f2fs: fix to update cached_en of extent tree properly

In f2fs_lookup_extent_tree, et->cached_en was read and updated with only
read lock held,
it could cause __lookup_extent_tree within return entirely wrong
extent_node, if other
thread update et->cached_en just before __lookup_extent_tree return.

However, there are two things about this patch that need to be noticed:
1. It does no good to arrange the order of concurrent read/write, the result
would still
be random in such case.
2. It's built on this assumption: the mix up of reads and writes on a single
pointer would
not make the pointer partially wrong at any time. Please let me know if I'm
wrong, thx.
Signed-off-by: Fan li <fanofcode.li@samsung.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/extent_cache.c

@@ -85,13 +85,13 @@ static struct extent_node *__lookup_extent_tree(struct extent_tree *et,
 							unsigned int fofs)
 {
 	struct rb_node *node = et->root.rb_node;
-	struct extent_node *en;
+	struct extent_node *en = et->cached_en;
 
-	if (et->cached_en) {
-		struct extent_info *cei = &et->cached_en->ei;
+	if (en) {
+		struct extent_info *cei = &en->ei;
 
 		if (cei->fofs <= fofs && cei->fofs + cei->len > fofs)
-			return et->cached_en;
+			return en;
 	}
 
 	while (node) {