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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2023-01-10 at 18:53 +0000, Chuck Lever III wrote:
> 
> > On Jan 10, 2023, at 1:46 PM, Dai Ngo <dai.ngo@oracle.com> wrote:
> > 
> > 
> > On 1/10/23 10:17 AM, Chuck Lever III wrote:
> > > 
> > > > On Jan 10, 2023, at 12:33 PM, Dai Ngo <dai.ngo@oracle.com> wrote:
> > > > 
> > > > 
> > > > 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@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.
> > > > 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.
> > > The delay argument of zero is interesting. If it's set to a value
> > > greater than zero, do you still see a problem?
> > 
> > I tried and tried but could not reproduce the problem that Mike
> > reported. I guess my VMs don't have fast enough cpus to make it
> > happen.
> 
> I'd prefer not to guess... it sounds like we don't have a clear
> root cause on this one yet.
> 
> I think I agree with Jeff: a spinlock shouldn't be required to
> make queuing work safe via this API.
> 
> 
> > As Jeff mentioned, delay 0 should be safe and we want to run
> > the shrinker as soon as possible when memory is low.
> 
> I suggested that because the !delay code paths seem to lead
> directly to the WARN_ONs in queue_work(). <shrug>
> 
> 


One of the WARNs in that Mike hit was this:

        WARN_ON_ONCE(timer->function != delayed_work_timer_fn);

nfsd isn't doing anything exotic with that function pointer, so that
really looks like something got corrupted. Given that this is happening
under low-memory conditions, then I have to wonder if we're just ending
up with a workqueue job that remained on the queue after the nfsd_net
got freed and recycled.

I'd start with a patch like this (note, untested):

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 2f4a2449b314..86da6663806e 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -8158,6 +8158,7 @@ nfs4_state_shutdown_net(struct net *net)
        struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 
        unregister_shrinker(&nn->nfsd_client_shrinker);
+       cancel_delayed_work_sync(&nn->nfsd_shrinker_work);
        cancel_delayed_work_sync(&nn->laundromat_work);
        locks_end_grace(&nn->nfsd4_manager);


Either way, I think longer nfsd_shrinker_work ought to be converted to a
normal work_struct since you don't ever use the delay.

> > -Dai
> > 
> > > 
> > > 
> > > > 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)
> > > --
> > > Chuck Lever
> 
> --
> Chuck Lever
> 
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>