nfs_localio: simplify interface to nfsd for getting nfsd_file

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: 86e0041 ("nfs: cache all open LOCALIO nfsd_file(s) in client")
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

Author: neilbrown

fs/nfs/localio.c

@@ -209,11 +209,6 @@ 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);
@@ -228,12 +223,13 @@ 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);
@@ -260,7 +256,7 @@ 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;
@@ -272,24 +268,9 @@ 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;
-		rcu_read_lock();
-		/* try to swap in the pointer */
-		nf = unrcu_pointer(cmpxchg(pnf, NULL, RCU_INITIALIZER(new)));
-		if (!nf)
-			swap(nf, new);
-	}
-	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);

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);

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