Re: [PATCH] scsi_dh_alua: avoid crash during alua_bus_detach()

[Bart Van Assche <bvanassche@acm.org>]

On 2020-09-24 03:45, Hannes Reinecke wrote:
> alua_bus_detach() might be running concurrently with alua_rtpg_work(),
> so we might trip over h->sdev == NULL and call BUG_ON().
> The correct way of handling it would be to not set h->sdev to NULL
> in alua_bus_detach(), and call rcu_synchronize() before the final
> delete to ensure that all concurrent threads have left the critical
> section.
> Then we can get rid of the BUG_ON(), and replace it with a simple
> if condition.
> 
> Cc: Brian Bunker <brian@purestorage.com>
> Signed-off-by: Hannes Reinecke <hare@suse.de>
> ---
>  drivers/scsi/device_handler/scsi_dh_alua.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/scsi/device_handler/scsi_dh_alua.c b/drivers/scsi/device_handler/scsi_dh_alua.c
> index f32da0ca529e..308bda2e9c00 100644
> --- a/drivers/scsi/device_handler/scsi_dh_alua.c
> +++ b/drivers/scsi/device_handler/scsi_dh_alua.c
> @@ -658,8 +658,8 @@ static int alua_rtpg(struct scsi_device *sdev, struct alua_port_group *pg)
>  					rcu_read_lock();
>  					list_for_each_entry_rcu(h,
>  						&tmp_pg->dh_list, node) {
> -						/* h->sdev should always be valid */
> -						BUG_ON(!h->sdev);
> +						if (!h->sdev)
> +							continue;
>  						h->sdev->access_state = desc[0];
>  					}
>  					rcu_read_unlock();
> @@ -705,7 +705,8 @@ static int alua_rtpg(struct scsi_device *sdev, struct alua_port_group *pg)
>  			pg->expiry = 0;
>  			rcu_read_lock();
>  			list_for_each_entry_rcu(h, &pg->dh_list, node) {
> -				BUG_ON(!h->sdev);
> +				if (!h->sdev)
> +					continue;
>  				h->sdev->access_state =
>  					(pg->state & SCSI_ACCESS_STATE_MASK);
>  				if (pg->pref)
> @@ -1147,7 +1148,6 @@ static void alua_bus_detach(struct scsi_device *sdev)
>  	spin_lock(&h->pg_lock);
>  	pg = rcu_dereference_protected(h->pg, lockdep_is_held(&h->pg_lock));
>  	rcu_assign_pointer(h->pg, NULL);
> -	h->sdev = NULL;
>  	spin_unlock(&h->pg_lock);
>  	if (pg) {
>  		spin_lock_irq(&pg->lock);
> @@ -1156,6 +1156,7 @@ static void alua_bus_detach(struct scsi_device *sdev)
>  		kref_put(&pg->kref, release_port_group);
>  	}
>  	sdev->handler_data = NULL;
> +	synchronize_rcu();
>  	kfree(h);
>  }

Hi Hannes,

Do you agree that the changes in alua_bus_detach() make the changes in
alua_rtpg() superfluous?

How about freezing command processing for 'sdev' while detaching a
device handler instead of inserting a synchronize_rcu() call in
alua_bus_detach()? I'm concerned that the alua_bus_detach() changes are
not sufficient to fix all possible races between detaching a device
handler and the following code from the SCSI error handler:

	if (sdev->handler && sdev->handler->check_sense) {
		int rc;

		rc = sdev->handler->check_sense(sdev, &sshdr);
		if (rc != SCSI_RETURN_NOT_HANDLED)
			return rc;
		/* handler does not care. Drop down to default handling */
	}

Thanks,

Bart.