From 6e7ca09b583de4be6c27d9d4b06e8c5dd46a58fa Mon Sep 17 00:00:00 2001
From: Nikolay Borisov <nborisov@suse.com>
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 <jthumshirn@suse.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
---
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 98fccce4208ca..393eceda57c8a 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);
--
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