[PATCH 5.17 0281/1126] sched/rt: Plug rt_mutex_setprio() vs push_rt_task() race

Greg Kroah-Hartman
2022-04-05T08:19:17Z

From: Valentin Schneider <valentin.schneider@arm.com>

[ Upstream commit 49bef33e4b87b743495627a529029156c6e09530 ]

John reported that push_rt_task() can end up invoking
find_lowest_rq(rq->curr) when curr is not an RT task (in this case a CFS
one), which causes mayhem down convert_prio().

This can happen when current gets demoted to e.g. CFS when releasing an
rt_mutex, and the local CPU gets hit with an rto_push_work irqwork before
getting the chance to reschedule. Exactly who triggers this work isn't
entirely clear to me - switched_from_rt() only invokes rt_queue_pull_task()
if there are no RT tasks on the local RQ, which means the local CPU can't
be in the rto_mask.

My current suspected sequence is something along the lines of the below,
with the demoted task being current.

mark_wakeup_next_waiter()
rt_mutex_adjust_prio()
rt_mutex_setprio() // deboost originally-CFS task
check_class_changed()
switched_from_rt() // Only rt_queue_pull_task() if !rq->rt.rt_nr_running
switched_to_fair() // Sets need_resched
__balance_callbacks() // if pull_rt_task(), tell_cpu_to_push() can't select local CPU per the above
raw_spin_rq_unlock(rq)

// need_resched is set, so task_woken_rt() can't
// invoke push_rt_tasks(). Best I can come up with is
// local CPU has rt_nr_migratory >= 2 after the demotion, so stays
// in the rto_mask, and then:

<some other CPU running rto_push_irq_work_func() queues rto_push_work on this CPU>
push_rt_task()
// breakage follows here as rq->curr is CFS

Move an existing check to check rq->curr vs the next pushable task's
priority before getting anywhere near find_lowest_rq(). While at it, add an
explicit sched_class of rq->curr check prior to invoking
find_lowest_rq(rq->curr). Align the DL logic to also reschedule regardless
of next_task's migratability.

Fixes: a7c81556ec4d ("sched: Fix migrate_disable() vs rt/dl balancing")
Reported-by: John Keeping <john@metanate.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: John Keeping <john@metanate.com>
Link: https://lore.kernel.org/r/20220127154059.974729-1-valentin.schneider@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/deadline.c | 12 ++++++------
kernel/sched/rt.c | 32 ++++++++++++++++++++++----------
2 files changed, 28 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index d2c072b0ef01..62f0cf842277 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2240,12 +2240,6 @@ static int push_dl_task(struct rq *rq)
return 0;

retry:
- if (is_migration_disabled(next_task))
- return 0;
-
- if (WARN_ON(next_task == rq->curr))
- return 0;
-
/*
* If next_task preempts rq->curr, and rq->curr
* can move away, it makes sense to just reschedule
@@ -2258,6 +2252,12 @@ static int push_dl_task(struct rq *rq)
return 0;
}

+ if (is_migration_disabled(next_task))
+ return 0;
+
+ if (WARN_ON(next_task == rq->curr))
+ return 0;
+
/* We might release rq lock */
get_task_struct(next_task);

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7b4f4fbbb404..14f273c29518 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2026,6 +2026,16 @@ static int push_rt_task(struct rq *rq, bool pull)
return 0;

retry:
+ /*
+ * It's possible that the next_task slipped in of
+ * higher priority than current. If that's the case
+ * just reschedule current.
+ */
+ if (unlikely(next_task->prio < rq->curr->prio)) {
+ resched_curr(rq);
+ return 0;
+ }
+
if (is_migration_disabled(next_task)) {
struct task_struct *push_task = NULL;
int cpu;
@@ -2033,6 +2043,18 @@ static int push_rt_task(struct rq *rq, bool pull)
if (!pull || rq->push_busy)
return 0;

+ /*
+ * Invoking find_lowest_rq() on anything but an RT task doesn't
+ * make sense. Per the above priority check, curr has to
+ * be of higher priority than next_task, so no need to
+ * reschedule when bailing out.
+ *
+ * Note that the stoppers are masqueraded as SCHED_FIFO
+ * (cf. sched_set_stop_task()), so we can't rely on rt_task().
+ */
+ if (rq->curr->sched_class != &rt_sched_class)
+ return 0;
+
cpu = find_lowest_rq(rq->curr);
if (cpu == -1 || cpu == rq->cpu)
return 0;
@@ -2057,16 +2079,6 @@ static int push_rt_task(struct rq *rq, bool pull)
if (WARN_ON(next_task == rq->curr))
return 0;

- /*
- * It's possible that the next_task slipped in of
- * higher priority than current. If that's the case
- * just reschedule current.
- */
- if (unlikely(next_task->prio < rq->curr->prio)) {
- resched_curr(rq);
- return 0;
- }
-
/* We might release rq lock */
get_task_struct(next_task);

--
2.34.1