From: Kirill Tkhai <tkhai@yandex.ru>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>,
Kirill Tkhai <ktkhai@parallels.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Vladimir Davydov <vdavydov@parallels.com>
Subject: Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign()
Date: Sun, 19 Oct 2014 12:20:59 +0400 [thread overview]
Message-ID: <5443746B.6060309@yandex.ru> (raw)
In-Reply-To: <20141018193612.GC23531@worktop.programming.kicks-ass.net>
On 18.10.2014 23:36, Peter Zijlstra wrote:
> On Sat, Oct 18, 2014 at 12:33:27PM +0400, Kirill Tkhai wrote:
>> How about this?
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index b78280c..d46427e 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1165,7 +1165,21 @@ static void task_numa_compare(struct task_numa_env *env,
>>
>> rcu_read_lock();
>> cur = ACCESS_ONCE(dst_rq->curr);
>> - if (cur->pid == 0) /* idle */
>> + /*
>> + * No need to move the exiting task, and this ensures that ->curr
>> + * wasn't reaped and thus get_task_struct() in task_numa_assign()
>> + * is safe; note that rcu_read_lock() can't protect from the final
>> + * put_task_struct() after the last schedule().
>> + */
>> + if (is_idle_task(cur) || (cur->flags & PF_EXITING))
>> + cur = NULL;
>> + /*
>> + * Check once again to be sure curr is still on dst_rq. Even if
>> + * it points on a new task, which is using the memory of freed
>> + * cur, it's OK, because we've locked RCU before
>> + * delayed_put_task_struct() callback is called to put its struct.
>> + */
>> + if (cur != ACCESS_ONCE(dst_rq->curr))
>> cur = NULL;
>>
>> /*
>
> So you worry about the refcount doing 0->1 ? In which case the above is
> still wrong and we should be using atomic_inc_not_zero() in order to
> acquire the reference count.
>
We can't use atomic_inc_not_zero(). The problem is that cur is pointing
to a memory, which may be not a task_struct even. No guarantees at all.
next prev parent reply other threads:[~2014-10-19 8:21 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-15 12:31 [PATCH RFC] sched: Revert delayed_put_task_struct() and fix use after free Kirill Tkhai
2014-10-15 15:06 ` Oleg Nesterov
2014-10-15 19:40 ` Oleg Nesterov
2014-10-15 21:46 ` Kirill Tkhai
2014-10-15 22:02 ` Kirill Tkhai
2014-10-16 7:59 ` Peter Zijlstra
2014-10-16 8:16 ` Kirill Tkhai
2014-10-16 9:43 ` Peter Zijlstra
2014-10-16 9:50 ` Kirill Tkhai
2014-10-16 9:51 ` Kirill Tkhai
2014-10-16 10:04 ` Kirill Tkhai
2014-10-17 21:34 ` Oleg Nesterov
2014-10-16 7:56 ` Peter Zijlstra
2014-10-16 8:01 ` Peter Zijlstra
2014-10-16 22:05 ` Oleg Nesterov
2014-10-17 21:36 ` [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Oleg Nesterov
2014-10-18 8:15 ` Kirill Tkhai
2014-10-18 8:33 ` Kirill Tkhai
2014-10-18 19:36 ` Peter Zijlstra
2014-10-18 21:18 ` Oleg Nesterov
2014-10-19 8:20 ` Kirill Tkhai [this message]
2014-10-18 20:56 ` Oleg Nesterov
2014-10-18 23:13 ` Kirill Tkhai
2014-10-19 19:24 ` Oleg Nesterov
2014-10-19 19:37 ` Oleg Nesterov
2014-10-19 19:43 ` Oleg Nesterov
2014-10-20 9:03 ` Kirill Tkhai
2014-10-20 9:13 ` Peter Zijlstra
2014-10-20 10:36 ` Kirill Tkhai
2014-10-20 9:00 ` Kirill Tkhai
2014-10-19 21:38 ` Peter Zijlstra
2014-10-20 8:56 ` Kirill Tkhai
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