Re: [PATCH] nvme: fix deadlock caused by ana update wrong locking

[Hannes Reinecke <hare@suse.de>]

On 4/2/20 6:18 PM, Sagi Grimberg wrote:
> The deadlock combines 4 flows in parallel:
> - ns scanning (triggered from reconnect)
> - request timeout
> - ANA update (triggered from reconnect)
> - I/O coming into the mpath device
> 
> (1) ns scanning triggers disk revalidation -> update disk info ->
>      freeze queue -> but blocked, due to (2)
> 
> (2) timeout handler reference the g_usage_counter - > but blocks in
>      the transport .timeout() handler, due to (3)
> 
> (3) the transport timeout handler (indirectly) calls nvme_stop_queue() ->
>      which takes the (down_read) namespaces_rwsem - > but blocks, due to (4)
> 
> (4) ANA update takes the (down_write) namespaces_rwsem -> calls
>      nvme_mpath_set_live() -> which synchronize the ns_head srcu
>      (see commit 504db087aacc) -> but blocks, due to (5)
> 
> (5) I/O came into nvme_mpath_make_request -> took srcu_read_lock ->
>      direct_make_request > blk_queue_enter -> but blocked, due to (1)
> 
> ==> the request queue is under freeze -> deadlock.
> 
> The fix is making ANA update take a read lock as the namespaces list
> is not manipulated, it is just the ns and ns->head (which is protected
> with the ns->head lock.
> 
> Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
> ---
>   drivers/nvme/host/multipath.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 61bf87592570..54603bd3e02d 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -510,7 +510,7 @@ static int nvme_update_ana_state(struct nvme_ctrl *ctrl,
>   	if (!nr_nsids)
>   		return 0;
>   
> -	down_write(&ctrl->namespaces_rwsem);
> +	down_read(&ctrl->namespaces_rwsem);
>   	list_for_each_entry(ns, &ctrl->namespaces, list) {
>   		unsigned nsid = le32_to_cpu(desc->nsids[n]);
>   
> @@ -521,7 +521,7 @@ static int nvme_update_ana_state(struct nvme_ctrl *ctrl,
>   		if (++n == nr_nsids)
>   			break;
>   	}
> -	up_write(&ctrl->namespaces_rwsem);
> +	up_read(&ctrl->namespaces_rwsem);
>   	return 0;
>   }
>   
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

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