[PATCH] sched/fair: Clear throttle state when leaving the fair class

[Daehyeon Ko <4ncienth@gmail.com>]

Task-based CFS bandwidth throttling removes a task's fair sched_entity
while the scheduler core still considers the task queued. p->throttled
carries the fair-specific state across dequeue/enqueue operations.

DEQUEUE_CLASS removes the task from cfs_rq->throttled_limbo_list
but retains the bit. If the last finite CFS quota is disabled while
the task runs in another class, it is absent from the unthrottle walk and
the bit survives.
Returning to fair while cfs_bandwidth_used() is false then performs a real
fair enqueue with p->throttled still set.

After bandwidth is re-enabled, a class-change dequeue treats the
stale bit as limbo state and skips __dequeue_task(). p->se.on_rq therefore
remains set, and put_prev_task_fair() places the entity in the EEVDF tree
while the task moves to RT. On exit, the fair picker can select the
TASK_DEAD task. __schedule() then returns to do_task_dead(),
which hits BUG().

Keep p->throttled across migrations and group moves, but clear it on
DEQUEUE_CLASS because fair-specific state must not outlive fair-class
membership. A later return to fair uses the normal enqueue path. That path
still respects a throttled cfs_rq.

A KASAN, LOCKDEP and DEBUG_LIST v7.3 kernel reproduced the do_task_dead()
BUG in 3/3 root and 3/3 delegated uid-1000 runs with CapPrm=CapEff=0 and
RLIMIT_RTPRIO=1. The fix passed both matrices plus a 3/3 class-switch
control with bandwidth continuously enabled, without a scheduler warning,
KASAN report, lockdep splat or crash.

An AI coding assistant analyzed the state mismatch and drafted the initial
reproducer, fix candidate and this changelog. The source paths, trigger,
controls and results were rechecked against the cited revisions.

Fixes: e1fad12dcb66 ("sched/fair: Switch to task based throttle model")
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Cc: stable@vger.kernel.org
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0feec172fd37eb..524fe88bbb381a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6957,8 +6957,8 @@ static void dequeue_throttled_task(struct task_struct *p, int flags)
 	WARN_ON_ONCE(p->se.on_rq);
 	list_del_init(&p->throttle_node);
 
-	/* task blocked after throttled */
-	if (flags & DEQUEUE_SLEEP) {
+	/* Task blocked after throttling or is leaving the fair class. */
+	if (flags & (DEQUEUE_SLEEP | DEQUEUE_CLASS)) {
 		p->throttled = false;
 		return;
 	}
-- 
2.55.0