From: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave@sr71.net>, Borislav Petkov <bp@alien8.de>,
LKML <linux-kernel@vger.kernel.org>,
Dave Jones <davej@redhat.com>,
dhillf@gmail.com, Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] CPU hotplug, smpboot: Fix crash in smpboot_thread_fn()
Date: Thu, 11 Apr 2013 15:49:41 +0530 [thread overview]
Message-ID: <51668E3D.1030305@linux.vnet.ibm.com> (raw)
In-Reply-To: <alpine.LFD.2.02.1304111007270.21884@ionos>
On 04/11/2013 01:40 PM, Thomas Gleixner wrote:
> On Wed, 10 Apr 2013, Srivatsa S. Bhat wrote:
>
>> Interestingly, in every single stack trace, the crashing task is the migration
>> thread. Now, migration thread belongs to the highest priority stop_task sched
>> class, and this particular sched class is very unique in the way it implements
>> its internal sched class functions, and I suspect this has a lot of bearing
>> on how functions like kthread_bind(), wake_up_process() etc react with it
>> (by looking at how it implements its functions such as select_task_rq(),
>> enqueue_task(), dequeue_task() etc).
>
> I don't think that's relevant. The migration thread can only be woken
> via try_to_wakeup and my previous patch which implements a separate
> task state makes sure that it cannot be woken accidentaly by anything
> else than unpark.
>
Hmm, but it got to be simpler than that, no? Given that it used to work fine
before...
>> But note that __kthread_bind() can wake up the task if the task is an RT
>> task. So it can be called only when the CPU (to which we want to bind the task)
>
> kthread_bind() does NOT wakeup anything. It merily sets the cpus
> allowed ptr without further ado.
>
Sorry, I was mistaken and was carried away by a bug in the code I was testing.
I had intended to move kthread_bind() to the body of kthread_create_on_cpu()
and place it after the call to kthread_park(), as shown below:
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 691dc2e..b485fc0 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -308,6 +308,9 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
to_kthread(p)->cpu = cpu;
/* Park the thread to get it out of TASK_UNINTERRUPTIBLE state */
kthread_park(p);
+
+ wait_task_inactive(p, TASK_INTERRUPTIBLE);
+ __kthread_bind(p, cpu);
return p;
}
But by mistake, I had written the code as:
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 691dc2e..b485fc0 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -308,6 +308,9 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
to_kthread(p)->cpu = cpu;
/* Park the thread to get it out of TASK_UNINTERRUPTIBLE state */
kthread_park(p);
+
+ if (!wait_task_inactive(p, TASK_INTERRUPTIBLE))
+ __kthread_bind(p, cpu);
return p;
}
So, no wonder it never actually bound the task to the CPU. So when I gave this a run,
I saw watchdog threads hitting the same BUG_ON(), and since watchdog threads are
of RT priority, and RT is the only class that implements ->set_cpus_allowed(), I
thought that those threads got woken up due to the bind. But I was mistaken of
course, because I had checked for the wrong return value of wait_task_inactive().
Regards,
Srivatsa S. Bhat
next prev parent reply other threads:[~2013-04-11 10:22 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-04-05 21:43 kernel BUG at kernel/smpboot.c:134! Dave Hansen
2013-04-06 7:12 ` Srivatsa S. Bhat
2013-04-06 8:31 ` Thomas Gleixner
2013-04-07 9:20 ` Thomas Gleixner
2013-04-07 9:50 ` Borislav Petkov
2013-04-08 9:24 ` Thomas Gleixner
2013-04-08 11:55 ` Borislav Petkov
2013-04-08 12:17 ` Thomas Gleixner
2013-04-09 14:38 ` [PATCH] kthread: Prevent unpark race which puts threads on the wrong cpu Thomas Gleixner
2013-04-09 15:55 ` Dave Hansen
2013-04-09 18:43 ` Thomas Gleixner
2013-04-09 19:30 ` Thomas Gleixner
2013-04-09 20:38 ` Dave Hansen
2013-04-09 20:54 ` Dave Hansen
2013-04-10 8:29 ` Thomas Gleixner
2013-04-10 10:51 ` Thomas Gleixner
2013-04-10 19:41 ` Dave Hansen
2013-04-11 10:19 ` Thomas Gleixner
2013-04-11 10:48 ` Srivatsa S. Bhat
2013-04-11 11:43 ` Srivatsa S. Bhat
2013-04-11 11:59 ` Srivatsa S. Bhat
2013-04-11 12:51 ` Thomas Gleixner
2013-04-11 12:54 ` Thomas Gleixner
2013-04-11 13:46 ` Thomas Gleixner
2013-04-11 18:07 ` Dave Hansen
2013-04-11 19:48 ` Thomas Gleixner
2013-04-10 14:03 ` [PATCH] CPU hotplug, smpboot: Fix crash in smpboot_thread_fn() Srivatsa S. Bhat
2013-04-11 8:10 ` Thomas Gleixner
2013-04-11 10:19 ` Srivatsa S. Bhat [this message]
2013-04-11 19:16 ` [PATCH] kthread: Prevent unpark race which puts threads on the wrong cpu Srivatsa S. Bhat
2013-04-11 20:47 ` Thomas Gleixner
2013-04-11 21:19 ` Srivatsa S. Bhat
2013-04-12 10:59 ` Thomas Gleixner
2013-04-12 11:26 ` Srivatsa S. Bhat
2013-04-15 19:49 ` Dave Hansen
2013-04-12 10:41 ` Peter Zijlstra
2013-04-12 12:32 ` [tip:core/urgent] " tip-bot for Thomas Gleixner
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