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Subject: Re: [PATCH 1/1] NFSv4: can_open_cached needs to be called with
 so_lock
To:     Trond Myklebust <trondmy@hammerspace.com>
Cc:     "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
References: <20210430050900.106851-1-dai.ngo@oracle.com>
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From:   dai.ngo@oracle.com
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On 4/30/21 10:56 AM, Trond Myklebust wrote:
> On Fri, 2021-04-30 at 10:24 -0700, dai.ngo@oracle.com wrote:
>> Hi Trond,
>>
>> I have a question below:
>>
>> On 4/30/21 5:42 AM, Trond Myklebust wrote:
>>> On Fri, 2021-04-30 at 01:09 -0400, Dai Ngo wrote:
>>>> Currently can_open_cached accesses the openstate's flags without
>>>> the
>>>> so_lock and also does not update the flags of the cached state.
>>>> This
>>>> results in the openstate's flags be out of sync which can cause
>>>> the
>>>> file to be closed prematurely.
>>>>
>>>> This patch adds the missing so_lock around the call to
>>>> can_open_cached
>>>> and also updates the openstate's flags if the cached openstate is
>>>> used.
>>>>
>>>> Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
>>>> ---
>>>>    fs/nfs/nfs4proc.c | 8 +++++++-
>>>>    1 file changed, 7 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
>>>> index c65c4b41e2c1..2464e77c51f9 100644
>>>> --- a/fs/nfs/nfs4proc.c
>>>> +++ b/fs/nfs/nfs4proc.c
>>>> @@ -2410,9 +2410,15 @@ static void nfs4_open_prepare(struct
>>>> rpc_task
>>>> *task, void *calldata)
>>>>           if (data->state != NULL) {
>>>>                   struct nfs_delegation *delegation;
>>>>    
>>>> +               spin_lock(&data->state->owner->so_lock);
>>>>                   if (can_open_cached(data->state, data-
>>>>> o_arg.fmode,
>>>> -                                       data->o_arg.open_flags,
>>>> claim))
>>>> +                               data->o_arg.open_flags, claim)) {
>>>> +                       update_open_stateflags(data->state, data-
>>>>> o_arg.fmode);
>>>> +                       spin_unlock(&data->state->owner-
>>>>> so_lock);
>>>>                           goto out_no_action;
>>>> +               }
>>>> +               spin_unlock(&data->state->owner->so_lock);
>>>> +
>>>>                   rcu_read_lock();
>>>>                   delegation = nfs4_get_valid_delegation(data-
>>>>> state-
>>>>> inode);
>>>>                   if (can_open_delegated(delegation, data-
>>>>> o_arg.fmode,
>>>> claim))
>>> This is going to introduce stateid leaks. The actual update of the
>>> open
>>> state flags happens in nfs4_try_open_cached(), which is called from
>>> nfs4_opendata_to_nfs4_state().
>>>
>>> While we could put spinlocks around the call to can_open_cached()
>>> here,
>>> there is little point in doing so, since this is just a read-only
>>> advisory check. The real check is performed, as I said, in
>>> nfs4_try_open_cached().
>> If we wait to update the flags in _nfs4_opendata_to_nfs4_state after
>> the
>> RPC thread decides to use the cached state, the file could be closed
>> by
>> another thread before _nfs4_opendata_to_nfs4_state is called by
>> another
>> thread. The client in this case will retry the open from nfs4_do_open
>> and
>> everything is ok.
>>
>> However, if we update the flags nfs4_open_prepare then it will
>> prevent
>> the file from being closed and this saves one CLOSE and one OPEN rpc
>> request to the server.  Is this correct and is it worth it to
>> consider
>> doing anything since this is a rare scenario?
> If you're in a scenario where several processes are accessing the same
> file on the same NFS client, you probably want to see the server hand
> out a delegation for that file rather than keep relying on OPEN/CLOSE.
> That's actually why we started using nfs4_try_open_cached(). The
> intention was that it mainly manages the delegated open case. We then
> added support for the non-delegated case mainly because the Linux
> server doesn't support write delegations and because there were corner
> cases where files were being opened/closed by multiple processes
> without a delegation.
>
> So what I'm saying is that ideally we really want to concentrate on
> fixing the Linux server to support write delegations so that we can
> relegate most of this code to handling corner cases.
>
> Make sense?

Yes, I agreed. It's not worth the effort to look and fix corner cases
on the client side that rarely happen. Support for write delegations
provides much more benefits and it's worth to spend the effort on.

Thanks,
-Dai