ext4: fix data races when using cached status extents

When using cached extent stored in extent status tree in tree->cache_es
another process holding ei->i_es_lock for reading can be racing with us
setting new value of tree->cache_es. If the compiler would decide to
refetch tree->cache_es at an unfortunate moment, it could result in a
bogus in_range() check. Fix the possible race by using READ_ONCE() when
using tree->cache_es only under ei->i_es_lock for reading.

Cc: [email protected]
Reported-by: [email protected]
Link: https://lore.kernel.org/all/[email protected]
Suggested-by: Dmitry Vyukov <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

Author: jankara

fs/ext4/extents_status.c

@@ -267,14 +267,12 @@ static void __es_find_extent_range(struct inode *inode,
 
 	/* see if the extent has been cached */
 	es->es_lblk = es->es_len = es->es_pblk = 0;
-	if (tree->cache_es) {
-		es1 = tree->cache_es;
-		if (in_range(lblk, es1->es_lblk, es1->es_len)) {
-			es_debug("%u cached by [%u/%u) %llu %x\n",
-				 lblk, es1->es_lblk, es1->es_len,
-				 ext4_es_pblock(es1), ext4_es_status(es1));
-			goto out;
-		}
+	es1 = READ_ONCE(tree->cache_es);
+	if (es1 && in_range(lblk, es1->es_lblk, es1->es_len)) {
+		es_debug("%u cached by [%u/%u) %llu %x\n",
+			 lblk, es1->es_lblk, es1->es_len,
+			 ext4_es_pblock(es1), ext4_es_status(es1));
+		goto out;
 	}
 
 	es1 = __es_tree_search(&tree->root, lblk);
@@ -293,7 +291,7 @@ static void __es_find_extent_range(struct inode *inode,
 	}
 
 	if (es1 && matching_fn(es1)) {
-		tree->cache_es = es1;
+		WRITE_ONCE(tree->cache_es, es1);
 		es->es_lblk = es1->es_lblk;
 		es->es_len = es1->es_len;
 		es->es_pblk = es1->es_pblk;
@@ -931,14 +929,12 @@ int ext4_es_lookup_extent(struct inode *inode, ext4_lblk_t lblk,
 
 	/* find extent in cache firstly */
 	es->es_lblk = es->es_len = es->es_pblk = 0;
-	if (tree->cache_es) {
-		es1 = tree->cache_es;
-		if (in_range(lblk, es1->es_lblk, es1->es_len)) {
-			es_debug("%u cached by [%u/%u)\n",
-				 lblk, es1->es_lblk, es1->es_len);
-			found = 1;
-			goto out;
-		}
+	es1 = READ_ONCE(tree->cache_es);
+	if (es1 && in_range(lblk, es1->es_lblk, es1->es_len)) {
+		es_debug("%u cached by [%u/%u)\n",
+			 lblk, es1->es_lblk, es1->es_len);
+		found = 1;
+		goto out;
 	}
 
 	node = tree->root.rb_node;