CIFS mounts hanging after CIFS server restart

[Robert Smith <Robert.Smith@gresearch.co.uk>]

Hi,

We have seen an issue where a CIFS server restarting has caused client processes to hang.

Server:
Dell Isilon NAS (accessed via DFS)

Client: 
Flatcar Container Linux by Kinvolk 2605.4.0 (Oklo)
Kernel: 5.4.65-flatcar

On the client, we run "touch --no-create /mnt/cfs/.cifskeepalive" regularly to keep the CIFS connections alive (workaround for another issue where idle connections die). These touch commands started hanging (along with all other users of CIFS). I sampled /proc/*/stack for several of the touch cmds, all gave the stacktrace below. We ended up with around 700 hung touch processes.

[<0>] d_alloc_parallel+0x420/0x480
[<0>] __lookup_slow+0x6e/0x150
[<0>] lookup_slow+0x35/0x50
[<0>] walk_component+0x1bf/0x330
[<0>] path_lookupat.isra.50+0x6d/0x220
[<0>] filename_lookup.part.66+0xa0/0x170
[<0>] do_utimes+0xd9/0x160
[<0>] __x64_sys_utimensat+0x7a/0xc0
[<0>] do_syscall_64+0x4e/0x120
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Also sampled a few cifs processes, these were all in

Robert Smith  1 day ago
# cat /proc/2477052/stack
[<0>] iterate_supers_type+0x6d/0xf0
[<0>] cifs_reconnect+0x8e/0xfd0 [cifs]
[<0>] cifs_reconnect+0xeed/0xfd0 [cifs]
[<0>] cifs_read_from_socket+0x4a/0x70 [cifs]
[<0>] cifs_handle_standard+0x298/0xd60 [cifs]
[<0>] kthread+0x112/0x130
[<0>] ret_from_fork+0x35/0x40

Sampling a few minutes apart showed voluntary_ctxt_switches increasing for a few of the 700 touch processes, so may not be totally hung.

Any ideas what's causing this?

Thanks for your help,

Rob


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