SUNRPC: Fix the batch tasks count wraparound.

mainline inclusion
from mainline-v5.14
commit fcb170a9d825d7db4a3fb870b0300f5a40a8d096
category: bugfix
bugzilla: 51898
CVE: NA

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

The 'queue->nr' will wraparound from 0 to 255 when only current
priority queue has tasks. This maybe lead a deadlock same as commit
dfe1fe75e00e ("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. When non-privileged task complete and release
the slot, a non-privileged maybe picked out. It maybe allocate slot
failed when the session on draining.

If the 'queue->nr' has wraparound to 255, and no enough slot to
service it, then the privileged delegreturn will lost to wake up.

So we should avoid the wraparound on 'queue->nr'.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 5fcdfacc ("NFSv4: Return delegations synchronously in evict_inode")
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>

net/sunrpc/sched.c

@@ -506,7 +506,8 @@ static struct rpc_task *__rpc_find_next_queued_priority(struct rpc_wait_queue *q
 	 * Service a batch of tasks from a single owner.
 	 */
 	q = &queue->tasks[queue->priority];
-	if (!list_empty(q) && --queue->nr) {
+	if (!list_empty(q) && queue->nr) {
+		queue->nr--;
 		task = list_first_entry(q, struct rpc_task, u.tk_wait.list);
 		goto out;
 	}