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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 2/5] svc: don't hold reference for poolstats, only mutex.
In-reply-to: <84354fd30d4b4a162b008067ad4e0d35a7d223da.camel@kernel.org>
References: <20231030011247.9794-1-neilb@suse.de>,
 <20231030011247.9794-3-neilb@suse.de>,
 <84354fd30d4b4a162b008067ad4e0d35a7d223da.camel@kernel.org>
Date:   Tue, 31 Oct 2023 08:48:56 +1100
Message-id: <169870253672.24305.8926736855317017757@noble.neil.brown.name>
Precedence: bulk

On Tue, 31 Oct 2023, Jeff Layton wrote:
> On Mon, 2023-10-30 at 12:08 +1100, NeilBrown wrote:
> > A future patch will remove refcounting on svc_serv as it is of little
> > use.
> > It is currently used to keep the svc around while the pool_stats file is
> > open.
> > Change this to get the pointer, protected by the mutex, only in
> > seq_start, and the release the mutex in seq_stop.
> > This means that if the nfsd server is stopped and restarted while the
> > pool_stats file it open, then some pool stats info could be from the
> > first instance and some from the second.  This might appear odd, but is
> > unlikely to be a problem in practice.
> >=20
> > Signed-off-by: NeilBrown <neilb@suse.de>
> > ---
> >  fs/nfsd/nfsctl.c           |  2 +-
> >  fs/nfsd/nfssvc.c           | 30 ++++++++---------------
> >  include/linux/sunrpc/svc.h |  5 +++-
> >  net/sunrpc/svc_xprt.c      | 49 ++++++++++++++++++++++++++++++++------
> >  4 files changed, 57 insertions(+), 29 deletions(-)
> >=20
> > diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> > index 79efb1075f38..d78ae4452946 100644
> > --- a/fs/nfsd/nfsctl.c
> > +++ b/fs/nfsd/nfsctl.c
> > @@ -179,7 +179,7 @@ static const struct file_operations pool_stats_operat=
ions =3D {
> >  	.open		=3D nfsd_pool_stats_open,
> >  	.read		=3D seq_read,
> >  	.llseek		=3D seq_lseek,
> > -	.release	=3D nfsd_pool_stats_release,
> > +	.release	=3D svc_pool_stats_release,
> >  };
> > =20
> >  DEFINE_SHOW_ATTRIBUTE(nfsd_reply_cache_stats);
> > diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> > index 6c968c02cc29..203e1cfc1cad 100644
> > --- a/fs/nfsd/nfssvc.c
> > +++ b/fs/nfsd/nfssvc.c
> > @@ -1072,30 +1072,20 @@ bool nfssvc_encode_voidres(struct svc_rqst *rqstp=
, struct xdr_stream *xdr)
> >  	return true;
> >  }
> > =20
> > -int nfsd_pool_stats_open(struct inode *inode, struct file *file)
> > +static struct svc_serv *nfsd_get_serv(struct seq_file *s, bool start)
> >  {
> > -	int ret;
> > -	struct nfsd_net *nn =3D net_generic(inode->i_sb->s_fs_info, nfsd_net_id=
);
> > -
> > -	mutex_lock(&nfsd_mutex);
> > -	if (nn->nfsd_serv =3D=3D NULL) {
> > +	struct nfsd_net *nn =3D net_generic(file_inode(s->file)->i_sb->s_fs_inf=
o,
> > +					  nfsd_net_id);
> > +	if (start) {
> > +		mutex_lock(&nfsd_mutex);
> > +		return nn->nfsd_serv;
> > +	} else {
> >  		mutex_unlock(&nfsd_mutex);
> > -		return -ENODEV;
> > +		return NULL;
> >  	}
> > -	svc_get(nn->nfsd_serv);
> > -	ret =3D svc_pool_stats_open(nn->nfsd_serv, file);
> > -	mutex_unlock(&nfsd_mutex);
> > -	return ret;
> >  }
> > =20
> > -int nfsd_pool_stats_release(struct inode *inode, struct file *file)
> > +int nfsd_pool_stats_open(struct inode *inode, struct file *file)
> >  {
> > -	struct seq_file *seq =3D file->private_data;
> > -	struct svc_serv *serv =3D seq->private;
> > -	int ret =3D seq_release(inode, file);
> > -
> > -	mutex_lock(&nfsd_mutex);
> > -	svc_put(serv);
> > -	mutex_unlock(&nfsd_mutex);
> > -	return ret;
> > +	return svc_pool_stats_open(nfsd_get_serv, file);
> >  }
> > diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> > index b10f987509cc..11acad6988a2 100644
> > --- a/include/linux/sunrpc/svc.h
> > +++ b/include/linux/sunrpc/svc.h
> > @@ -433,7 +433,10 @@ void		   svc_exit_thread(struct svc_rqst *);
> >  struct svc_serv *  svc_create_pooled(struct svc_program *, unsigned int,
> >  				     int (*threadfn)(void *data));
> >  int		   svc_set_num_threads(struct svc_serv *, struct svc_pool *, int);
> > -int		   svc_pool_stats_open(struct svc_serv *serv, struct file *file);
> > +int		   svc_pool_stats_open(struct svc_serv *(*get_serv)(struct seq_file=
 *, bool),
> > +				       struct file *file);
> > +int		   svc_pool_stats_release(struct inode *inode,
> > +					  struct file *file);
> >  void		   svc_process(struct svc_rqst *rqstp);
> >  void		   svc_process_bc(struct rpc_rqst *req, struct svc_rqst *rqstp);
> >  int		   svc_register(const struct svc_serv *, struct net *, const int,
> > diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> > index fee83d1024bc..2f99f7475b7b 100644
> > --- a/net/sunrpc/svc_xprt.c
> > +++ b/net/sunrpc/svc_xprt.c
> > @@ -1366,26 +1366,38 @@ EXPORT_SYMBOL_GPL(svc_xprt_names);
> > =20
> >  /*----------------------------------------------------------------------=
------*/
> > =20
> > +struct pool_private {
> > +	struct svc_serv *(*get_serv)(struct seq_file *, bool);
>=20
> This bool is pretty ugly. I think I'd rather see two operations here
> (get_serv/put_serv). Also, this could use a kerneldoc comment.

I agree that bool is ugly, but two function pointers as function args
seemed ugly, and stashing them in 'struct svc_serv' seemed ugly.
So I picked one.  I'd be keen to find an approach that didn't require a
function pointer.

Maybe sunrpc could declare

   struct svc_ref {
         struct mutex mutex;
         struct svc_serv *serv;
   }

and nfsd could use one of those instead of nfsd_mutex and nfsd_serv, and
pass a pointer to it to the open function.

But then the mutex would have to be in the per-net structure.  And maybe
that isn't a bad idea, but it is a change...

I guess I could pass pointers to nfsd_mutex and nn->nfsd_serv to the
open function....

Any other ideas?

Thanks,
NeilBrown

>=20
> > +	struct svc_serv *serv;
> > +};
> > +
> >  static void *svc_pool_stats_start(struct seq_file *m, loff_t *pos)
> >  {
> >  	unsigned int pidx =3D (unsigned int)*pos;
> > -	struct svc_serv *serv =3D m->private;
> > +	struct pool_private *pp =3D m->private;
> > =20
> >  	dprintk("svc_pool_stats_start, *pidx=3D%u\n", pidx);
> > =20
> > +	pp->serv =3D pp->get_serv(m, true);
> > +
> >  	if (!pidx)
> >  		return SEQ_START_TOKEN;
> > -	return (pidx > serv->sv_nrpools ? NULL : &serv->sv_pools[pidx-1]);
> > +	if (!pp->serv)
> > +		return NULL;
> > +	return (pidx > pp->serv->sv_nrpools ? NULL : &pp->serv->sv_pools[pidx-1=
]);
> >  }
> > =20
> >  static void *svc_pool_stats_next(struct seq_file *m, void *p, loff_t *po=
s)
> >  {
> >  	struct svc_pool *pool =3D p;
> > -	struct svc_serv *serv =3D m->private;
> > +	struct pool_private *pp =3D m->private;
> > +	struct svc_serv *serv =3D pp->serv;
> > =20
> >  	dprintk("svc_pool_stats_next, *pos=3D%llu\n", *pos);
> > =20
> > -	if (p =3D=3D SEQ_START_TOKEN) {
> > +	if (!serv) {
> > +		pool =3D NULL;
> > +	} else if (p =3D=3D SEQ_START_TOKEN) {
> >  		pool =3D &serv->sv_pools[0];
> >  	} else {
> >  		unsigned int pidx =3D (pool - &serv->sv_pools[0]);
> > @@ -1400,6 +1412,9 @@ static void *svc_pool_stats_next(struct seq_file *m=
, void *p, loff_t *pos)
> > =20
> >  static void svc_pool_stats_stop(struct seq_file *m, void *p)
> >  {
> > +	struct pool_private *pp =3D m->private;
> > +
> > +	pp->get_serv(m, false);
> >  }
> > =20
> >  static int svc_pool_stats_show(struct seq_file *m, void *p)
> > @@ -1427,15 +1442,35 @@ static const struct seq_operations svc_pool_stats=
_seq_ops =3D {
> >  	.show	=3D svc_pool_stats_show,
> >  };
> > =20
> > -int svc_pool_stats_open(struct svc_serv *serv, struct file *file)
> > +int svc_pool_stats_open(struct svc_serv *(*get_serv)(struct seq_file *, =
bool),
> > +			struct file *file)
> >  {
> > +	struct pool_private *pp;
> >  	int err;
> > =20
> > +	pp =3D kmalloc(sizeof(*pp), GFP_KERNEL);
> > +	if (!pp)
> > +		return -ENOMEM;
> > +
> >  	err =3D seq_open(file, &svc_pool_stats_seq_ops);
> > -	if (!err)
> > -		((struct seq_file *) file->private_data)->private =3D serv;
> > +	if (!err) {
> > +		pp->get_serv =3D get_serv;
> > +		((struct seq_file *) file->private_data)->private =3D pp;
> > +	} else
> > +		kfree(pp);
> > +
> >  	return err;
> >  }
> >  EXPORT_SYMBOL(svc_pool_stats_open);
> > =20
> > +int svc_pool_stats_release(struct inode *inode, struct file *file)
> > +{
> > +	struct seq_file *seq =3D file->private_data;
> > +
> > +	kfree(seq->private);
> > +	seq->private =3D NULL;
> > +	return seq_release(inode, file);
> > +}
> > +EXPORT_SYMBOL(svc_pool_stats_release);
> > +
> >  /*----------------------------------------------------------------------=
------*/
>=20
> --=20
> Jeff Layton <jlayton@kernel.org>
>=20