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From:   "NeilBrown" <neilb@suse.de>
To:     "Jeff Layton" <jlayton@kernel.org>
Cc:     "Chuck Lever" <chuck.lever@oracle.com>, linux-nfs@vger.kernel.org,
        "Olga Kornievskaia" <kolga@netapp.com>,
        "Dai Ngo" <Dai.Ngo@oracle.com>, "Tom Talpey" <tom@talpey.com>
Subject: Re: [PATCH 1/9] nfsd: hold ->cl_lock for hash_delegation_locked()
In-reply-to: <40e1bf417c635ce303f9e42ddb0e3dbd90022477.camel@kernel.org>
References: <20231117022121.23310-1-neilb@suse.de>,
 <20231117022121.23310-2-neilb@suse.de>,
 <40e1bf417c635ce303f9e42ddb0e3dbd90022477.camel@kernel.org>
Date:   Mon, 20 Nov 2023 08:37:55 +1100
Message-id: <170042987584.19300.7721851585544522693@noble.neil.brown.name>
Precedence: bulk

On Fri, 17 Nov 2023, Jeff Layton wrote:
> On Fri, 2023-11-17 at 13:18 +1100, NeilBrown wrote:
> > The protocol for creating a new state in nfsd is to allocated the state
> > leaving it largely uninitialised, add that state to the ->cl_stateids
> > idr so as to reserve a state id, then complete initialisation of the
> > state and only set ->sc_type to non-zero once the state is fully
> > initialised.
> >=20
> > If a state is found in the idr with ->sc_type =3D=3D 0, it is ignored.
> > The ->cl_lock list is used to avoid races - it is held while checking
> > sc_type during lookup, and held when a non-zero value is stored in
> > ->sc_type.
> >=20
> > ... except... hash_delegation_locked() finalises the initialisation of a
> > delegation state, but does NOT hold ->cl_lock.
> >=20
> > So this patch takes ->cl_lock at the appropriate time w.r.t other locks,
> > and so ensures there are no races (which are extremely unlikely in any
> > case).
> >=20
> > Signed-off-by: NeilBrown <neilb@suse.de>
> > ---
> >  fs/nfsd/nfs4state.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >=20
> > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > index 65fd5510323a..6368788a7d4e 100644
> > --- a/fs/nfsd/nfs4state.c
> > +++ b/fs/nfsd/nfs4state.c
> > @@ -1317,6 +1317,7 @@ hash_delegation_locked(struct nfs4_delegation *dp, =
struct nfs4_file *fp)
> > =20
> >  	lockdep_assert_held(&state_lock);
> >  	lockdep_assert_held(&fp->fi_lock);
> > +	lockdep_assert_held(&clp->cl_lock);
> > =20
> >  	if (nfs4_delegation_exists(clp, fp))
> >  		return -EAGAIN;
> > @@ -5609,12 +5610,14 @@ nfs4_set_delegation(struct nfsd4_open *open, stru=
ct nfs4_ol_stateid *stp,
> >  		goto out_unlock;
> > =20
> >  	spin_lock(&state_lock);
> > +	spin_lock(&clp->cl_lock);
> >  	spin_lock(&fp->fi_lock);
> >  	if (fp->fi_had_conflict)
> >  		status =3D -EAGAIN;
> >  	else
> >  		status =3D hash_delegation_locked(dp, fp);
> >  	spin_unlock(&fp->fi_lock);
> > +	spin_unlock(&clp->cl_lock);
> >  	spin_unlock(&state_lock);
> > =20
> >  	if (status)
>=20
> I know it's (supposedly) an unlikely race, but should we send this to
> stable?

I don't know.  Once upon a time, "stable" meant something.  There was a
clear list of rules.  Those seem to have been torn up.  Now it seems to
be whatever some machine-learning tool chooses.
If that tool chooses this patch (which I suspect it will), I won't
object.  But I don't think it is worth encouraging it.


>=20
> Reviewed-by: Jeff Layton <jlayton@kernel.org>
>=20
Thanks,

NelBrown