SUNRPC: Fix the batch tasks count wraparound.

The 'queue->nr' will wraparound from 0 to 255 when only current
priority queue has tasks. This maybe lead a deadlock same as commit
e4abb4a0415e ("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: 348fe89b656b ("NFSv4: Return delegations synchronously in evict_inode")
Fixes: 1da177e4c3f4 ("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>

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 39ed0e0afe6d9bc25069659e853fbde29326a062..71993adb076258e009b65e0defca481d906d72b4 100644 (file)
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -595,7 +595,8 @@ 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;
        }