[PATCH v4] f2fs: use post-decrement count for cp_wait wakeup

[Wenjie Qi]

f2fs_write_end_io() decrements the writeback page counter and then
reads it again with get_pages() to decide whether the last
F2FS_WB_CP_DATA completion should wake cp_wait.

Use atomic_dec_return() for F2FS_WB_CP_DATA completions so the wakeup
decision is made from the value produced by the decrement itself. Keep
the existing dec_page_count() path for other writeback counters.

Fixes: ce2739e482bc ("f2fs: fix to avoid UAF in f2fs_write_end_io()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Wenjie Qi <qiwenjie@xxxxxxxxxx>
---
Changes in v4:
- Add Fixes and Cc stable tags.

 fs/f2fs/data.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index d83a21998ec2..58d23eb74ec2 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -392,15 +392,17 @@ static void f2fs_write_end_io(struct bio *bio)
 		if (f2fs_in_warm_node_list(folio))
 			f2fs_del_fsync_node_entry(sbi, folio);
 
-		dec_page_count(sbi, type);
-
 		/*
 		 * we should access sbi before folio_end_writeback() to
 		 * avoid racing w/ kill_f2fs_super()
 		 */
-		if (type == F2FS_WB_CP_DATA && !get_pages(sbi, type) &&
-				wq_has_sleeper(&sbi->cp_wait))
-			wake_up(&sbi->cp_wait);
+		if (type == F2FS_WB_CP_DATA) {
+			if (!atomic_dec_return(&sbi->nr_pages[type]) &&
+			    wq_has_sleeper(&sbi->cp_wait))
+				wake_up(&sbi->cp_wait);
+		} else {
+			dec_page_count(sbi, type);
+		}
 
 		folio_clear_f2fs_gcing(folio);
 		folio_end_writeback(folio);
-- 
2.43.0