linux-kernel - [PATCH v2 1/2] sched: schedule_tail() should disable preemption

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Message-ID: <20141008193644.GA32055@redhat.com>
Date:	Wed, 8 Oct 2014 21:36:44 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Kirill Tkhai <tkhai@...dex.ru>
Subject: [PATCH v2 1/2] sched: schedule_tail() should disable preemption

On 10/08, Oleg Nesterov wrote:
>
> Another problem is that finish_task_switch() itself runs with preempt
> enabled after finish_lock_switch(). If nothing else this means that
> ->sched_in() notifier can't trust its "cpu" arg.

OOPS, this obviously can't happen, ->preempt_notifiers must be empty.
Remove this part from the changelog, please see v2.

I am not sure about finish_arch_post_lock_switch() ... but probably
it should be fine without preempt_disable.

-------------------------------------------------------------------------------
Subject: [PATCH v2 1/2] sched: schedule_tail() should disable preemption

finish_task_switch() enables preemption, so post_schedule(rq) can be
called on the wrong (and even dead) CPU. Afaics, nothing really bad
can happen, but in this case we can wrongly clear rq->post_schedule
on that CPU. And this simply looks wrong in any case.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/sched/core.c |   11 +++++------
 1 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 703c7e6..3f267e8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2277,15 +2277,14 @@ static inline void post_schedule(struct rq *rq)
 asmlinkage __visible void schedule_tail(struct task_struct *prev)
 	__releases(rq->lock)
 {
-	struct rq *rq = this_rq();
+	struct rq *rq;
 
+	/* finish_task_switch() drops rq->lock and enables preemtion */
+	preempt_disable();
+	rq = this_rq();
 	finish_task_switch(rq, prev);
-
-	/*
-	 * FIXME: do we need to worry about rq being invalidated by the
-	 * task_switch?
-	 */
 	post_schedule(rq);
+	preempt_enable();
 
 	if (current->set_child_tid)
 		put_user(task_pid_vnr(current), current->set_child_tid);
-- 
1.5.5.1


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