SUNRPC: Should wake up the privileged task firstly.

mainline inclusion
from mainline-v5.14
commit 5483b904
category: bugfix
bugzilla: 51898 https://gitee.com/openeuler/kernel/issues/I4DD74

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5483b904bf336948826594610af4c9bbb0d9e3aa

---------------------------

When find a task from wait queue to wake up, a non-privileged task may
be found out, rather than the privileged. This maybe lead a deadlock
same as commit dfe1fe75 ("NFSv4: Fix deadlock between nfs4_evict_inode()
and nfs4_opendata_get_inode()"):

Privileged delegreturn task is queued to privileged list because all
the slots are assigned. If there has no enough slot to wake up the
non-privileged batch tasks(session less than 8 slot), then the privileged
delegreturn task maybe lost waked up because the found out task can't
get slot since the session is on draining.

So we should treate the privileged task as the emergency task, and
execute it as for as we can.
Reported-by: NHulk Robot <hulkci@huawei.com>
Fixes: 5fcdfacc ("NFSv4: Return delegations synchronously in evict_inode")
Cc: stable@vger.kernel.org
Signed-off-by: NZhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: NChenXiaoSong <chenxiaosong2@huawei.com>
Reviewed-by: NYue Haibing <yuehaibing@huawei.com>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

net/sunrpc/sched.c

@@ -591,6 +591,15 @@ static struct rpc_task *__rpc_find_next_queued_priority(struct rpc_wait_queue *q
 	struct list_head *q;
 	struct rpc_task *task;
 
+	/*
+	 * Service the privileged queue.
+	 */
+	q = &queue->tasks[RPC_NR_PRIORITY - 1];
+	if (queue->maxpriority > RPC_PRIORITY_PRIVILEGED && !list_empty(q)) {
+		task = list_first_entry(q, struct rpc_task, u.tk_wait.list);
+		goto out;
+	}
+
 	/*
 	 * Service a batch of tasks from a single owner.
 	 */