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

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On 1/10/23 2:30 AM, Jeff Layton wrote:
On Mon, 2023-01-09 at 22:48 -0800, Dai Ngo wrote:
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;
+	}
Is this extra machinery really necessary? The bool and spinlock don't
seem to be needed. Typically there is no issue with calling
queued_delayed_work when the work is already queued. It just returns
false in that case without doing anything.

When there are multiple calls to mod_delayed_work/queue_delayed_work
we hit the WARN_ON_ONCE's in __queue_delayed_work and __queue_work if
the work is queued but not execute yet.

This problem was reported by Mike. I initially tried with only the
bool but that was not enough that was why the spinlock was added.
Mike verified that the patch fixed the problem.

-Dai


  	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)



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