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

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commit c6c15e1ed3 upstream.

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>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>
This commit is contained in:
Trond Myklebust 2014-11-19 12:47:50 -05:00 committed by Zefan Li
parent 63ed3573ab
commit b198b71168
1 changed files with 6 additions and 2 deletions
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8
fs/nfsd/nfs4callback.c
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@ -784,8 +784,12 @@ 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;
}