Re: [PATCH] nfsd: decrease cl_cb_inflight if fail to queue cb_work

[Chuck Lever <chuck.lever@oracle.com>]

On 2/18/25 9:40 AM, Jeff Layton wrote:
> On Tue, 2025-02-18 at 09:31 -0500, Chuck Lever wrote:
>> On 2/18/25 9:29 AM, Jeff Layton wrote:
>>> On Tue, 2025-02-18 at 08:58 -0500, Jeff Layton wrote:
>>>> On Tue, 2025-02-18 at 21:54 +0800, Li Lingfeng wrote:
>>>>> In nfsd4_run_cb, cl_cb_inflight is increased before attempting to queue
>>>>> cb_work to callback_wq. This count can be decreased in three situations:
>>>>> 1) If queuing fails in nfsd4_run_cb, the count will be decremented
>>>>> accordingly.
>>>>> 2) After cb_work is running, the count is decreased in the exception
>>>>> branch of nfsd4_run_cb_work via nfsd41_destroy_cb.
>>>>> 3) The count is decreased in the release callback of rpc_task — either
>>>>> directly calling nfsd41_cb_inflight_end in nfsd4_cb_probe_release, or
>>>>> calling nfsd41_destroy_cb in 	.
>>>>>
>>>>> However, in nfsd4_cb_release, if the current cb_work needs to restart, the
>>>>> count will not be decreased, with the expectation that it will be reduced
>>>>> once cb_work is running.
>>>>> If queuing fails here, then the count will leak, ultimately causing the
>>>>> nfsd service to be unable to exit as shown below:
>>>>> [root@nfs_test2 ~]# cat /proc/2271/stack
>>>>> [<0>] nfsd4_shutdown_callback+0x22b/0x290
>>>>> [<0>] __destroy_client+0x3cd/0x5c0
>>>>> [<0>] nfs4_state_destroy_net+0xd2/0x330
>>>>> [<0>] nfs4_state_shutdown_net+0x2ad/0x410
>>>>> [<0>] nfsd_shutdown_net+0xb7/0x250
>>>>> [<0>] nfsd_last_thread+0x15f/0x2a0
>>>>> [<0>] nfsd_svc+0x388/0x3f0
>>>>> [<0>] write_threads+0x17e/0x2b0
>>>>> [<0>] nfsctl_transaction_write+0x91/0xf0
>>>>> [<0>] vfs_write+0x1c4/0x750
>>>>> [<0>] ksys_write+0xcb/0x170
>>>>> [<0>] do_syscall_64+0x70/0x120
>>>>> [<0>] entry_SYSCALL_64_after_hwframe+0x78/0xe2
>>>>> [root@nfs_test2 ~]#
>>>>>
>>>>> Fix this by decreasing cl_cb_inflight if the restart fails.
>>>>>
>>>>> Fixes: cba5f62b1830 ("nfsd: fix callback restarts")
>>>>> Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
>>>>> ---
>>>>>  fs/nfsd/nfs4callback.c | 10 +++++++---
>>>>>  1 file changed, 7 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
>>>>> index 484077200c5d..8a7d24efdd08 100644
>>>>> --- a/fs/nfsd/nfs4callback.c
>>>>> +++ b/fs/nfsd/nfs4callback.c
>>>>> @@ -1459,12 +1459,16 @@ static void nfsd4_cb_done(struct rpc_task *task, void *calldata)
>>>>>  static void nfsd4_cb_release(void *calldata)
>>>>>  {
>>>>>  	struct nfsd4_callback *cb = calldata;
>>>>> +	struct nfs4_client *clp = cb->cb_clp;
>>>>> +	int queued;
>>>>>  
>>>>>  	trace_nfsd_cb_rpc_release(cb->cb_clp);
>>>>>  
>>>>> -	if (cb->cb_need_restart)
>>>>> -		nfsd4_queue_cb(cb);
>>>>> -	else
>>>>> +	if (cb->cb_need_restart) {
>>>>> +		queued = nfsd4_queue_cb(cb);
>>>>> +		if (!queued)
>>>>> +			nfsd41_cb_inflight_end(clp);
>>>>> +	} else
>>>>>  		nfsd41_destroy_cb(cb);
>>>>>  
>>>>>  }
>>>>
>>>> Good catch!
>>>>
>>>> Reviewed-by: Jeff Layton <jlayton@kernel.org>
>>>>
>>>
>>> Actually, I think this is not quite right. It's a bit more subtle than
>>> it first appears. The problem of course is that the callback workqueue
>>> jobs run in a different task than the RPC workqueue jobs, so they can
>>> race.
>>>
>>> cl_cb_inflight gets bumped when the callback is first queued, and only
>>> gets released in nfsd41_destroy_cb(). If it fails to be queued, it's
>>> because something else has queued the workqueue job in the meantime.
>>>
>>> There are two places that can occur: nfsd4_cb_release() and
>>> nfsd4_run_cb(). Since this is occurring in nfsd4_cb_release(), the only
>>> other option is that something raced in and queued it via
>>> nfsd4_run_cb().
>>
>> What would be the "something" that raced in?
>>
> 
> I think we may be able to get there via multiple __break_lease() calls
> on the same layout or delegation. That could mean multiple calls to the
> ->lm_break operation on the same object.

Makes sense.

Out of curiosity, what would be the complexity/performance cost of
serializing the lm_break calls, or having each lm_break call register
an interest in the CB_RECALL callback? Maybe not worth it.


>>> That will have incremented cl_cb_inflight() an extra
>>> time and so your patch will make sense for that.
>>>
>>> Unfortunately, the slot may leak in that case if nothing released it
>>> earlier. I think this probably needs to call nfsd41_destroy_cb() if the
>>> job can't be queued. That might, however, race with the callback
>>> workqueue job running.
>>>
>>> I think we might need to consider adding some sort of "this callback is
>>> running" atomic flag: do a test_and_set on the flag in nfsd4_run_cb()
>>> and only queue the workqueue job if that comes back false. Then, we can
>>> clear the bit in nfsd41_destroy_cb().
>>>
>>> That should ensure that you never fail to queue the workqueue job from
>>> nfsd4_cb_release().
>>>
>>> Thoughts?
>>
>>
> 


-- 
Chuck Lever