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Subject[PATCH] sched_rt: RCU-protect for_each_domain() within find_lowest_rq()
Protect for_each_domain() within find_lowest_rq() with RCU read-side critical 
section, suppressing suspicious rcu_dereference_check():

[ 2752.503488] ===================================================
[ 2752.503494] [ INFO: suspicious rcu_dereference_check() usage. ]
[ 2752.503497] ---------------------------------------------------
[ 2752.503501] kernel/sched_rt.c:1266 invoked rcu_dereference_check() without protection!
[ 2752.503504]
[ 2752.503505] other info that might help us debug this:
[ 2752.503506]
[ 2752.503510] rcu_scheduler_active = 1, debug_locks = 0
[ 2752.503514] 3 locks held by rcub0/9:
[ 2752.503517] #0: (&lock->wait_lock){+.+...}, at: [<ffffffff81479294>] rt_mutex_slowlock+0x34/0x154
[ 2752.503534] #1: (&p->pi_lock){-.-.-.}, at: [<ffffffff810768ab>] task_blocks_on_rt_mutex+0x124/0x1d1
[ 2752.503547] #2: (&rq->lock){-.-.-.}, at: [<ffffffff8103844b>] rt_mutex_setprio+0x5f/0x252
[ 2752.503559]
[ 2752.503569] Call Trace:
[ 2752.503579] [<ffffffff8106e5ab>] lockdep_rcu_dereference+0xa7/0xaf
[ 2752.503587] [<ffffffff8102b089>] find_lowest_rq+0x131/0x1ad
[ 2752.503593] [<ffffffff810370d4>] push_rt_task.part.133+0x89/0x247
[ 2752.503599] [<ffffffff810372bc>] switched_to_rt+0x2a/0x65
[ 2752.503605] [<ffffffff810385b4>] rt_mutex_setprio+0x1c8/0x252
[ 2752.503610] [<ffffffff81076398>] __rt_mutex_adjust_prio+0x1d/0x21
[ 2752.503616] [<ffffffff810768e7>] task_blocks_on_rt_mutex+0x160/0x1d1
[ 2752.503622] [<ffffffff8147931c>] rt_mutex_slowlock+0xbc/0x154
[ 2752.503629] [<ffffffff814793c6>] rt_mutex_lock+0x12/0x14
[ 2752.503637] [<ffffffff810a4402>] rcu_boost+0xad/0xe1
[ 2752.503644] [<ffffffff81479fe2>] ? _raw_spin_unlock_irqrestore+0x56/0x74
[ 2752.503652] [<ffffffff8105d883>] ? finish_wait+0x5e/0x67
[ 2752.503657] [<ffffffff810a4509>] rcu_boost_kthread+0xd3/0xfc
[ 2752.503663] [<ffffffff8105d7c6>] ? __init_waitqueue_head+0x46/0x46
[ 2752.503669] [<ffffffff810a4436>] ? rcu_boost+0xe1/0xe1
[ 2752.503675] [<ffffffff8105cf6e>] kthread+0x9a/0xa2
[ 2752.503683] [<ffffffff814810e4>] kernel_thread_helper+0x4/0x10
[ 2752.503689] [<ffffffff8102d80c>] ? finish_task_switch+0x76/0xf0
[ 2752.503695] [<ffffffff8147a4d8>] ? retint_restore_args+0x13/0x13
[ 2752.503701] [<ffffffff8105ced4>] ? __init_kthread_worker+0x53/0x53
[ 2752.503707] [<ffffffff814810e0>] ? gs_change+0x13/0x13


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

---

kernel/sched_rt.c | 18 ++++++++++++++----
1 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 64b2a37..242e476 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1263,6 +1263,7 @@ static int find_lowest_rq(struct task_struct *task)
if (!cpumask_test_cpu(this_cpu, lowest_mask))
this_cpu = -1; /* Skip this_cpu opt if not among lowest */

+ rcu_read_lock();
for_each_domain(cpu, sd) {
if (sd->flags & SD_WAKE_AFFINE) {
int best_cpu;
@@ -1272,15 +1273,20 @@ static int find_lowest_rq(struct task_struct *task)
* remote processor.
*/
if (this_cpu != -1 &&
- cpumask_test_cpu(this_cpu, sched_domain_span(sd)))
- return this_cpu;
+ cpumask_test_cpu(this_cpu, sched_domain_span(sd))) {
+ cpu = this_cpu;
+ goto out_unlock;
+ }

best_cpu = cpumask_first_and(lowest_mask,
sched_domain_span(sd));
- if (best_cpu < nr_cpu_ids)
- return best_cpu;
+ if (best_cpu < nr_cpu_ids) {
+ cpu = best_cpu;
+ goto out_unlock;
+ }
}
}
+ rcu_read_unlock();

/*
* And finally, if there were no matches within the domains
@@ -1294,6 +1300,10 @@ static int find_lowest_rq(struct task_struct *task)
if (cpu < nr_cpu_ids)
return cpu;
return -1;
+
+out_unlock:
+ rcu_read_unlock();
+ return cpu;
}

/* Will lock the rq it finds */


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