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Subject[PATCH 3.4 009/176] nfsd: Fix slot wake up race in the nfsv4.1 callback code
Date
From: Trond Myklebust <trond.myklebust@primarydata.com>

3.4.107-rc1 review patch. If anyone has any objections, please let me know.

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


commit c6c15e1ed303ffc47e696ea1c9a9df1761c1f603 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>
---
fs/nfsd/nfs4callback.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index c8e9f63..ebf3f59 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -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;
}
--
1.9.1


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