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.

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

Replace mod_delayed_work in nfsd4_state_shrinker_count with queue_work.

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 <redacted>
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
---
v2:
  . Change nfsd_shrinker_work from delayed_work to work_struct
  . Replace mod_delayed_work in nfsd4_state_shrinker_count with queue_work
  . Cancel work_struct nfsd_shrinker_work after unregistering shrinker
v3:
  . set nfsd_shrinker_active earlier in nfsd4_state_shrinker_count

 fs/nfsd/netns.h     |  3 ++-
 fs/nfsd/nfs4state.c | 24 +++++++++++++++++++-----
 2 files changed, 21 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..35ec4cba88b3 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c

@@ -4407,11 +4407,22 @@ 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;
+       }
+       nn->nfsd_shrinker_active = true;
        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) {
+               spin_unlock(&nn->client_lock);
+               queue_work(laundry_wq, &nn->nfsd_shrinker_work);
+       } else {
+               nn->nfsd_shrinker_active = false;
+               spin_unlock(&nn->client_lock);
+       }
        return (unsigned long)count;
 }
 

@@ -6233,12 +6244,14 @@ 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 = false;
+       spin_unlock(&nn->client_lock);
 }
 
 static inline __be32 nfs4_check_fh(struct svc_fh *fhp, struct nfs4_stid *stp)

@@ -8064,7 +8077,7 @@ 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 +8184,7 @@ 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);