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

[Jeff Layton <jlayton@kernel.org>]

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@gmx.de>
> Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
> ---
>  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.

> 
>  	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)

-- 
Jeff Layton <jlayton@kernel.org>