Commit c6c15e1e authored Nov 19, 2014 by Trond Myklebust Committed by J. Bruce Fields Nov 19, 2014

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>

parent 093a1468

fs/nfsd/nfs4callback.c

+6 −2

Original line number	Diff line number	Diff line
		@@ -774,9 +774,13 @@ 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);
		/* 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;
		}

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