Patch "sched/rt: Fix bad task migration for rt tasks" has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    sched/rt: Fix bad task migration for rt tasks

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sched-rt-fix-bad-task-migration-for-rt-tasks.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit eee32781c5c40f03714450753341e0ca6a0b5b19
Author: Schspa Shi <schspa@xxxxxxxxx>
Date:   Mon Aug 29 01:03:02 2022 +0800

    sched/rt: Fix bad task migration for rt tasks
    
    [ Upstream commit feffe5bb274dd3442080ef0e4053746091878799 ]
    
    Commit 95158a89dd50 ("sched,rt: Use the full cpumask for balancing")
    allows find_lock_lowest_rq() to pick a task with migration disabled.
    The purpose of the commit is to push the current running task on the
    CPU that has the migrate_disable() task away.
    
    However, there is a race which allows a migrate_disable() task to be
    migrated. Consider:
    
      CPU0                                    CPU1
      push_rt_task
        check is_migration_disabled(next_task)
    
                                              task not running and
                                              migration_disabled == 0
    
        find_lock_lowest_rq(next_task, rq);
          _double_lock_balance(this_rq, busiest);
            raw_spin_rq_unlock(this_rq);
            double_rq_lock(this_rq, busiest);
              <<wait for busiest rq>>
                                                  <wakeup>
                                              task become running
                                              migrate_disable();
                                                <context out>
        deactivate_task(rq, next_task, 0);
        set_task_cpu(next_task, lowest_rq->cpu);
          WARN_ON_ONCE(is_migration_disabled(p));
    
    Fixes: 95158a89dd50 ("sched,rt: Use the full cpumask for balancing")
    Signed-off-by: Schspa Shi <schspa@xxxxxxxxx>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
    Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
    Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
    Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>
    Tested-by: Dwaine Gonyier <dgonyier@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 9ae8f41e3372f..f7d381b6c3133 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2246,6 +2246,7 @@ static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq)
 				     !cpumask_test_cpu(later_rq->cpu, &task->cpus_mask) ||
 				     task_on_cpu(rq, task) ||
 				     !dl_task(task) ||
+				     is_migration_disabled(task) ||
 				     !task_on_rq_queued(task))) {
 				double_unlock_balance(rq, later_rq);
 				later_rq = NULL;
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 0a11f44adee57..4f5796dd26a56 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2000,11 +2000,15 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 			 * the mean time, task could have
 			 * migrated already or had its affinity changed.
 			 * Also make sure that it wasn't scheduled on its rq.
+			 * It is possible the task was scheduled, set
+			 * "migrate_disabled" and then got preempted, so we must
+			 * check the task migration disable flag here too.
 			 */
 			if (unlikely(task_rq(task) != rq ||
 				     !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_mask) ||
 				     task_on_cpu(rq, task) ||
 				     !rt_task(task) ||
+				     is_migration_disabled(task) ||
 				     !task_on_rq_queued(task))) {
 
 				double_unlock_balance(rq, lowest_rq);



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