Re: [PATCH] sched: RCU-protect __set_task_cpu() in set_task_cpu()

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

On (06/03/11 17:37), Peter Zijlstra wrote:
> On Tue, 2011-05-31 at 20:26 +0300, Sergey Senozhatsky wrote:
> >  [  152.262791] kernel/sched.c:619 invoked rcu_dereference_check() without protection!
> >  [  152.262795] 
> >  [  152.262841] stack backtrace:
> >  [  152.262846] Pid: 16, comm: watchdog/1 Not tainted 3.0.0-rc1-dbg-00441-g1d5f9cc-dirty #599
> >  [  152.262851] Call Trace:
> >  [  152.262860]  [<ffffffff8106e17b>] lockdep_rcu_dereference+0xa7/0xaf
> >  [  152.262868]  [<ffffffff810369f4>] set_task_cpu+0x1ed/0x3ce
> >  [  152.262876]  [<ffffffff8123a5d7>] ? plist_check_head+0x94/0x98
> >  [  152.262883]  [<ffffffff8123a72d>] ? plist_del+0x82/0x89
> >  [  152.262889]  [<ffffffff8102b139>] ? dequeue_task_rt+0x33/0x38
> >  [  152.262895]  [<ffffffff8102e3ac>] ? dequeue_task+0x82/0x89
> >  [  152.262902]  [<ffffffff81036fc0>] push_rt_task.part.131+0x1bb/0x247
> >  [  152.262909]  [<ffffffff81037138>] post_schedule_rt+0x1b/0x24
> >  [  152.262918]  [<ffffffff81477c1c>] schedule+0x989/0xa9e 
> 
> Does the below cure the issue? (completely untested)
>
Hello,

I believe it should, rq->lock is grabbed in post_schedule() and in 
find_lock_lowest_rq(). For now I grab task->pi_lock  in push_rt_task(), 
but your approach is definitely better.

Will test and report back.


	Sergey


> ---
> Subject: sched: Fix/clarify set_task_cpu() locking rules
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Fri Jun 03 17:28:08 CEST 2011
> 
> Sergey reported a CONFIG_PROVE_RCU warning in push_rt_task where
> set_task_cpu() was called with both relevant rq->locks held, which
> should be sufficient for running tasks since holding its rq->lock will
> serialize against sched_move_task().
> 
> Update the comments and fix the task_group() lockdep test.
> 
> Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Link: http://lkml.kernel.org/n/tip-k3lie1tjkcp3626dn5r5ihge@git.kernel.org
> ---
>  kernel/sched.c |   21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -605,10 +605,10 @@ static inline int cpu_of(struct rq *rq)
>  /*
>   * Return the group to which this tasks belongs.
>   *
> - * We use task_subsys_state_check() and extend the RCU verification
> - * with lockdep_is_held(&p->pi_lock) because cpu_cgroup_attach()
> - * holds that lock for each task it moves into the cgroup. Therefore
> - * by holding that lock, we pin the task to the current cgroup.
> + * We use task_subsys_state_check() and extend the RCU verification with
> + * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each
> + * task it moves into the cgroup. Therefore by holding either of those locks,
> + * we pin the task to the current cgroup.
>   */
>  static inline struct task_group *task_group(struct task_struct *p)
>  {
> @@ -616,7 +616,8 @@ static inline struct task_group *task_gr
>  	struct cgroup_subsys_state *css;
>  
>  	css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
> -			lockdep_is_held(&p->pi_lock));
> +			lockdep_is_held(&p->pi_lock) ||
> +			lockdep_is_held(&task_rq(p)->lock));
>  	tg = container_of(css, struct task_group, css);
>  
>  	return autogroup_task_group(p, tg);
> @@ -2200,6 +2201,16 @@ void set_task_cpu(struct task_struct *p,
>  			!(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));
>  
>  #ifdef CONFIG_LOCKDEP
> +	/*
> +	 * The caller should hold either p->pi_lock or rq->lock, when changing
> +	 * a task's CPU.
> +	 *
> +	 * sched_move_task() holds both and thus holding either pins the cgroup,
> +	 * see set_task_rq().
> +	 *
> +	 * Furthermore, all task_rq users should acquire both locks, see
> +	 * task_rq_lock().
> +	 */
>  	WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
>  				      lockdep_is_held(&task_rq(p)->lock)));
>  #endif
>