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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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