missing
something.

Mikulas (or Hannes or NEC guys), would welcome your take on this.

> @@ -1164,7 +1167,7 @@ static int pg_init_limit_reached(struct 
>  
>  	spin_lock_irqsave(&m->lock, flags);
>  
> -	if (m->pg_init_count <= m->pg_init_retries)
> +	if (m->pg_init_count <= m->pg_init_retries && !m->dtr_in_progress)
>  		m->pg_init_required = 1;
>  	else
>  		limit_reached = 1;
> --