Date:	Fri, 22 Jun 2012 17:06:49 +0200
From:	Stefan Bader <stefan.bader@...onical.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...nel.org, Oleg Nesterov <oleg@...hat.com>,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>,
	Andrew Vagin <avagin@...nvz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [RFC][PATCH] sched: Fix race in task_group()

On 22.06.2012 13:36, Peter Zijlstra wrote:
> Stefan reported a crash on a kernel before a3e5d1091c1 ("sched: Don't
> call task_group() too many times in set_task_rq()"), he found the reason
> to be that the multiple task_group() invocations in set_task_rq()
> returned different values.
> 
> Looking at all that I found a lack of serialization and plain wrong
> comments.
> 
> The below tries to fix it using an extra pointer which is updated under
> the appropriate scheduler locks. Its not pretty, but I can't really see
> another way given how all the cgroup stuff works.
> 
> Anybody else got a better idea?
> 
> 
> Reported-by: Stefan Bader <stefan.bader@...onical.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  include/linux/init_task.h |   12 +++++++++++-
>  include/linux/sched.h     |    5 ++++-
>  kernel/sched/core.c       |    9 ++++++++-
>  kernel/sched/sched.h      |   23 ++++++++++-------------
>  4 files changed, 33 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> index 4e4bc1a..53be033 100644
> --- a/include/linux/init_task.h
> +++ b/include/linux/init_task.h
> @@ -123,8 +123,17 @@ extern struct group_info init_groups;
>  
>  extern struct cred init_cred;
>  
> +extern struct task_group root_task_group;
> +
> +#ifdef CONFIG_CGROUP_SCHED
> +# define INIT_CGROUP_SCHED(tsk)						\
> +	.sched_task_group = &root_task_group,
> +#else
> +# define INIT_CGROUP_SCHED(tsk)
> +#endif
> +
>  #ifdef CONFIG_PERF_EVENTS
> -# define INIT_PERF_EVENTS(tsk)					\
> +# define INIT_PERF_EVENTS(tsk)						\
>  	.perf_event_mutex = 						\
>  		 __MUTEX_INITIALIZER(tsk.perf_event_mutex),		\
>  	.perf_event_list = LIST_HEAD_INIT(tsk.perf_event_list),
> @@ -168,6 +177,7 @@ extern struct cred init_cred;
>  	},								\
>  	.tasks		= LIST_HEAD_INIT(tsk.tasks),			\
>  	INIT_PUSHABLE_TASKS(tsk)					\
> +	INIT_CGROUP_SCHED(tsk)						\
>  	.ptraced	= LIST_HEAD_INIT(tsk.ptraced),			\
>  	.ptrace_entry	= LIST_HEAD_INIT(tsk.ptrace_entry),		\
>  	.real_parent	= &tsk,						\
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 32157b9..77437d4 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1246,6 +1246,9 @@ struct task_struct {
>  	const struct sched_class *sched_class;
>  	struct sched_entity se;
>  	struct sched_rt_entity rt;
> +#ifdef CONFIG_CGROUP_SCHED
> +	struct task_struct *sched_task_group;
> +#endif
>  
>  #ifdef CONFIG_NUMA
>  	unsigned long	 numa_contrib;
> @@ -2741,7 +2744,7 @@ extern int sched_group_set_rt_period(struct task_group *tg,
>  extern long sched_group_rt_period(struct task_group *tg);
>  extern int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
>  #endif
> -#endif
> +#endif /* CONFIG_CGROUP_SCHED */
>  
>  extern int task_can_switch_user(struct user_struct *up,
>  					struct task_struct *tsk);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9bb7d28..9adb9a0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1096,7 +1096,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>  	 * a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.
>  	 *
>  	 * sched_move_task() holds both and thus holding either pins the cgroup,
> -	 * see set_task_rq().
> +	 * see task_group().
>  	 *
>  	 * Furthermore, all task_rq users should acquire both locks, see
>  	 * task_rq_lock().
> @@ -7581,6 +7581,8 @@ void sched_destroy_group(struct task_group *tg)
>   */
>  void sched_move_task(struct task_struct *tsk)
>  {
> +	struct cgroup_subsys_state *css;
> +	struct task_group *tg;
>  	int on_rq, running;
>  	unsigned long flags;
>  	struct rq *rq;
> @@ -7595,6 +7597,11 @@ void sched_move_task(struct task_struct *tsk)
>  	if (unlikely(running))
>  		tsk->sched_class->put_prev_task(rq, tsk);
>  
> +	tg = container_of(task_subsys_state(p, cpu_cgroup_subsys_id),
s/p/tsk/
> +			  struct task_group, css);
> +	tg = autogroup_task_group(p, tg);
s/p/tsk/
> +	tsk->sched_task_group = tg;
> +
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>  	if (tsk->sched_class->task_move_group)
>  		tsk->sched_class->task_move_group(tsk, on_rq);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 4134d37..c26378c 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -554,22 +554,19 @@ extern int group_balance_cpu(struct sched_group *sg);
>  /*
>   * Return the group to which this tasks belongs.
>   *
> - * 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.
> + * We cannot use task_subsys_state() and friends because the cgroup
> + * subsystem changes that value before the cgroup_subsys::attach() method
> + * is called, therefore we cannot pin it and might observe the wrong value.
> + *
> + * The same is true for autogroup's p->signal->autogroup->tg, the autogroup
> + * core changes this before calling sched_move_task().
> + *
> + * Instead we use a 'copy' which is updated from sched_move_task() while
> + * holding both task_struct::pi_lock and rq::lock.
>   */
>  static inline struct task_group *task_group(struct task_struct *p)
>  {
> -	struct task_group *tg;
> -	struct cgroup_subsys_state *css;
> -
> -	css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
> -			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);
> +	return p->sched_task_group;
>  }
>  
>  /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
> 

Tried out a backported (to 3.2) version of above patch which mainly differs in
having to move sched/sched.h changes back into sched.c and got this warning on boot:

[    2.648099] ===============================
[    2.648205] [ INFO: suspicious RCU usage. ]
[    2.648338] -------------------------------
[    2.648465] /home/smb/precise-amd64/ubuntu-2.6/include/linux/cgroup.h:548
suspicious rcu_dereference_check() usage!
[    2.648775]
[    2.648777] other info that might help us debug this:
[    2.648780]
[    2.649010]
[    2.649012] rcu_scheduler_active = 1, debug_locks = 0
[    2.649205] 3 locks held by udevd/91:
[    2.649296]  #0:  (&(&sighand->siglock)->rlock){......}, at:
[<ffffffff8107ff24>] __lock_task_sighand+0x94/0x1b0
[    2.649824]  #1:  (&p->pi_lock){-.-.-.}, at: [<ffffffff8104ee90>]
task_rq_lock+0x40/0xb0
[    2.650071]  #2:  (&rq->lock){-.-.-.}, at: [<ffffffff8104eeab>]
task_rq_lock+0x5b/0xb0
[    2.650297]
[    2.650299] stack backtrace:
[    2.650439] Pid: 91, comm: udevd Not tainted 3.2.0-26-generic #41+lp999755v7
[    2.650562] Call Trace:
[    2.650562]  [<ffffffff810a5507>] lockdep_rcu_suspicious+0xd7/0xe0
[    2.650562]  [<ffffffff81065ea5>] sched_move_task+0x165/0x230
[    2.650562]  [<ffffffff8107feb3>] ? __lock_task_sighand+0x23/0x1b0
[    2.650562]  [<ffffffff8106607f>] autogroup_move_group+0xbf/0x160
[    2.650562]  [<ffffffff8106620e>] sched_autogroup_create_attach+0xce/0x150
[    2.650562]  [<ffffffff81084ca4>] sys_setsid+0xd4/0xf0
[    2.650562]  [<ffffffff816affc2>] system_call_fastpath+0x16/0x1b

Will see how well it survives the test but thought to let you know.

-Stefan


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