[PATCH 3/3] Revert "task_work: remove fifo ordering guarantee"

[Oleg Nesterov <oleg@redhat.com>]

This reverts commit c82199061009d1561e31e17fca5e47a87cb7ff4c.

Now that fput() can't abuse ->task_works list, we can restore the FIFO
ordering. Yes, currently there are no in-kernel users which need this,
but I think task_work_add() will have more users and FIFO makes more
sense if (unlike fput/mntput) the callbacks change the task's state.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/task_work.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 53fa971..8727032 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -18,8 +18,6 @@ static struct callback_head work_exited; /* all we need is ->next == NULL */
  * This is like the signal handler which runs in kernel mode, but it doesn't
  * try to wake up the @task.
  *
- * Note: there is no ordering guarantee on works queued here.
- *
  * RETURNS:
  * 0 if succeeds or -ESRCH.
  */
@@ -110,6 +108,16 @@ void task_work_run(void)
 		raw_spin_unlock_wait(&task->pi_lock);
 		smp_mb();
 
+		/* Reverse the list to run the works in fifo order */
+		head = NULL;
+		do {
+			next = work->next;
+			work->next = head;
+			head = work;
+			work = next;
+		} while (work);
+
+		work = head;
 		do {
 			next = work->next;
 			work->func(work);
-- 
2.4.3