linux-kernel - [PATCH 4.2.y-ckt 015/211] task_work: remove fifo ordering guarantee

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1452023119-25647-16-git-send-email-kamal@canonical.com>
Date:	Tue,  5 Jan 2016 11:42:03 -0800
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Eric Dumazet <edumazet@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 4.2.y-ckt 015/211] task_work: remove fifo ordering guarantee

4.2.8-ckt1 -stable review patch.  If anyone has any objections, please let me know.

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

From: Eric Dumazet <edumazet@...gle.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@...gle.com>
Reported-by: Maciej Żenczykowski <maze@...gle.com>
Acked-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal@...onical.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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/