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


Tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

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
>


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