[PATCH 3.19.y-ckt 148/156] task_work: remove fifo ordering guarantee

[Kamal Mostafa]

3.19.8-ckt8 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet at google.com>

commit c82199061009d1561e31e17fca5e47a87cb7ff4c upstream.

In commit f341861fb0b ("task_work: add a scheduling point in
task_work_run()") I fixed a latency problem adding a cond_resched()
call.

Later, commit ac3d0da8f329 added yet another loop to reverse a list,
bringing back the latency spike :

I've seen in some cases this loop taking 275 ms, if for example a
process with 2,000,000 files is killed.

We could add yet another cond_resched() in the reverse loop, or we
can simply remove the reversal, as I do not think anything
would depend on order of task_work_add() submitted works.

Fixes: ac3d0da8f329 ("task_work: Make task_work_add() lockless")
Signed-off-by: Eric Dumazet <edumazet at google.com>
Reported-by: Maciej Żenczykowski <maze at google.com>
Acked-by: Al Viro <viro at zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/task_work.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 8727032..53fa971 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -18,6 +18,8 @@ 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.
  */
@@ -108,16 +110,6 @@ 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);
-- 
1.9.1