[PATCH 4/5] sched_ext: Fix scx_sched_lock / rq lock ordering

[Tejun Heo <tj@kernel.org>]

There are two sites that nest rq lock inside scx_sched_lock:

- scx_bypass() takes scx_sched_lock then rq lock per CPU to propagate
  per-cpu bypass flags and re-enqueue tasks.

- sysrq_handle_sched_ext_dump() takes scx_sched_lock to iterate all
  scheds, scx_dump_state() then takes rq lock per CPU for dump.

And scx_claim_exit() takes scx_sched_lock to propagate exits to
descendants. It can be reached from scx_tick(), BPF kfuncs, and many
other paths with rq lock already held, creating the reverse ordering:

  rq lock -> scx_sched_lock vs. scx_sched_lock -> rq lock

Fix by flipping scx_bypass() to take rq lock first, and dropping
scx_sched_lock from sysrq_handle_sched_ext_dump() as scx_sched_all is
already RCU-traversable and scx_dump_lock now prevents dumping a dead
sched. This makes the consistent ordering rq lock -> scx_sched_lock.

Reported-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Link: http://lkml.kernel.org/r/20260309163025.2240221-1-yphbchou0911@gmail.com
Fixes: ebeca1f930ea ("sched_ext: Introduce cgroup sub-sched support")
Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index cf28a8f62ad0..677c1c6c64bf 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5097,8 +5097,8 @@ static void scx_bypass(struct scx_sched *sch, bool bypass)
 		struct rq *rq = cpu_rq(cpu);
 		struct task_struct *p, *n;
 
-		raw_spin_lock(&scx_sched_lock);
 		raw_spin_rq_lock(rq);
+		raw_spin_lock(&scx_sched_lock);
 
 		scx_for_each_descendant_pre(pos, sch) {
 			struct scx_sched_pcpu *pcpu = per_cpu_ptr(pos->pcpu, cpu);
@@ -7240,8 +7240,6 @@ static void sysrq_handle_sched_ext_dump(u8 key)
 	struct scx_exit_info ei = { .kind = SCX_EXIT_NONE, .reason = "SysRq-D" };
 	struct scx_sched *sch;
 
-	guard(raw_spinlock_irqsave)(&scx_sched_lock);
-
 	list_for_each_entry_rcu(sch, &scx_sched_all, all)
 		scx_dump_state(sch, &ei, 0, false);
 }
-- 
2.53.0