[PATCH 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 with queue_delayed_work since we
don't expect to modify the delay of any pending work.

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>
---
 fs/nfsd/netns.h     |  1 +
 fs/nfsd/nfs4state.c | 16 ++++++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
index 8c854ba3285b..801d70926442 100644
--- a/fs/nfsd/netns.h
+++ b/fs/nfsd/netns.h
@@ -196,6 +196,7 @@ struct nfsd_net {
 	atomic_t		nfsd_courtesy_clients;
 	struct shrinker		nfsd_client_shrinker;
 	struct delayed_work	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 ee56c9466304..e00551af6a11 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_delayed_work(laundry_wq, &nn->nfsd_shrinker_work, 0);
+	} else
+		spin_unlock(&nn->client_lock);
 	return (unsigned long)count;
 }
 
@@ -6239,6 +6248,9 @@ nfsd4_state_shrinker_worker(struct work_struct *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)
-- 
2.9.5