[PATCH v2 1/1] NFSD: fix WARN_ON_ONCE in __queue_delayed_work

[Dai Ngo <dai.ngo@xxxxxxxxxx>]

Currently nfsd4_state_shrinker_worker can be schduled multiple times
from nfsd4_state_shrinker_count when memory is low. This causes
the WARN_ON_ONCE in __queue_delayed_work to trigger.

This patch allows only one instance of nfsd4_state_shrinker_worker
at a time using the nfsd_shrinker_active flag, protected by the
client_lock.

Replace mod_delayed_work in nfsd4_state_shrinker_count with queue_work.

Change nfsd_shrinker_work from delayed_work to work_struct since we
don't use the delay.

Cancel work_struct nfsd_shrinker_work after unregistering shrinker
in nfs4_state_shutdown_net

Fixes: 44df6f439a17 ("NFSD: add delegation reaper to react to low memory condition")
Reported-by: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Dai Ngo <dai.ngo@xxxxxxxxxx>
---
v2:
  . Replace mod_delayed_work in nfsd4_state_shrinker_count with queue_work
  . Change nfsd_shrinker_work from delayed_work to work_struct
  . Cancel work_struct nfsd_shrinker_work after unregistering shrinker

 fs/nfsd/netns.h     |  3 ++-
 fs/nfsd/nfs4state.c | 22 +++++++++++++++++-----
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
index 8c854ba3285b..b0c7b657324b 100644
--- a/fs/nfsd/netns.h
+++ b/fs/nfsd/netns.h
@@ -195,7 +195,8 @@ struct nfsd_net {
 
 	atomic_t		nfsd_courtesy_clients;
 	struct shrinker		nfsd_client_shrinker;
-	struct delayed_work	nfsd_shrinker_work;
+	struct work_struct	nfsd_shrinker_work;
+	bool			nfsd_shrinker_active;
 };
 
 /* Simple check to find out if a given net was properly initialized */
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index a7cfefd7c205..6508f9c79315 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -4407,11 +4407,20 @@ nfsd4_state_shrinker_count(struct shrinker *shrink, struct shrink_control *sc)
 	struct nfsd_net *nn = container_of(shrink,
 			struct nfsd_net, nfsd_client_shrinker);
 
+	spin_lock(&nn->client_lock);
+	if (nn->nfsd_shrinker_active) {
+		spin_unlock(&nn->client_lock);
+		return 0;
+	}
 	count = atomic_read(&nn->nfsd_courtesy_clients);
 	if (!count)
 		count = atomic_long_read(&num_delegations);
-	if (count)
-		mod_delayed_work(laundry_wq, &nn->nfsd_shrinker_work, 0);
+	if (count) {
+		nn->nfsd_shrinker_active = true;
+		spin_unlock(&nn->client_lock);
+		queue_work(laundry_wq, &nn->nfsd_shrinker_work);
+	} else
+		spin_unlock(&nn->client_lock);
 	return (unsigned long)count;
 }
 
@@ -6233,12 +6242,14 @@ deleg_reaper(struct nfsd_net *nn)
 static void
 nfsd4_state_shrinker_worker(struct work_struct *work)
 {
-	struct delayed_work *dwork = to_delayed_work(work);
-	struct nfsd_net *nn = container_of(dwork, struct nfsd_net,
+	struct nfsd_net *nn = container_of(work, struct nfsd_net,
 				nfsd_shrinker_work);
 
 	courtesy_client_reaper(nn);
 	deleg_reaper(nn);
+	spin_lock(&nn->client_lock);
+	nn->nfsd_shrinker_active = 0;
+	spin_unlock(&nn->client_lock);
 }
 
 static inline __be32 nfs4_check_fh(struct svc_fh *fhp, struct nfs4_stid *stp)
@@ -8064,7 +8075,7 @@ static int nfs4_state_create_net(struct net *net)
 	INIT_LIST_HEAD(&nn->blocked_locks_lru);
 
 	INIT_DELAYED_WORK(&nn->laundromat_work, laundromat_main);
-	INIT_DELAYED_WORK(&nn->nfsd_shrinker_work, nfsd4_state_shrinker_worker);
+	INIT_WORK(&nn->nfsd_shrinker_work, nfsd4_state_shrinker_worker);
 	get_net(net);
 
 	nn->nfsd_client_shrinker.scan_objects = nfsd4_state_shrinker_scan;
@@ -8171,6 +8182,7 @@ nfs4_state_shutdown_net(struct net *net)
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 
 	unregister_shrinker(&nn->nfsd_client_shrinker);
+	cancel_work(&nn->nfsd_shrinker_work);
 	cancel_delayed_work_sync(&nn->laundromat_work);
 	locks_end_grace(&nn->nfsd4_manager);
 
-- 
2.9.5