2014-11-19 17:47:57

by Trond Myklebust

Subject: [PATCH] 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 <[email protected]>
Cc: [email protected]
---
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 ed2b1151b171..7cbdf1b2e4ab 100644
--- 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;
}
--
1.9.3

2014-11-19 20:49:14

by J. Bruce Fields

Subject: Re: [PATCH] nfsd: Fix slot wake up race in the nfsv4.1 callback code

On Wed, Nov 19, 2014 at 12:47:50PM -0500, Trond Myklebust wrote:
> 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.

Makes sense, thanks. Applying for 3.18 and stable.

--b.

>
> 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 <[email protected]>
> Cc: [email protected]
> ---
> 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 ed2b1151b171..7cbdf1b2e4ab 100644
> --- 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;
> }
> --
> 1.9.3
>