Re: [iscsi] Deadlock occurred when network is in error

[Bart Van Assche]

On Mon, 2017-08-14 at 11:23 +0000, Tangchen (UVP) wrote:
> Problem 2:
> 
> ***************
> [What it looks like]
> ***************
> When remove a scsi device, and the network error happens, __blk_drain_queue() could hang forever.
> 
> # cat /proc/19160/stack 
> [<ffffffff8005886d>] msleep+0x1d/0x30
> [<ffffffff80201a84>] __blk_drain_queue+0xe4/0x160
> [<ffffffff80202766>] blk_cleanup_queue+0x106/0x2e0
> [<ffffffffa000fb02>] __scsi_remove_device+0x52/0xc0 [scsi_mod]
> [<ffffffffa000fb9b>] scsi_remove_device+0x2b/0x40 [scsi_mod]
> [<ffffffffa000fbc0>] sdev_store_delete_callback+0x10/0x20 [scsi_mod]
> [<ffffffff801a4e75>] sysfs_schedule_callback_work+0x15/0x80
> [<ffffffff80062d69>] process_one_work+0x169/0x340
> [<ffffffff800667e3>] worker_thread+0x183/0x490
> [<ffffffff8006a526>] kthread+0x96/0xa0
> [<ffffffff8041ebb4>] kernel_thread_helper+0x4/0x10
> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> The request queue of this device was stopped. So the following check will be true forever:
> __blk_run_queue()
> {
>         if (unlikely(blk_queue_stopped(q)))
>                 return;
> 
>         __blk_run_queue_uncond(q);
> }
> 
> So __blk_run_queue_uncond() will never be called, and the process hang.
> 
> [ ... ]
>
> ****************
> [How to reproduce]
> ****************
> Unfortunately I cannot reproduce it in the latest kernel. 
> The script below will help to reproduce, but not very often.
> 
> # create network error
> tc qdisc add dev eth1 root netem loss 60%
> 
> # restart iscsid and rescan scsi bus again and again
> while [ 1 ]
> do
> systemctl restart iscsid
> rescan-scsi-bus        (http://manpages.ubuntu.com/manpages/trusty/man8/rescan-scsi-bus.8.html)
> done

This should have been fixed by commit 36e3cf273977 ("scsi: Avoid that SCSI
queues get stuck"). The first mainline kernel that includes this commit is
kernel v4.11.

> void __blk_run_queue(struct request_queue *q)
> {
> -       if (unlikely(blk_queue_stopped(q)))
> +       if (unlikely(blk_queue_stopped(q)) && unlikely(!blk_queue_dying(q)))
>                 return;
> 
>         __blk_run_queue_uncond(q);

Are you aware that the single queue block layer is on its way out and will
be removed sooner or later? Please focus your testing on scsi-mq. 

Regarding the above patch: it is wrong because it will cause lockups during
path removal for other block drivers. Please drop this patch.

Bart.