btrfs_bio_counter_sub() seems to be missing a memory barrier which might
cause the waker to not notice the waiter and miss sending a wake_up as
in the following figure.
btrfs_bio_counter_sub btrfs_rm_dev_replace_blocked
------------------------------------------------------------------------
if (waitqueue_active(&fs_info->replace_wait))
/* The CPU might reorder the test for
the waitqueue up here, before
prior writes complete */
/* wait_event */
/* __wait_event */
/* ___wait_event */
long __int =
prepare_to_wait_event(&wq,
&__wait, state);
if
(!percpu_counter_sum(&fs_info->bio_counter))
percpu_counter_sub(&fs_info->bio_counter,
amount);
schedule()
------------------------------------------------------------------------
This patch removes the call to waitqueue_active() leaving just wake_up()
behind. This fixes the problem because the call to spin_lock_irqsave()
in wake_up() will be an ACQUIRE operation.
I found this issue when I was looking through the linux source code
for places calling waitqueue_active() before wake_up*(), but without
preceding memory barriers, after sending a patch to fix a similar
issue in drivers/tty/n_tty.c (Details about the original issue can be
found here: https://lkml.org/lkml/2015/9/28/849).
Signed-off-by: Kosuke Tatsukawa <ta...@ab.jp.nec.com>
---
fs/btrfs/dev-replace.c | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)
diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index e54dd59..ecb3e71 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -918,9 +918,7 @@ void btrfs_bio_counter_inc_noblocked(struct btrfs_fs_info
*fs_info)
void btrfs_bio_counter_sub(struct btrfs_fs_info *fs_info, s64 amount)
{
percpu_counter_sub(&fs_info->bio_counter, amount);
-
- if (waitqueue_active(&fs_info->replace_wait))
- wake_up(&fs_info->replace_wait);
+ wake_up(&fs_info->replace_wait);
}
void btrfs_bio_counter_inc_blocked(struct btrfs_fs_info *fs_info)
--
1.7.1
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