nfsd: Fix slot wake up race in the nfsv4.1 callback code

The currect code for nfsd41_cb_get_slot() and nfsd4_cb_done() has no
locking in order to guarantee atomicity, and so allows for races of
the form.

Task 1                                  Task 2
======                                  ======
if (test_and_set_bit(0) != 0) {
                                        clear_bit(0)
                                        rpc_wake_up_next(queue)
        rpc_sleep_on(queue)
        return false;
}

This patch breaks the race condition by adding a retest of the bit
after the call to rpc_sleep_on().

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index ed2b115..7cbdf1b 100644 (file)
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -774,8 +774,12 @@ static bool nfsd41_cb_get_slot(struct nfs4_client *clp, struct rpc_task *task)
 {
        if (test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
                rpc_sleep_on(&clp->cl_cb_waitq, task, NULL);
-               dprintk("%s slot is busy\n", __func__);
-               return false;
+               /* Race breaker */
+               if (test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
+                       dprintk("%s slot is busy\n", __func__);
+                       return false;
+               }
+               rpc_wake_up_queued_task(&clp->cl_cb_waitq, task);
        }
        return true;
 }