commit e877a88d1f upstream.
If sunrpc.tcp_max_slot_table_entries is small and there are tasks
on the backlog queue, then when a request completes it is freed and the
first task on the queue is woken. The expectation is that it will wake
and claim that request. However if it was a sync task and the waiting
process was killed at just that moment, it will wake and NOT claim the
request.
As long as TASK_CONGESTED remains set, requests can only be claimed by
tasks woken from the backlog, and they are woken only as requests are
freed, so when a task doesn't claim a request, no other task can ever
get that request until TASK_CONGESTED is cleared. Each time this
happens the number of available requests is decreased by one.
With a sufficiently high workload and sufficiently low setting of
max_slot (16 in the case where this was seen), TASK_CONGESTED can remain
set for an extended period, and the above scenario (of a process being
killed just as its task was woken) can repeat until no requests can be
allocated. Then traffic stops.
This patch addresses the problem by introducing a positive handover of a
request from a completing task to a backlog task - the request is never
freed when there is a backlog.
When a task is woken it might not already have a request attached in
which case it is *not* freed (as with current code) but is initialised
(if needed) and used. If it isn't used it will eventually be freed by
rpc_exit_task(). xprt_release() is enhanced to be able to correctly
release an uninitialised request.
Fixes: ba60eb25ff ("SUNRPC: Fix a livelock problem in the xprt->backlog queue")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
5343fcfc6c