From 6e7ca09b583de4be6c27d9d4b06e8c5dd46a58fa Mon Sep 17 00:00:00 2001 From: Nikolay Borisov Date: Thu, 25 Jul 2019 11:27:29 +0300 Subject: [PATCH] btrfs: Fix deadlock caused by missing memory barrier Commit 06297d8cefca ("btrfs: switch extent_buffer blocking_writers from atomic to int") changed the type of blocking_writers but forgot to adjust relevant code in btrfs_tree_unlock by converting the smp_mb__after_atomic to smp_mb. This opened up the possibility of a deadlock due to re-ordering of setting blocking_writers and checking/waking up the waiter. This particular lockup is explained in a comment above waitqueue_active() function. Fix it by converting the memory barrier to a full smp_mb, accounting for the fact that blocking_writers is a simple integer. Fixes: 06297d8cefca ("btrfs: switch extent_buffer blocking_writers from atomic to int") Tested-by: Johannes Thumshirn Signed-off-by: Nikolay Borisov Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/locking.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c index 98fccce..393eced 100644 --- a/fs/btrfs/locking.c +++ b/fs/btrfs/locking.c @@ -346,9 +346,12 @@ void btrfs_tree_unlock(struct extent_buffer *eb) if (blockers) { btrfs_assert_no_spinning_writers(eb); eb->blocking_writers--; - /* Use the lighter barrier after atomic */ - smp_mb__after_atomic(); - cond_wake_up_nomb(&eb->write_lock_wq); + /* + * We need to order modifying blocking_writers above with + * actually waking up the sleepers to ensure they see the + * updated value of blocking_writers + */ + cond_wake_up(&eb->write_lock_wq); } else { btrfs_assert_spinning_writers_put(eb); write_unlock(&eb->lock); -- 2.7.4