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

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

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?


> 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