From: Zhou Chengming <zhouchengmi...@huawei.com>

push_rt_task() pick the first pushable task and find an eligible
lowest_rq, then double_lock_balance(rq, lowest_rq). So if
double_lock_balance() unlock the rq (when double_lock_balance() return 1),
we have to check if this task is still on the rq.

The problem is that the check conditions are not sufficient:

if (unlikely(task_rq(task) != rq ||
             !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_allowed) ||
             task_running(rq, task) ||
             !rt_task(task) ||
             !task_on_rq_queued(task))) {

cpu2                            cpu1                    cpu0
push_rt_task(rq1)
  pick task_A on rq1
  find rq0
    double_lock_balance(rq1, rq0)
      unlock(rq1)
                                rq1 __schedule
                                  pick task_A run
                                task_A sleep (dequeued)
      lock(rq0)
      lock(rq1)
    do_above_check(task_A)
      task_rq(task_A) == rq1
      cpus_allowed unchanged
      task_running == false
      rt_task(task_A) == true
                                                        try_to_wake_up(task_A)
                                                          select_cpu = cpu3
                                                          enqueue(rq3, task_A)
                                                          task_A->on_rq = 1
      task_on_rq_queued(task_A)
    above_check passed, return rq0
    ...
    migrate task_A from rq1 to rq0

So we can't rely on these checks of task_A to make sure the task_A is
still on the rq1, even though we hold the rq1->lock. This patch will
repick the first pushable task to be sure the task is still on the rq.

Signed-off-by: Zhou Chengming <zhouchengmi...@huawei.com>
Signed-off-by: Li Bin <huawei.li...@huawei.com>
---
 kernel/sched/rt.c | 49 +++++++++++++++++++++++--------------------------
 1 file changed, 23 insertions(+), 26 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index aad49451..e51d574 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1695,6 +1695,26 @@ static int find_lowest_rq(struct task_struct *task)
        return -1;
 }
 
+static struct task_struct *pick_next_pushable_task(struct rq *rq)
+{
+       struct task_struct *p;
+
+       if (!has_pushable_tasks(rq))
+               return NULL;
+
+       p = plist_first_entry(&rq->rt.pushable_tasks,
+                             struct task_struct, pushable_tasks);
+
+       BUG_ON(rq->cpu != task_cpu(p));
+       BUG_ON(task_current(rq, p));
+       BUG_ON(p->nr_cpus_allowed <= 1);
+
+       BUG_ON(!task_on_rq_queued(p));
+       BUG_ON(!rt_task(p));
+
+       return p;
+}
+
 /* Will lock the rq it finds */
 static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 {
@@ -1726,13 +1746,10 @@ static struct rq *find_lock_lowest_rq(struct 
task_struct *task, struct rq *rq)
                         * We had to unlock the run queue. In
                         * the mean time, task could have
                         * migrated already or had its affinity changed.
-                        * Also make sure that it wasn't scheduled on its rq.
                         */
-                       if (unlikely(task_rq(task) != rq ||
-                                    !cpumask_test_cpu(lowest_rq->cpu, 
&task->cpus_allowed) ||
-                                    task_running(rq, task) ||
-                                    !rt_task(task) ||
-                                    !task_on_rq_queued(task))) {
+                       struct task_struct *next_task = 
pick_next_pushable_task(rq);
+                       if (unlikely(next_task != task ||
+                                    !cpumask_test_cpu(lowest_rq->cpu, 
&task->cpus_allowed))) {
 
                                double_unlock_balance(rq, lowest_rq);
                                lowest_rq = NULL;
@@ -1752,26 +1769,6 @@ static struct rq *find_lock_lowest_rq(struct task_struct 
*task, struct rq *rq)
        return lowest_rq;
 }
 
-static struct task_struct *pick_next_pushable_task(struct rq *rq)
-{
-       struct task_struct *p;
-
-       if (!has_pushable_tasks(rq))
-               return NULL;
-
-       p = plist_first_entry(&rq->rt.pushable_tasks,
-                             struct task_struct, pushable_tasks);
-
-       BUG_ON(rq->cpu != task_cpu(p));
-       BUG_ON(task_current(rq, p));
-       BUG_ON(p->nr_cpus_allowed <= 1);
-
-       BUG_ON(!task_on_rq_queued(p));
-       BUG_ON(!rt_task(p));
-
-       return p;
-}
-
 /*
  * If the current CPU has more than one RT task, see if the non
  * running task can migrate over to a CPU that is running a task
-- 
1.7.12.4

Reply via email to