Re: [PATCH 2/2] nvme-multipath: don't block on blk_queue_enter of the underlying device

[Sagi Grimberg <sagi@grimberg.me>]

On 3/22/21 7:57 PM, Sagi Grimberg wrote:
> 
>> When we reset/teardown a controller, we must freeze and quiesce the
>> namespaces request queues to make sure that we safely stop inflight I/O
>> submissions. Freeze is mandatory because if our hctx map changed between
>> reconnects, blk_mq_update_nr_hw_queues will immediately attempt to freeze
>> the queue, and if it still has pending submissions (that are still
>> quiesced) it will hang.
>>
>> However, by freezing the namespaces request queues, and only unfreezing
>> them when we successfully reconnect, inflight submissions that are
>> running concurrently can now block grabbing the nshead srcu until either
>> we successfully reconnect or ctrl_loss_tmo expired (or the user
>> explicitly disconnected).
>>
>> This caused a deadlock when a different controller (different path on the
>> same subsystem) became live (i.e. optimized/non-optimized). This is
>> because nvme_mpath_set_live needs to synchronize the nshead srcu before
>> requeueing I/O in order to make sure that current_path is visible to
>> future (re-)submisions. However the srcu lock is taken by a blocked
>> submission on a frozen request queue, and we have a deadlock.
>>
>> In order to fix this use the blk_mq_submit_bio_direct API to submit the
>> bio to the low-level driver, which does not block on the queue free
>> but instead allows nvme-multipath to pick another path or queue up the
>> bio.
> 
> Almost...
> 
> This still has the same issue but instead of blocking on
> blk_queue_enter() it is blocked on blk_mq_get_tag():
> -- 
>   __schedule+0x22b/0x6e0
>   schedule+0x46/0xb0
>   io_schedule+0x42/0x70
>   blk_mq_get_tag+0x11d/0x270
>   ? blk_bio_segment_split+0x235/0x2a0
>   ? finish_wait+0x80/0x80
>   __blk_mq_alloc_request+0x65/0xe0
>   blk_mq_submit_bio+0x144/0x500
>   blk_mq_submit_bio_direct+0x78/0xa0
>   nvme_ns_head_submit_bio+0xc3/0x2f0 [nvme_core]
>   __submit_bio_noacct+0xcf/0x2e0
>   __blkdev_direct_IO+0x413/0x440
>   ? __io_complete_rw.constprop.0+0x150/0x150
>   generic_file_read_iter+0x92/0x160
>   io_iter_do_read+0x1a/0x40
>   io_read+0xc5/0x350
>   ? common_interrupt+0x14/0xa0
>   ? update_load_avg+0x7a/0x5e0
>   io_issue_sqe+0xa28/0x1020
>   ? lock_timer_base+0x61/0x80
>   io_wq_submit_work+0xaa/0x120
>   io_worker_handle_work+0x121/0x330
>   io_wqe_worker+0xb6/0x190
>   ? io_worker_handle_work+0x330/0x330
>   ret_from_fork+0x22/0x30
> -- 
> 
> -- 
>   ? usleep_range+0x80/0x80
>   __schedule+0x22b/0x6e0
>   ? usleep_range+0x80/0x80
>   schedule+0x46/0xb0
>   schedule_timeout+0xff/0x140
>   ? del_timer_sync+0x67/0xb0
>   ? __prepare_to_swait+0x4b/0x70
>   __wait_for_common+0xb3/0x160
>   __synchronize_srcu.part.0+0x75/0xe0
>   ? __bpf_trace_rcu_utilization+0x10/0x10
>   nvme_mpath_set_live+0x61/0x130 [nvme_core]
>   nvme_update_ana_state+0xd7/0x100 [nvme_core]
>   nvme_parse_ana_log+0xa5/0x160 [nvme_core]
>   ? nvme_mpath_set_live+0x130/0x130 [nvme_core]
>   nvme_read_ana_log+0x7b/0xe0 [nvme_core]
>   process_one_work+0x1e6/0x380
>   worker_thread+0x49/0x300
> -- 
> 
> 
> 
> If I were to always start the queues in nvme_tcp_teardown_ctrl
> right after I cancel the tagset inflights like:
> -- 
> @@ -1934,8 +1934,7 @@ static void nvme_tcp_teardown_io_queues(struct 
> nvme_ctrl *ctrl,
>          nvme_sync_io_queues(ctrl);
>          nvme_tcp_stop_io_queues(ctrl);
>          nvme_cancel_tagset(ctrl);
> -       if (remove)
> -               nvme_start_queues(ctrl);
> +       nvme_start_queues(ctrl);
>          nvme_tcp_destroy_io_queues(ctrl, remove);
> -- 
> 
> then a simple reset during traffic bricks the host on infinite loop
> because in the setup sequence we freeze the queue in
> nvme_update_ns_info, so the queue is frozen but we still have an
> available path (because the controller is back to live!) so nvme-mpath
> keeps calling blk_mq_submit_bio_direct and fails, and
> nvme_update_ns_info cannot properly freeze the queue..
> -> deadlock.
> 
> So this is obviously incorrect.
> 
> Also, if we make nvme-mpath submit a REQ_NOWAIT we basically
> will fail as soon as we run out of tags, even in the normal path...
> 
> So I'm not exactly sure what we should do to fix this...

It's still not too late to go with my original approach... ;)

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