SUNRPC in case of backlog, hand free slots directly to waiting task

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: ba60eb25ff6b ("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>

1 files changed, 0 insertions, 7 deletions

diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index f555d335e910..42623d6b8f0e 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -1677,13 +1677,6 @@ call_reserveresult(struct rpc_task *task)
 		return;
 	}
 
-	/*
-	 * Even though there was an error, we may have acquired
-	 * a request slot somehow.  Make sure not to leak it.
-	 */
-	if (task->tk_rqstp)
-		xprt_release(task);
-
 	switch (status) {
 	case -ENOMEM:
 		rpc_delay(task, HZ >> 2);