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SubjectRe: [syzbot] WARNING in p9_client_destroy
syzbot wrote on Sun, Feb 27, 2022 at 04:53:29PM -0800:
> kmem_cache_destroy 9p-fcall-cache: Slab cache still has objects when
> called from p9_client_destroy+0x213/0x370 net/9p/client.c:1100

hmm, there is no previous "Packet with tag %d has still references"
(sic) message, so this is probably because p9_tag_cleanup only relies on
rcu read lock for consistency, so even if the connection has been closed
above (clnt->trans_mode->close) there could have been a request sent
(= tag added) just before that which isn't visible on the destroying

I guess adding an rcu_barrier() is what makes most sense here to protect
this case?
I'll send a patch in the next few days unless it was a stupid idea.

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