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

[Jeff Layton <jlayton@kernel.org>]

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(). 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?
-- 
Jeff Layton <jlayton@kernel.org>