Re: [PATCH] NFS: Fix an LOCK/OPEN race when unlinking an open file

[Anna Schumaker <Anna.Schumaker@netapp.com>]

On 04/28/2016 10:06 AM, Chuck Lever wrote:
> 
>> On Apr 28, 2016, at 9:13 AM, Anna Schumaker <Anna.Schumaker@netapp.com> wrote:
>>
>> The patch looks pretty straightforward to me, and it sounds like it fixes a few problems that people are seeing.  One question (below):
>>
>> On 04/28/2016 08:43 AM, William Dauchy wrote:
>>> Hello Anna,
>>>
>>> Could you have a look at this one please?
>>>
>>> Thanks,
>>>
>>> William
>>>
>>> On Tue, Apr 26, 2016 at 4:24 PM, Olga Kornievskaia <aglo@umich.edu> wrote:
>>>> I believe this patch also helps with a race between a LOCK and
>>>> CB_RECALL. Application does a lock as the delegation is being
>>>> recalled. The lock thread sees the delegated state and acquires a
>>>> local lock. At the same time delegation doesn't see it the lock yet
>>>> and returns the delegation. Application proceeds to do IO. It ends up
>>>> using an open stateid for the IO (as there is no delegation stateid or
>>>> lock stateid). The server is unaware of the lock so it can give that
>>>> lock to somebody else. Yet this client thinks it has a local lock. It
>>>> leads to inconsistent data between clients.
>>>>
>>>> On Mon, Apr 11, 2016 at 4:20 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
>>>>> At Connectathon 2016, we found that recent upstream Linux clients
>>>>> would occasionally send a LOCK operation with a zero stateid. This
>>>>> appeared to happen in close proximity to another thread returning
>>>>> a delegation before unlinking the same file while it remained open.
>>>>>
>>>>> Earlier, the client received a write delegation on this file and
>>>>> returned the open stateid. Now, as it is getting ready to unlink the
>>>>> file, it returns the write delegation. But there is still an open
>>>>> file descriptor on that file, so the client must OPEN the file
>>>>> again before it returns the delegation.
>>>>>
>>>>> Since commit 24311f884189 ('NFSv4: Recovery of recalled read
>>>>> delegations is broken'), nfs_open_delegation_recall() clears the
>>>>> NFS_DELEGATED_STATE flag _before_ it sends the OPEN. This allows a
>>>>> racing LOCK on the same inode to be put on the wire before the OPEN
>>>>> operation has returned a valid open stateid.
>>>>>
>>>>> To eliminate this race, serialize delegation return with the
>>>>> acquisition of a file lock on the same file. Adopt the same approach
>>>>> as is used in the unlock path.
>>>>>
>>>>> Fixes: 24311f884189 ('NFSv4: Recovery of recalled read ... ')
>>>>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>>>>> ---
>>>>> Hi-
>>>>>
>>>>> This fix appears to be both safe and effective. Please consider
>>>>> it for v4.7 and for stable. Thanks!
>>>>>
>>>>>
>>>>> fs/nfs/nfs4proc.c |    4 ++++
>>>>> 1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
>>>>> index 01bef06..c40f1b6 100644
>>>>> --- a/fs/nfs/nfs4proc.c
>>>>> +++ b/fs/nfs/nfs4proc.c
>>>>> @@ -6054,6 +6054,7 @@ static int nfs41_lock_expired(struct nfs4_state *state, struct file_lock *reques
>>>>> static int _nfs4_proc_setlk(struct nfs4_state *state, int cmd, struct file_lock *request)
>>>>> {
>>>>>        struct nfs_inode *nfsi = NFS_I(state->inode);
>>>>> +       struct nfs4_state_owner *sp = state->owner;
>>>>>        unsigned char fl_flags = request->fl_flags;
>>>>>        int status = -ENOLCK;
>>>>>
>>>>> @@ -6068,6 +6069,7 @@ static int _nfs4_proc_setlk(struct nfs4_state *state, int cmd, struct file_lock
>>>>>        status = do_vfs_lock(state->inode, request);
>>>>>        if (status < 0)
>>>>>                goto out;
>>>>> +       mutex_lock(&sp->so_delegreturn_mutex);
>>
>> From what I can tell, the first call to do_vfs_lock() in this function is used to test if we can take the lock locally.  Do we need to worry about this racing with delegreturn, too?
> 
> When I included that call in the critical section,
> cthon04 locking tests deadlocked.

Got it.  Thanks for checking!

Anna

> 
> 
>> Thanks,
>> Anna
>>
>>>>>        down_read(&nfsi->rwsem);
>>>>>        if (test_bit(NFS_DELEGATED_STATE, &state->flags)) {
>>>>>                /* Yes: cache locks! */
>>>>> @@ -6075,9 +6077,11 @@ static int _nfs4_proc_setlk(struct nfs4_state *state, int cmd, struct file_lock
>>>>>                request->fl_flags = fl_flags & ~FL_SLEEP;
>>>>>                status = do_vfs_lock(state->inode, request);
>>>>>                up_read(&nfsi->rwsem);
>>>>> +               mutex_unlock(&sp->so_delegreturn_mutex);
>>>>>                goto out;
>>>>>        }
>>>>>        up_read(&nfsi->rwsem);
>>>>> +       mutex_unlock(&sp->so_delegreturn_mutex);
>>>>>        status = _nfs4_do_setlk(state, cmd, request, NFS_LOCK_NEW);
>>>>> out:
>>>>>        request->fl_flags = fl_flags;
> 
> --
> Chuck Lever
> 
> 
>