btrfs: Fix deadlock caused by missing memory barrier
authorNikolay Borisov <nborisov@suse.com>
Thu, 25 Jul 2019 08:27:29 +0000 (11:27 +0300)
committerDavid Sterba <dsterba@suse.com>
Thu, 25 Jul 2019 15:34:08 +0000 (17:34 +0200)
commit6e7ca09b583de4be6c27d9d4b06e8c5dd46a58fa
treea08365bc6b1370f162158b1518061f689dc924a8
parent373c3b80e459cb57c34381b928588a3794eb5bbd
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