[PATCH] f2fs: fix to update cached_en of extent tree properly

[Fan Li <fanofcode.li@samsung.com>]

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>
---
 fs/f2fs/extent_cache.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/f2fs/extent_cache.c b/fs/f2fs/extent_cache.c index
32fae8a..cea5813 100644
--- a/fs/f2fs/extent_cache.c
+++ b/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) {
--
1.7.9.5


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