(2011/06/20 19:20), Peter Zijlstra wrote:
> On Sat, 2011-05-28 at 16:35 +0000, tip-bot for KOSAKI Motohiro wrote:
>> +++ b/kernel/kthread.c
>> @@ -202,8 +202,8 @@ void kthread_bind(struct task_struct *p, unsigned int cpu)
>> return;
>> }
>>
>> - p->cpus_allowed = cpumask_of_cpu(cpu);
>> - p->rt.nr_cpus_allowed = 1;
>> + /* It's safe because the task is inactive. */
>> + do_set_cpus_allowed(p, cpumask_of(cpu));
>> p->flags |= PF_THREAD_BOUND;
>> }
>
>
> I just happened to be staring at this stuff again, and I'm wondering
> how and why this is correct. After kthread_create() the thread exists
> and is exposed in the pid-hash, therefore userspace can come and do
> sys_sched_setaffinity() on it, and since we're not holding any locks and
> set PF_THREAD_BOUND _after_ setting cpus_allowed, things can end up
> funny.
>
> Hmm?
Can't we take just either rq lock or pi_lock? Layer violation?
>From 1c0874b9157f47e22d0f6499612f0f78b830f018 Mon Sep 17 00:00:00 2001
From: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Date: Tue, 21 Jun 2011 18:15:19 +0900
Subject: [PATCH] kthread: fix kthread_bind() race
Peter Zijlstra reported kthread_bind() has a race. It doesn't hold
any locks and set PF_THREAD_BOUND _after_ setting cpus_allowed. So,
following race can be happen.
CPU0 CPU1
----------------------------------------------------
do_set_cpus_allowed()
sys_sched_setaffinity()
p->flags |= PF_THREAD_BOUND;
The solution is to take either rq lock or pi_lock. They prevent
a race because set_cpus_allowed_ptr() take both locks. This patch
choose to use latter way.
Reported-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
---
kernel/kthread.c | 5 +++++
1 files changed, 5 insertions(+), 0 deletions(-)
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 4ba7ccc..92e3083 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -196,15 +196,20 @@ EXPORT_SYMBOL(kthread_create_on_node);
*/
void kthread_bind(struct task_struct *p, unsigned int cpu)
{
+ unsigned long flags;
+
/* Must have done schedule() in kthread() before we set_task_cpu */
if (!wait_task_inactive(p, TASK_UNINTERRUPTIBLE)) {
WARN_ON(1);
return;
}
+ /* protect from a race against set_cpus_allowed_ptr() */
+ raw_spin_lock_irqsave(&p->pi_lock, flags);
/* It's safe because the task is inactive. */
do_set_cpus_allowed(p, cpumask_of(cpu));
p->flags |= PF_THREAD_BOUND;
+ raw_spin_unlock_irqrestore(&p->pi_lock, flags);
}
EXPORT_SYMBOL(kthread_bind);
--
1.7.3.1
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