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Message-ID: <9de70a3e0c3e6ae62811351f6c6fd46a58259d1c.camel@kernel.org>
Subject: Re: [PATCH v2] nfsd: drop st_mutex and rp_mutex before calling
 move_to_close_lru()
From: Jeff Layton <jlayton@kernel.org>
To: NeilBrown <neilb@suse.de>, Chuck Lever <chuck.lever@oracle.com>, "J.
	Bruce Fields" <bfields@fieldses.org>
Cc: Olga Kornievskaia <kolga@netapp.com>, Dai Ngo <Dai.Ngo@oracle.com>, Tom
	Talpey <tom@talpey.com>, linux-nfs@vger.kernel.org
Date: Wed, 28 Feb 2024 14:30:18 -0500
In-Reply-To: <6926f1be34dfb66fc5395a7465c2f3970ac7652a.camel@kernel.org>
References: <170546328406.23031.11217818844350800811@noble.neil.brown.name>
	 <6926f1be34dfb66fc5395a7465c2f3970ac7652a.camel@kernel.org>
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On Wed, 2024-02-28 at 12:40 -0500, Jeff Layton wrote:
> On Wed, 2024-01-17 at 14:48 +1100, NeilBrown wrote:
> > move_to_close_lru() is currently called with ->st_mutex and .rp_mutex h=
eld.
> > This can lead to a deadlock as move_to_close_lru() waits for sc_count t=
o
> > drop to 2, and some threads holding a reference might be waiting for ei=
ther
> > mutex.  These references will never be dropped so sc_count will never
> > reach 2.
> >=20
> > There can be no harm in dropping ->st_mutex to before
> > move_to_close_lru() because the only place that takes the mutex is
> > nfsd4_lock_ol_stateid(), and it quickly aborts if sc_type is
> > NFS4_CLOSED_STID, which it will be before move_to_close_lru() is called=
.
> >=20
> > Similarly dropping .rp_mutex is safe after the state is closed and so
> > no longer usable.  Another way to look at this is that nothing
> > significant happens between when nfsd4_close() now calls
> > nfsd4_cstate_clear_replay(), and where nfsd4_proc_compound calls
> > nfsd4_cstate_clear_replay() a little later.
> >=20
> > See also
> >  https://lore.kernel.org/lkml/4dd1fe21e11344e5969bb112e954affb@jd.com/T=
/
> > where this problem was raised but not successfully resolved.
> >=20
> > Signed-off-by: NeilBrown <neilb@suse.de>
> > ---
> >  fs/nfsd/nfs4state.c | 18 ++++++++++++++----
> >  1 file changed, 14 insertions(+), 4 deletions(-)
> >=20
> > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > index 40415929e2ae..0850191f9920 100644
> > --- a/fs/nfsd/nfs4state.c
> > +++ b/fs/nfsd/nfs4state.c
> > @@ -7055,7 +7055,7 @@ nfsd4_open_downgrade(struct svc_rqst *rqstp,
> >  	return status;
> >  }
> > =20
> > -static void nfsd4_close_open_stateid(struct nfs4_ol_stateid *s)
> > +static bool nfsd4_close_open_stateid(struct nfs4_ol_stateid *s)
> >  {
> >  	struct nfs4_client *clp =3D s->st_stid.sc_client;
> >  	bool unhashed;
> > @@ -7072,11 +7072,11 @@ static void nfsd4_close_open_stateid(struct nfs=
4_ol_stateid *s)
> >  		list_for_each_entry(stp, &reaplist, st_locks)
> >  			nfs4_free_cpntf_statelist(clp->net, &stp->st_stid);
> >  		free_ol_stateid_reaplist(&reaplist);
> > +		return false;
> >  	} else {
> >  		spin_unlock(&clp->cl_lock);
> >  		free_ol_stateid_reaplist(&reaplist);
> > -		if (unhashed)
> > -			move_to_close_lru(s, clp->net);
> > +		return unhashed;
> >  	}
> >  }
> > =20
> > @@ -7092,6 +7092,7 @@ nfsd4_close(struct svc_rqst *rqstp, struct nfsd4_=
compound_state *cstate,
> >  	struct nfs4_ol_stateid *stp;
> >  	struct net *net =3D SVC_NET(rqstp);
> >  	struct nfsd_net *nn =3D net_generic(net, nfsd_net_id);
> > +	bool need_move_to_close_list;
> > =20
> >  	dprintk("NFSD: nfsd4_close on file %pd\n",=20
> >  			cstate->current_fh.fh_dentry);
> > @@ -7114,8 +7115,17 @@ nfsd4_close(struct svc_rqst *rqstp, struct nfsd4=
_compound_state *cstate,
> >  	 */
> >  	nfs4_inc_and_copy_stateid(&close->cl_stateid, &stp->st_stid);
> > =20
> > -	nfsd4_close_open_stateid(stp);
> > +	need_move_to_close_list =3D nfsd4_close_open_stateid(stp);
> >  	mutex_unlock(&stp->st_mutex);
> > +	if (need_move_to_close_list) {
> > +		/* Drop the replay mutex early as move_to_close_lru()
> > +		 * can wait for other threads which hold that mutex.
> > +		 * This call is idempotent, so that fact that it will
> > +		 * be called twice is harmless.
> > +		 */
> > +		nfsd4_cstate_clear_replay(cstate);
> > +		move_to_close_lru(stp, net);
> > +	}
> > =20
> >  	/* v4.1+ suggests that we send a special stateid in here, since the
> >  	 * clients should just ignore this anyway. Since this is not useful
>=20
> There is a recent regression in pynfs test CLOSE12 in Chuck's nfsd-next
> branch. In the attached capture, there is an extra 40 bytes on the end
> of the CLOSE response in frame 112.
>=20
> A bisect landed on this patch, though I don't see the cause just yet.
>=20
> Thoughts?

To follow up with what I found so far:

The problem seems to be with the rp_buflen in nfsd4_encode_replay. With
this patch applied, rp_buflen is 56 bytes when nfsd4_encode_replay is
called. With this patch reverted, it's 16 bytes. That accounts for the
extra 40 bytes.

Still trying to determine why this is occurring though...
--=20
Jeff Layton <jlayton@kernel.org>