Btrfs: fix deadlock in btrfs_commit_transaction
We calculate timeout (either 1 or MAX_SCHEDULE_TIMEOUT) based on whether
num_writers > 1 or should_grow at the top of the loop. Then, much much
later, we wait for that timeout if either num_writers or should_grow is
true. However, it's possible for a racing process (calling
btrfs_end_transaction()) to decrement num_writers such that we wait
forever instead of for 1.
Fix this by deciding how long to wait when we wait. Include a smp_mb()
before checking if the waitqueue is active to ensure the num_writers
is visible.
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 325d9a5..700dc4b 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -402,6 +402,7 @@
WARN_ON(cur_trans->num_writers < 1);
cur_trans->num_writers--;
+ smp_mb();
if (waitqueue_active(&cur_trans->writer_wait))
wake_up(&cur_trans->writer_wait);
put_transaction(cur_trans);
@@ -1010,7 +1011,6 @@
struct btrfs_root *root)
{
unsigned long joined = 0;
- unsigned long timeout = 1;
struct btrfs_transaction *cur_trans;
struct btrfs_transaction *prev_trans = NULL;
DEFINE_WAIT(wait);
@@ -1081,11 +1081,6 @@
snap_pending = 1;
WARN_ON(cur_trans != trans->transaction);
- if (cur_trans->num_writers > 1)
- timeout = MAX_SCHEDULE_TIMEOUT;
- else if (should_grow)
- timeout = 1;
-
mutex_unlock(&root->fs_info->trans_mutex);
if (flush_on_commit || snap_pending) {
@@ -1107,8 +1102,10 @@
TASK_UNINTERRUPTIBLE);
smp_mb();
- if (cur_trans->num_writers > 1 || should_grow)
- schedule_timeout(timeout);
+ if (cur_trans->num_writers > 1)
+ schedule_timeout(MAX_SCHEDULE_TIMEOUT);
+ else if (should_grow)
+ schedule_timeout(1);
mutex_lock(&root->fs_info->trans_mutex);
finish_wait(&cur_trans->writer_wait, &wait);
