nfs_localio: use cmpxchg() to install new nfs_file_localio

Rather than using nfs_uuid.lock to protect installing
a new ro_file or rw_file, change to use cmpxchg().
Removing the file already uses xchg() so this improves symmetry
and also makes the code a little simpler.

Also remove the optimisation of not taking the lock, and not removing
the nfs_file_localio from the linked list, when both ->ro_file and
->rw_file are already NULL.  Given that ->nfs_uuid was not NULL, it is
extremely unlikely that neither ->ro_file or ->rw_file is NULL so
this optimisation can be of little value and it complicates
understanding of the code - why can the list_del_init() be skipped?

Finally, move the assignment of NULL to ->nfs_uuid until after
the last action on the nfs_file_localio (the list_del_init).  As soon as
this is NULL a racing nfs_close_local_fh() can bypass all the locking
and go on to free the nfs_file_localio, so we must be certain to be
finished with it first.

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

@@ -282,14 +282,9 @@ nfs_local_open_fh(struct nfs_client *clp, const struct cred *cred,
 			return NULL;
 		rcu_read_lock();
 		/* try to swap in the pointer */
-		spin_lock(&clp->cl_uuid.lock);
-		nf = rcu_dereference_protected(*pnf, 1);
-		if (!nf) {
-			nf = new;
-			new = NULL;
-			rcu_assign_pointer(*pnf, nf);
-		}
-		spin_unlock(&clp->cl_uuid.lock);
+		nf = unrcu_pointer(cmpxchg(pnf, NULL, RCU_INITIALIZER(new)));
+		if (!nf)
+			swap(nf, new);
 	}
 	nf = nfs_local_file_get(nf);
 	rcu_read_unlock();

fs/nfs_common/nfslocalio.c

@@ -273,8 +273,8 @@ EXPORT_SYMBOL_GPL(nfs_open_local_fh);
 
 void nfs_close_local_fh(struct nfs_file_localio *nfl)
 {
-	struct nfsd_file *ro_nf = NULL;
-	struct nfsd_file *rw_nf = NULL;
+	struct nfsd_file *ro_nf;
+	struct nfsd_file *rw_nf;
 	nfs_uuid_t *nfs_uuid;
 
 	rcu_read_lock();
@@ -285,28 +285,23 @@ void nfs_close_local_fh(struct nfs_file_localio *nfl)
 		return;
 	}
 
-	ro_nf = rcu_access_pointer(nfl->ro_file);
-	rw_nf = rcu_access_pointer(nfl->rw_file);
-	if (ro_nf || rw_nf) {
-		spin_lock(&nfs_uuid->lock);
-		if (ro_nf)
-			ro_nf = rcu_dereference_protected(xchg(&nfl->ro_file, NULL), 1);
-		if (rw_nf)
-			rw_nf = rcu_dereference_protected(xchg(&nfl->rw_file, NULL), 1);
-
-		/* Remove nfl from nfs_uuid->files list */
-		RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
-		list_del_init(&nfl->list);
-		spin_unlock(&nfs_uuid->lock);
-		rcu_read_unlock();
+	ro_nf = unrcu_pointer(xchg(&nfl->ro_file, NULL));
+	rw_nf = unrcu_pointer(xchg(&nfl->rw_file, NULL));
 
-		if (ro_nf)
-			nfs_to_nfsd_file_put_local(ro_nf);
-		if (rw_nf)
-			nfs_to_nfsd_file_put_local(rw_nf);
-		return;
-	}
+	spin_lock(&nfs_uuid->lock);
+	/* Remove nfl from nfs_uuid->files list */
+	list_del_init(&nfl->list);
+	spin_unlock(&nfs_uuid->lock);
 	rcu_read_unlock();
+	/* Now we can allow racing nfs_close_local_fh() to
+	 * skip the locking.
+	 */
+	RCU_INIT_POINTER(nfl->nfs_uuid, NULL);
+
+	if (ro_nf)
+		nfs_to_nfsd_file_put_local(ro_nf);
+	if (rw_nf)
+		nfs_to_nfsd_file_put_local(rw_nf);
 }
 EXPORT_SYMBOL_GPL(nfs_close_local_fh);