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

[Jeff Layton]

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@xxxxxxxxxx>
> > > > ---
> > > >  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@xxxxxxxxxx>
> > > 
> > 
> > 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.

> 
> > 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@xxxxxxxxxx>