Re: [PATCH v2] nfsd: Fix races between nfsd4_cb_release() and nfsd4_shutdown_callback()

[Trond Myklebust <trondmy@hammerspace.com>]

On Fri, 2019-10-25 at 10:51 -0400, J. Bruce Fields wrote:
> On Wed, Oct 23, 2019 at 05:43:18PM -0400, Trond Myklebust wrote:
> > When we're destroying the client lease, and we call
> > nfsd4_shutdown_callback(), we must ensure that we do not return
> > before all outstanding callbacks have terminated and have
> > released their payloads.
> 
> This is great, thanks!  We've seen what I'm fairly sure is the same
> bug
> from Red Hat users.  I think my blind spot was an assumption that
> rpc tasks wouldn't outlive rpc_shutdown_client().
> 
> However, it's causing xfstests runs to hang, and I haven't worked out
> why yet.
> 
> I'll spend some time on it this afternoon and let you know what I
> figure
> out.
> 

Is that happening with v2 or with v1? With v1 there is definitely a
hang in __destroy_client() due to the refcount leak that I believe I
fixed in v2.

> --b.
> 
> > Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> > ---
> > v2 - Fix a leak of clp->cl_cb_inflight when running null probes
> > 
> >  fs/nfsd/nfs4callback.c | 97 +++++++++++++++++++++++++++++++++-----
> > ----
> >  fs/nfsd/state.h        |  1 +
> >  2 files changed, 79 insertions(+), 19 deletions(-)
> > 
> > diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
> > index 524111420b48..a3c9517bcc64 100644
> > --- a/fs/nfsd/nfs4callback.c
> > +++ b/fs/nfsd/nfs4callback.c
> > @@ -826,6 +826,31 @@ static int max_cb_time(struct net *net)
> >  	return max(nn->nfsd4_lease/10, (time_t)1) * HZ;
> >  }
> >  
> > +static struct workqueue_struct *callback_wq;
> > +
> > +static bool nfsd4_queue_cb(struct nfsd4_callback *cb)
> > +{
> > +	return queue_work(callback_wq, &cb->cb_work);
> > +}
> > +
> > +static void nfsd41_cb_inflight_begin(struct nfs4_client *clp)
> > +{
> > +	atomic_inc(&clp->cl_cb_inflight);
> > +}
> > +
> > +static void nfsd41_cb_inflight_end(struct nfs4_client *clp)
> > +{
> > +
> > +	if (atomic_dec_and_test(&clp->cl_cb_inflight))
> > +		wake_up_var(&clp->cl_cb_inflight);
> > +}
> > +
> > +static void nfsd41_cb_inflight_wait_complete(struct nfs4_client
> > *clp)
> > +{
> > +	wait_var_event(&clp->cl_cb_inflight,
> > +			!atomic_read(&clp->cl_cb_inflight));
> > +}
> > +
> >  static const struct cred *get_backchannel_cred(struct nfs4_client
> > *clp, struct rpc_clnt *client, struct nfsd4_session *ses)
> >  {
> >  	if (clp->cl_minorversion == 0) {
> > @@ -937,14 +962,21 @@ static void nfsd4_cb_probe_done(struct
> > rpc_task *task, void *calldata)
> >  		clp->cl_cb_state = NFSD4_CB_UP;
> >  }
> >  
> > +static void nfsd4_cb_probe_release(void *calldata)
> > +{
> > +	struct nfs4_client *clp = container_of(calldata, struct
> > nfs4_client, cl_cb_null);
> > +
> > +	nfsd41_cb_inflight_end(clp);
> > +
> > +}
> > +
> >  static const struct rpc_call_ops nfsd4_cb_probe_ops = {
> >  	/* XXX: release method to ensure we set the cb channel down if
> >  	 * necessary on early failure? */
> >  	.rpc_call_done = nfsd4_cb_probe_done,
> > +	.rpc_release = nfsd4_cb_probe_release,
> >  };
> >  
> > -static struct workqueue_struct *callback_wq;
> > -
> >  /*
> >   * Poke the callback thread to process any updates to the callback
> >   * parameters, and send a null probe.
> > @@ -975,9 +1007,12 @@ void nfsd4_change_callback(struct nfs4_client
> > *clp, struct nfs4_cb_conn *conn)
> >   * If the slot is available, then mark it busy.  Otherwise, set
> > the
> >   * thread for sleeping on the callback RPC wait queue.
> >   */
> > -static bool nfsd41_cb_get_slot(struct nfs4_client *clp, struct
> > rpc_task *task)
> > +static bool nfsd41_cb_get_slot(struct nfsd4_callback *cb, struct
> > rpc_task *task)
> >  {
> > -	if (test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
> > +	struct nfs4_client *clp = cb->cb_clp;
> > +
> > +	if (!cb->cb_holds_slot &&
> > +	    test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
> >  		rpc_sleep_on(&clp->cl_cb_waitq, task, NULL);
> >  		/* Race breaker */
> >  		if (test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
> > @@ -985,10 +1020,32 @@ static bool nfsd41_cb_get_slot(struct
> > nfs4_client *clp, struct rpc_task *task)
> >  			return false;
> >  		}
> >  		rpc_wake_up_queued_task(&clp->cl_cb_waitq, task);
> > +		cb->cb_holds_slot = true;
> >  	}
> >  	return true;
> >  }
> >  
> > +static void nfsd41_cb_release_slot(struct nfsd4_callback *cb)
> > +{
> > +	struct nfs4_client *clp = cb->cb_clp;
> > +
> > +	if (cb->cb_holds_slot) {
> > +		cb->cb_holds_slot = false;
> > +		clear_bit(0, &clp->cl_cb_slot_busy);
> > +		rpc_wake_up_next(&clp->cl_cb_waitq);
> > +	}
> > +}
> > +
> > +static void nfsd41_destroy_cb(struct nfsd4_callback *cb)
> > +{
> > +	struct nfs4_client *clp = cb->cb_clp;
> > +
> > +	nfsd41_cb_release_slot(cb);
> > +	if (cb->cb_ops && cb->cb_ops->release)
> > +		cb->cb_ops->release(cb);
> > +	nfsd41_cb_inflight_end(clp);
> > +}
> > +
> >  /*
> >   * TODO: cb_sequence should support referring call lists,
> > cachethis, multiple
> >   * slots, and mark callback channel down on communication errors.
> > @@ -1005,11 +1062,8 @@ static void nfsd4_cb_prepare(struct rpc_task
> > *task, void *calldata)
> >  	 */
> >  	cb->cb_seq_status = 1;
> >  	cb->cb_status = 0;
> > -	if (minorversion) {
> > -		if (!cb->cb_holds_slot && !nfsd41_cb_get_slot(clp,
> > task))
> > -			return;
> > -		cb->cb_holds_slot = true;
> > -	}
> > +	if (minorversion && !nfsd41_cb_get_slot(cb, task))
> > +		return;
> >  	rpc_call_start(task);
> >  }
> >  
> > @@ -1076,9 +1130,7 @@ static bool nfsd4_cb_sequence_done(struct
> > rpc_task *task, struct nfsd4_callback
> >  			cb->cb_seq_status);
> >  	}
> >  
> > -	cb->cb_holds_slot = false;
> > -	clear_bit(0, &clp->cl_cb_slot_busy);
> > -	rpc_wake_up_next(&clp->cl_cb_waitq);
> > +	nfsd41_cb_release_slot(cb);
> >  	dprintk("%s: freed slot, new seqid=%d\n", __func__,
> >  		clp->cl_cb_session->se_cb_seq_nr);
> >  
> > @@ -1091,8 +1143,10 @@ static bool nfsd4_cb_sequence_done(struct
> > rpc_task *task, struct nfsd4_callback
> >  		ret = false;
> >  	goto out;
> >  need_restart:
> > -	task->tk_status = 0;
> > -	cb->cb_need_restart = true;
> > +	if (!test_bit(NFSD4_CLIENT_CB_KILL, &clp->cl_flags)) {
> > +		task->tk_status = 0;
> > +		cb->cb_need_restart = true;
> > +	}
> >  	return false;
> >  }
> >  
> > @@ -1134,9 +1188,9 @@ static void nfsd4_cb_release(void *calldata)
> >  	struct nfsd4_callback *cb = calldata;
> >  
> >  	if (cb->cb_need_restart)
> > -		nfsd4_run_cb(cb);
> > +		nfsd4_queue_cb(cb);
> >  	else
> > -		cb->cb_ops->release(cb);
> > +		nfsd41_destroy_cb(cb);
> >  
> >  }
> >  
> > @@ -1170,6 +1224,7 @@ void nfsd4_shutdown_callback(struct
> > nfs4_client *clp)
> >  	 */
> >  	nfsd4_run_cb(&clp->cl_cb_null);
> >  	flush_workqueue(callback_wq);
> > +	nfsd41_cb_inflight_wait_complete(clp);
> >  }
> >  
> >  /* requires cl_lock: */
> > @@ -1255,8 +1310,7 @@ nfsd4_run_cb_work(struct work_struct *work)
> >  	clnt = clp->cl_cb_client;
> >  	if (!clnt) {
> >  		/* Callback channel broken, or client killed; give up:
> > */
> > -		if (cb->cb_ops && cb->cb_ops->release)
> > -			cb->cb_ops->release(cb);
> > +		nfsd41_destroy_cb(cb);
> >  		return;
> >  	}
> >  
> > @@ -1265,6 +1319,7 @@ nfsd4_run_cb_work(struct work_struct *work)
> >  	 */
> >  	if (!cb->cb_ops && clp->cl_minorversion) {
> >  		clp->cl_cb_state = NFSD4_CB_UP;
> > +		nfsd41_destroy_cb(cb);
> >  		return;
> >  	}
> >  
> > @@ -1290,5 +1345,9 @@ void nfsd4_init_cb(struct nfsd4_callback *cb,
> > struct nfs4_client *clp,
> >  
> >  void nfsd4_run_cb(struct nfsd4_callback *cb)
> >  {
> > -	queue_work(callback_wq, &cb->cb_work);
> > +	struct nfs4_client *clp = cb->cb_clp;
> > +
> > +	nfsd41_cb_inflight_begin(clp);
> > +	if (!nfsd4_queue_cb(cb))
> > +		nfsd41_cb_inflight_end(clp);
> >  }
> > diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
> > index 46f56afb6cb8..d61b83b9654c 100644
> > --- a/fs/nfsd/state.h
> > +++ b/fs/nfsd/state.h
> > @@ -367,6 +367,7 @@ struct nfs4_client {
> >  	struct net		*net;
> >  	struct list_head	async_copies;	/* list of async copies */
> >  	spinlock_t		async_lock;	/* lock for async copies */
> > +	atomic_t		cl_cb_inflight;	/* Outstanding callbacks */
> >  };
> >  
> >  /* struct nfs4_client_reset
> > -- 
> > 2.21.0
> > 
-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com