cifs: don't take exclusive lock for updating target hints

Avoid contention while updating dfs target hints.  This should be
perfectly fine to update them under shared locks.
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/dfs_cache.c

@@ -269,7 +269,7 @@ static int dfscache_proc_show(struct seq_file *m, void *v)
 			list_for_each_entry(t, &ce->tlist, list) {
 				seq_printf(m, "  %s%s\n",
 					   t->name,
-					   ce->tgthint == t ? " (target hint)" : "");
+					   READ_ONCE(ce->tgthint) == t ? " (target hint)" : "");
 			}
 		}
 	}
@@ -321,7 +321,7 @@ static inline void dump_tgts(const struct cache_entry *ce)
 	cifs_dbg(FYI, "target list:\n");
 	list_for_each_entry(t, &ce->tlist, list) {
 		cifs_dbg(FYI, "  %s%s\n", t->name,
-			 ce->tgthint == t ? " (target hint)" : "");
+			 READ_ONCE(ce->tgthint) == t ? " (target hint)" : "");
 	}
 }
 
@@ -427,7 +427,7 @@ static int cache_entry_hash(const void *data, int size, unsigned int *hash)
 /* Return target hint of a DFS cache entry */
 static inline char *get_tgt_name(const struct cache_entry *ce)
 {
-	struct cache_dfs_tgt *t = ce->tgthint;
+	struct cache_dfs_tgt *t = READ_ONCE(ce->tgthint);
 
 	return t ? t->name : ERR_PTR(-ENOENT);
 }
@@ -470,6 +470,7 @@ static struct cache_dfs_tgt *alloc_target(const char *name, int path_consumed)
 static int copy_ref_data(const struct dfs_info3_param *refs, int numrefs,
 			 struct cache_entry *ce, const char *tgthint)
 {
+	struct cache_dfs_tgt *target;
 	int i;
 
 	ce->ttl = max_t(int, refs[0].ttl, CACHE_MIN_TTL);
@@ -496,8 +497,9 @@ static int copy_ref_data(const struct dfs_info3_param *refs, int numrefs,
 		ce->numtgts++;
 	}
 
-	ce->tgthint = list_first_entry_or_null(&ce->tlist,
-					       struct cache_dfs_tgt, list);
+	target = list_first_entry_or_null(&ce->tlist, struct cache_dfs_tgt,
+					  list);
+	WRITE_ONCE(ce->tgthint, target);
 
 	return 0;
 }
@@ -712,14 +714,15 @@ void dfs_cache_destroy(void)
 static int update_cache_entry_locked(struct cache_entry *ce, const struct dfs_info3_param *refs,
 				     int numrefs)
 {
+	struct cache_dfs_tgt *target;
+	char *th = NULL;
 	int rc;
-	char *s, *th = NULL;
 
 	WARN_ON(!rwsem_is_locked(&htable_rw_lock));
 
-	if (ce->tgthint) {
-		s = ce->tgthint->name;
-		th = kstrdup(s, GFP_ATOMIC);
+	target = READ_ONCE(ce->tgthint);
+	if (target) {
+		th = kstrdup(target->name, GFP_ATOMIC);
 		if (!th)
 			return -ENOMEM;
 	}
@@ -896,7 +899,7 @@ static int get_targets(struct cache_entry *ce, struct dfs_cache_tgt_list *tl)
 		}
 		it->it_path_consumed = t->path_consumed;
 
-		if (ce->tgthint == t)
+		if (READ_ONCE(ce->tgthint) == t)
 			list_add(&it->it_list, head);
 		else
 			list_add_tail(&it->it_list, head);
@@ -1052,23 +1055,14 @@ int dfs_cache_update_tgthint(const unsigned int xid, struct cifs_ses *ses,
 		goto out_free_path;
 	}
 
-	up_read(&htable_rw_lock);
-	down_write(&htable_rw_lock);
-
-	ce = lookup_cache_entry(npath);
-	if (IS_ERR(ce)) {
-		rc = PTR_ERR(ce);
-		goto out_unlock;
-	}
-
-	t = ce->tgthint;
+	t = READ_ONCE(ce->tgthint);
 
 	if (likely(!strcasecmp(it->it_name, t->name)))
 		goto out_unlock;
 
 	list_for_each_entry(t, &ce->tlist, list) {
 		if (!strcasecmp(t->name, it->it_name)) {
-			ce->tgthint = t;
+			WRITE_ONCE(ce->tgthint, t);
 			cifs_dbg(FYI, "%s: new target hint: %s\n", __func__,
 				 it->it_name);
 			break;
@@ -1076,7 +1070,7 @@ int dfs_cache_update_tgthint(const unsigned int xid, struct cifs_ses *ses,
 	}
 
 out_unlock:
-	up_write(&htable_rw_lock);
+	up_read(&htable_rw_lock);
 out_free_path:
 	kfree(npath);
 	return rc;
@@ -1106,21 +1100,20 @@ void dfs_cache_noreq_update_tgthint(const char *path, const struct dfs_cache_tgt
 
 	cifs_dbg(FYI, "%s: path: %s\n", __func__, path);
 
-	if (!down_write_trylock(&htable_rw_lock))
-		return;
+	down_read(&htable_rw_lock);
 
 	ce = lookup_cache_entry(path);
 	if (IS_ERR(ce))
 		goto out_unlock;
 
-	t = ce->tgthint;
+	t = READ_ONCE(ce->tgthint);
 
 	if (unlikely(!strcasecmp(it->it_name, t->name)))
 		goto out_unlock;
 
 	list_for_each_entry(t, &ce->tlist, list) {
 		if (!strcasecmp(t->name, it->it_name)) {
-			ce->tgthint = t;
+			WRITE_ONCE(ce->tgthint, t);
 			cifs_dbg(FYI, "%s: new target hint: %s\n", __func__,
 				 it->it_name);
 			break;
@@ -1128,7 +1121,7 @@ void dfs_cache_noreq_update_tgthint(const char *path, const struct dfs_cache_tgt
 	}
 
 out_unlock:
-	up_write(&htable_rw_lock);
+	up_read(&htable_rw_lock);
 }
 
 /**