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Date: Tue, 22 Dec 2009 15:41:38 +0800
Message-ID: <7b6bb4a50912212341q1e35689cuc40394ae31f96bd0@mail.gmail.com>
Subject: Re: 2.6.33-rc1 unusable due to scheduler issues, circular locking, 
	WARNs and BUGs
From: Xiaotian Feng <xtfeng@gmail.com>
To: =?UTF-8?Q?Am=C3=A9rico_Wang?= <xiyou.wangcong@gmail.com>
Cc: Eric Paris <eparis@redhat.com>, linux-kernel@vger.kernel.org,
       mingo@elte.hu, peterz@infradead.org, efault@gmx.de
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On Tue, Dec 22, 2009 at 3:19 PM, Américo Wang <xiyou.wangcong@gmail.com> wrote:
> [Fix top-posting]
>
> On Tue, Dec 22, 2009 at 1:42 PM, Xiaotian Feng <xtfeng@gmail.com> wrote:
>>
>> On Tue, Dec 22, 2009 at 8:17 AM, Eric Paris <eparis@redhat.com> wrote:
>>> Trying to build a kernel on a 48 core x86_64 box using make -j 64 and
>>> I'm exploding in the scheduler.  I'm running (and building) kernel
>>> f7b84a6ba7eaeba4e1df8feddca1473a7db369a5  There are three distinct
>>> signatures of problems.  Some boots I'll see all 3 of these failures
>>> sometimes only 1 or 2 of them.  That's the reason they are kinda split
>>> up in dmesg.
>>>
>>> 1) gcc/3141 is trying to acquire lock:
>>>  (&(&sem->wait_lock)->rlock){......}, at: [<ffffffff81223234>] __down_read_trylock+0x13/0x46
>>>
>>> but task is already holding lock:
>>>  (&rq->lock){-.-.-.}, at: [<ffffffff8103dd2d>] task_rq_lock+0x51/0x83
>>>
>>> 2) WARN() in kernel/sched_fair.c:1001 hrtick_start_fair()
>>>
>>> 3) NULL pointer dereference at 0000000000000168 in check_preempt_wakeup
>>>      kernel/sched_fair.c
>>>
>>> Full backtraces are in the attached dmesg.
>>>
>> Does a revert of cd29fe6f2637cc2ccbda5ac65f5332d6bf5fa3c6 fix this problem?
>
>
> I don't think so...
>
> I think the most suspicious commit here is ab19cb23. It kicked
> "local_irq_save()"
> out, which means if the task is selected to run on another cpu which doesn't
> disable irq, we will have a page fault, thun we will try to hold mm->mmap_sem
> while we are holding rq->lock already.

The page fault is from kernel  NULL pointer deref.  You should connect
the lockdep warning and kernel BUG together.

>
> Does the following untested patch fix the problem?
>
> NOT-signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
>
> ------
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 87f1f47..221ab59 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2408,13 +2408,13 @@ static int try_to_wake_up(struct task_struct
> *p, unsigned int state,
>        if (p->sched_class->task_waking)
>                p->sched_class->task_waking(rq, p);
>
> -       __task_rq_unlock(rq);
> +       task_rq_unlock(rq);
>
>        cpu = select_task_rq(p, SD_BALANCE_WAKE, wake_flags);
>        if (cpu != orig_cpu)
>                set_task_cpu(p, cpu);
>
> -       rq = __task_rq_lock(p);
> +       rq = task_rq_lock(p);
>        update_rq_clock(rq);
>
>        WARN_ON(p->state != TASK_WAKING);
>
--
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