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sched/rt: Fix bad task migration for rt tasks

Valentin Schneider requested to merge vschneid/centos-stream-9:redhat/bz/2182900/mr into main

sched/rt: Fix bad task migration for rt tasks

Bugzilla: https://bugzilla.redhat.com/2182900 Upstream-status: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git

commit feffe5bb274dd3442080ef0e4053746091878799 Author: Schspa Shi schspa@gmail.com Date: Mon Aug 29 01:03:02 2022 +0800

sched/rt: Fix bad task migration for rt tasks

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@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Tested-by: Dwaine Gonyier <dgonyier@redhat.com>

Signed-off-by: Valentin Schneider vschneid@redhat.com

Edited by Valentin Schneider

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