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Subject[PATCH] sched: RCU-protect __set_task_cpu() in set_task_cpu()
Wrap __set_task_cpu() with RCU read-side critical section. 
__set_task_cpu() calls task_group() that performs rcu dereference check in
task_subsys_state_check(), causing:

[ 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
[ 152.262923] [<ffffffff814775e6>] ? schedule+0x353/0xa9e
[ 152.262931] [<ffffffff8147de58>] ? sub_preempt_count+0x8f/0xa3
[ 152.262938] [<ffffffff8109fd2c>] ? watchdog_enable+0x195/0x195
[ 152.262946] [<ffffffff810072e5>] ? native_sched_clock+0x38/0x65
[ 152.262953] [<ffffffff81062c0c>] ? cpu_clock+0x4a/0x5f
[ 152.262958] [<ffffffff8109fd2c>] ? watchdog_enable+0x195/0x195
[ 152.262965] [<ffffffff81071a15>] ? trace_hardirqs_on_caller+0x10d/0x131
[ 152.262971] [<ffffffff81071a46>] ? trace_hardirqs_on+0xd/0xf
[ 152.262977] [<ffffffff8109fd2c>] ? watchdog_enable+0x195/0x195
[ 152.262983] [<ffffffff8109fd94>] watchdog+0x68/0xab
[ 152.262990] [<ffffffff8105cb82>] kthread+0x9a/0xa2
[ 152.262999] [<ffffffff81481e24>] kernel_thread_helper+0x4/0x10
[ 152.263005] [<ffffffff8102d6bf>] ? finish_task_switch+0x76/0xf0
[ 152.263012] [<ffffffff8147b258>] ? retint_restore_args+0x13/0x13
[ 152.263019] [<ffffffff8105cae8>] ? __init_kthread_worker+0x53/0x53
[ 152.263024] [<ffffffff81481e20>] ? gs_change+0x13/0x13

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

---
kernel/sched.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index cbb3a0e..cf0dd8a 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2212,7 +2212,9 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, 1, NULL, 0);
}

+ rcu_read_lock();
__set_task_cpu(p, new_cpu);
+ rcu_read_unlock();
}

struct migration_arg {


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