linux-kernel - [PATCH 0/1] hung_task: fix the broken rcu_lock

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Date:	Fri, 17 Feb 2012 19:00:11 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	apw@...onical.com, arjan@...ux.intel.com, fhrbata@...hat.com,
	john.johansen@...onical.com, rientjes@...gle.com,
	rusty@...tcorp.com.au, tj@...nel.org, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mandeep Singh Baines <msb@...omium.org>
Subject: [PATCH 0/1] hung_task: fix the broken rcu_lock_break() logic

On 02/17, Oleg Nesterov wrote:
>
> On 02/17, Tetsuo Handa wrote:
> >
> > Oleg Nesterov wrote:
> > >     int main(void)
> > >     {
> > >             vfork();
> > >             pause();
> > >     }
> >^
> > Wow!
>
> and this reminds me... check_hung_uninterruptible_tasks() is very broken,
> it can crash the kernel itself. I'll resend my ancient fixes once again
> today, maybe this resend will be successful ;)

Please apply.

The patch was sent more than a year ago, I preserved the acks I got.

I was also going to add the PF_FREEZER_SKIP check in check_hung_task(),
but this was already done: f9fab10bbd768b0e5254e53a4a8477a94bfc4b96.
This means I lied, the program above no longer creates the problem
for check_hung_uninterruptible_tasks(). But that was not the point
for make-it-killable anyway.

I simply can't understand why do we have sysctl_hung_task_check_count,
may be someone can explain... OK, we need to do rcu_read_unlock()
from time to time even if need_resched() is not set, that is why
we have HUNG_TASK_BATCHING. Although perhaps it makes sense to do

	-	if (!--batch_count)
	+	if (!--batch_count || need_resched())
			rcu_lock_break();

but max_count? I guess I missed something obvious.

Oleg.

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