Re: [PATCH 1/1] NFSv4: can_open_cached needs to be called with so_lock

From: dai.ngo@oracle.com
To: Trond Myklebust <trondmy@hammerspace.com>
Cc: "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH 1/1] NFSv4: can_open_cached needs to be called with so_lock
Date: Fri, 30 Apr 2021 10:24:11 -0700	[thread overview]
Message-ID: <6ecdca6f-85a8-87d5-a5ce-069b98533a10@oracle.com> (raw)
In-Reply-To: <8fadf7c12b188eacf5c2bb577a2fbf938e51ebaa.camel@hammerspace.com>

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?

Thanks,
-Dai

>