linux-kernel - [PATCH 3.12 75/76] sched: Remove lockdep check in sched_move

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Date:	Thu, 19 May 2016 11:08:37 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Kirill Tkhai <ktkhai@...allels.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 75/76] sched: Remove lockdep check in sched_move_task()

From: Kirill Tkhai <ktkhai@...allels.com>

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

===============

commit f7b8a47da17c9ee4998f2ca2018fcc424e953c0e upstream.

sched_move_task() is the only interface to change sched_task_group:
cpu_cgrp_subsys methods and autogroup_move_group() use it.

Everything is synchronized by task_rq_lock(), so cpu_cgroup_attach()
is ordered with other users of sched_move_task(). This means we do no
need RCU here: if we've dereferenced a tg here, the .attach method
hasn't been called for it yet.

Thus, we should pass "true" to task_css_check() to silence lockdep
warnings.

Fixes: eeb61e53ea19 ("sched: Fix race between task_group and sched_task_group")
Reported-by: Oleg Nesterov <oleg@...hat.com>
Reported-by: Fengguang Wu <fengguang.wu@...el.com>
Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: http://lkml.kernel.org/r/1414473874.8574.2.camel@tkhai
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 kernel/sched/core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index dd794a9b6850..e382c14652d0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6865,8 +6865,12 @@ void sched_move_task(struct task_struct *tsk)
 	if (unlikely(running))
 		tsk->sched_class->put_prev_task(rq, tsk);
 
-	tg = container_of(task_css_check(tsk, cpu_cgroup_subsys_id,
-				lockdep_is_held(&tsk->sighand->siglock)),
+	/*
+	 * All callers are synchronized by task_rq_lock(); we do not use RCU
+	 * which is pointless here. Thus, we pass "true" to task_css_check()
+	 * to prevent lockdep warnings.
+	 */
+	tg = container_of(task_css_check(tsk, cpu_cgroup_subsys_id, true),
 			  struct task_group, css);
 	tg = autogroup_task_group(tsk, tg);
 	tsk->sched_task_group = tg;
-- 
2.8.2