[PATCH 3/6] nfs_localio: simplify interface to nfsd for getting nfsd_file

NeilBrown
2025-05-09T00:49:15Z

The nfsd_localio_operations structure contains nfsd_file_get() to get a
reference to an nfsd_file. This is only used in one place, where
nfsd_open_local_fh() is also used.

This patch combines the two, calling nfsd_open_local_fh() passing a
pointer to where the nfsd_file pointer might be stored. If there is a
pointer there an nfsd_file_get() can get a reference, that reference is
returned. If not a new nfsd_file is acquired, stored at the pointer,
and returned. When we store a reference we also increase the refcount
on the net, as that refcount is decrements when we clear the stored
pointer.

We now get an extra reference *before* storing the new nfsd_file at the
given location. This avoids possible races with the nfsd_file being
freed before the final reference can be taken.

This patch moves the rcu_dereference() needed after fetching from
ro_file or rw_file into the nfsd code where the 'struct nfs_file' is
fully defined. This avoids an error reported by older versions of gcc
such as gcc-8 which complain about rcu_dereference() use in contexts
where the structure (which will supposedly be accessed) is not fully
defined.

Reported-by: Pali Rohár <pali@kernel.org>
Reported-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Fixes: 86e00412254a ("nfs: cache all open LOCALIO nfsd_file(s) in client")
Signed-off-by: NeilBrown <neil@brown.name>
---
fs/nfs/localio.c | 31 ++++--------------
fs/nfs_common/nfslocalio.c | 3 +-
fs/nfsd/localio.c | 65 +++++++++++++++++++++++++++-----------
include/linux/nfslocalio.h | 6 ++--
4 files changed, 57 insertions(+), 48 deletions(-)

diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
index 27d5fff9747b..030a54c8c9d8 100644
--- a/fs/nfs/localio.c
+++ b/fs/nfs/localio.c
@@ -207,11 +207,6 @@ void nfs_local_probe_async(struct nfs_client *clp)
}
EXPORT_SYMBOL_GPL(nfs_local_probe_async);

-static inline struct nfsd_file *nfs_local_file_get(struct nfsd_file *nf)
-{
- return nfs_to->nfsd_file_get_local(nf);
-}
-
static inline void nfs_local_file_put(struct nfsd_file *nf)
{
nfs_to_nfsd_file_put_local(nf);
@@ -226,12 +221,13 @@ static inline void nfs_local_file_put(struct nfsd_file *nf)
static struct nfsd_file *
__nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
struct nfs_fh *fh, struct nfs_file_localio *nfl,
+ struct nfsd_file __rcu **pnf,
const fmode_t mode)
{
struct nfsd_file *localio;

localio = nfs_open_local_fh(&clp->cl_uuid, clp->cl_rpcclient,
- cred, fh, nfl, mode);
+ cred, fh, nfl, pnf, mode);
if (IS_ERR(localio)) {
int status = PTR_ERR(localio);
trace_nfs_local_open_fh(fh, mode, status);
@@ -258,7 +254,7 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
struct nfs_fh *fh, struct nfs_file_localio *nfl,
const fmode_t mode)
{
- struct nfsd_file *nf, *new, __rcu **pnf;
+ struct nfsd_file *nf, __rcu **pnf;

if (!nfs_server_is_local(clp))
return NULL;
@@ -270,24 +266,9 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
else
pnf = &nfl->ro_file;

- new = NULL;
- rcu_read_lock();
- nf = rcu_dereference(*pnf);
- if (!nf) {
- rcu_read_unlock();
- new = __nfs_local_open_fh(clp, cred, fh, nfl, mode);
- if (IS_ERR(new))
- return NULL;
- /* try to swap in the pointer */
- nf = unrcu_pointer(cmpxchg(pnf, NULL, RCU_INITIALIZER(new)));
- if (!nf)
- swap(nf, new);
- rcu_read_lock();
- }
- nf = nfs_local_file_get(nf);
- rcu_read_unlock();
- if (new)
- nfs_to_nfsd_file_put_local(new);
+ nf = __nfs_local_open_fh(clp, cred, fh, nfl, pnf, mode);
+ if (IS_ERR(nf))
+ return NULL;
return nf;
}
EXPORT_SYMBOL_GPL(nfs_local_open_fh);
diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c
index f6821b2c87a2..503f85f64b76 100644
--- a/fs/nfs_common/nfslocalio.c
+++ b/fs/nfs_common/nfslocalio.c
@@ -237,6 +237,7 @@ static void nfs_uuid_add_file(nfs_uuid_t *nfs_uuid, struct nfs_file_localio *nfl
struct nfsd_file *nfs_open_local_fh(nfs_uuid_t *uuid,
struct rpc_clnt *rpc_clnt, const struct cred *cred,
const struct nfs_fh *nfs_fh, struct nfs_file_localio *nfl,
+ struct nfsd_file __rcu **pnf,
const fmode_t fmode)
{
struct net *net;
@@ -261,7 +262,7 @@ struct nfsd_file *nfs_open_local_fh(nfs_uuid_t *uuid,
rcu_read_unlock();
/* We have an implied reference to net thanks to nfsd_net_try_get */
localio = nfs_to->nfsd_open_local_fh(net, uuid->dom, rpc_clnt,
- cred, nfs_fh, fmode);
+ cred, nfs_fh, pnf, fmode);
nfs_to_nfsd_net_put(net);
if (!IS_ERR(localio))
nfs_uuid_add_file(uuid, nfl);
diff --git a/fs/nfsd/localio.c b/fs/nfsd/localio.c
index 2c0afd1067ea..80d9ff6608a7 100644
--- a/fs/nfsd/localio.c
+++ b/fs/nfsd/localio.c
@@ -24,20 +24,6 @@
#include "filecache.h"
#include "cache.h"

-static const struct nfsd_localio_operations nfsd_localio_ops = {
- .nfsd_net_try_get = nfsd_net_try_get,
- .nfsd_net_put = nfsd_net_put,
- .nfsd_open_local_fh = nfsd_open_local_fh,
- .nfsd_file_put_local = nfsd_file_put_local,
- .nfsd_file_get_local = nfsd_file_get_local,
- .nfsd_file_file = nfsd_file_file,
-};
-
-void nfsd_localio_ops_init(void)
-{
- nfs_to = &nfsd_localio_ops;
-}
-
/**
* nfsd_open_local_fh - lookup a local filehandle @nfs_fh and map to nfsd_file
*
@@ -46,6 +32,7 @@ void nfsd_localio_ops_init(void)
* @rpc_clnt: rpc_clnt that the client established
* @cred: cred that the client established
* @nfs_fh: filehandle to lookup
+ * @nfp: place to find the nfsd_file, or store it if it was non-NULL
* @fmode: fmode_t to use for open
*
* This function maps a local fh to a path on a local filesystem.
@@ -56,10 +43,11 @@ void nfsd_localio_ops_init(void)
* set. Caller (NFS client) is responsible for calling nfsd_net_put and
* nfsd_file_put (via nfs_to_nfsd_file_put_local).
*/
-struct nfsd_file *
+static struct nfsd_file *
nfsd_open_local_fh(struct net *net, struct auth_domain *dom,
struct rpc_clnt *rpc_clnt, const struct cred *cred,
- const struct nfs_fh *nfs_fh, const fmode_t fmode)
+ const struct nfs_fh *nfs_fh, struct nfsd_file __rcu **pnf,
+ const fmode_t fmode)
{
int mayflags = NFSD_MAY_LOCALIO;
struct svc_cred rq_cred;
@@ -73,6 +61,12 @@ nfsd_open_local_fh(struct net *net, struct auth_domain *dom,
if (!nfsd_net_try_get(net))
return ERR_PTR(-ENXIO);

+ rcu_read_lock();
+ localio = nfsd_file_get(rcu_dereference(*pnf));
+ rcu_read_unlock();
+ if (localio)
+ return localio;
+
/* nfs_fh -> svc_fh */
fh_init(&fh, NFS4_FHSIZE);
fh.fh_handle.fh_size = nfs_fh->size;
@@ -94,12 +88,47 @@ nfsd_open_local_fh(struct net *net, struct auth_domain *dom,
if (rq_cred.cr_group_info)
put_group_info(rq_cred.cr_group_info);

- if (IS_ERR(localio))
+ if (!IS_ERR(localio)) {
+ struct nfsd_file *new;
+ if (!nfsd_net_try_get(net)) {
+ nfsd_file_put(localio);
+ nfsd_net_put(net);
+ return ERR_PTR(-ENXIO);
+ }
+ nfsd_file_get(localio);
+ again:
+ new = unrcu_pointer(cmpxchg(pnf, NULL, RCU_INITIALIZER(localio)));
+ if (new) {
+ /* Some other thread installed an nfsd_file */
+ if (nfsd_file_get(new) == NULL)
+ goto again;
+ /*
+ * Drop the ref we were going to install and the
+ * one we were going to return.
+ */
+ nfsd_file_put(localio);
+ nfsd_file_put(localio);
+ localio = new;
+ }
+ } else
nfsd_net_put(net);

return localio;
}
-EXPORT_SYMBOL_GPL(nfsd_open_local_fh);
+
+static const struct nfsd_localio_operations nfsd_localio_ops = {
+ .nfsd_net_try_get = nfsd_net_try_get,
+ .nfsd_net_put = nfsd_net_put,
+ .nfsd_open_local_fh = nfsd_open_local_fh,
+ .nfsd_file_put_local = nfsd_file_put_local,
+ .nfsd_file_get_local = nfsd_file_get_local,
+ .nfsd_file_file = nfsd_file_file,
+};
+
+void nfsd_localio_ops_init(void)
+{
+ nfs_to = &nfsd_localio_ops;
+}

/*
* UUID_IS_LOCAL XDR functions
diff --git a/include/linux/nfslocalio.h b/include/linux/nfslocalio.h
index c3f34bae60e1..e6cd6ec447f5 100644
--- a/include/linux/nfslocalio.h
+++ b/include/linux/nfslocalio.h
@@ -50,10 +50,6 @@ void nfs_localio_invalidate_clients(struct list_head *nn_local_clients,
spinlock_t *nn_local_clients_lock);

/* localio needs to map filehandle -> struct nfsd_file */
-extern struct nfsd_file *
-nfsd_open_local_fh(struct net *, struct auth_domain *, struct rpc_clnt *,
- const struct cred *, const struct nfs_fh *,
- const fmode_t) __must_hold(rcu);
void nfs_close_local_fh(struct nfs_file_localio *);

struct nfsd_localio_operations {
@@ -64,6 +60,7 @@ struct nfsd_localio_operations {
struct rpc_clnt *,
const struct cred *,
const struct nfs_fh *,
+ struct nfsd_file __rcu **pnf,
const fmode_t);
struct net *(*nfsd_file_put_local)(struct nfsd_file *);
struct nfsd_file *(*nfsd_file_get_local)(struct nfsd_file *);
@@ -76,6 +73,7 @@ extern const struct nfsd_localio_operations *nfs_to;
struct nfsd_file *nfs_open_local_fh(nfs_uuid_t *,
struct rpc_clnt *, const struct cred *,
const struct nfs_fh *, struct nfs_file_localio *,
+ struct nfsd_file __rcu **pnf,
const fmode_t);

static inline void nfs_to_nfsd_net_put(struct net *net)
--
2.49.0