[PATCH] sched/ext: convert scx_tasks_lock to raw spinlock

[Emil Tsalapatis <etsal@meta.com>]

Update scx_task_locks so that it's safe to lock/unlock in a
non-sleepable context in PREEMPT_RT kernels. scx_task_locks is
(non-raw) spinlock used to protect the list of tasks under SCX.
This list is updated during from finish_task_switch(), which
cannot sleep. Regular spinlocks can be locked in such a context
in non-RT kernels, but are sleepable under when CONFIG_PREEMPT_RT=y.

Convert scx_task_locks into a raw spinlock, which is not sleepable
even on RT kernels.

Sample backtrace:

<TASK>
dump_stack_lvl+0x83/0xa0
__might_resched+0x14a/0x200
rt_spin_lock+0x61/0x1c0
? sched_ext_dead+0x2d/0xf0
? lock_release+0xc6/0x280
sched_ext_dead+0x2d/0xf0
? srso_alias_return_thunk+0x5/0xfbef5
finish_task_switch.isra.0+0x254/0x360
__schedule+0x584/0x11d0
? srso_alias_return_thunk+0x5/0xfbef5
? srso_alias_return_thunk+0x5/0xfbef5
? tick_nohz_idle_exit+0x7e/0x120
schedule_idle+0x23/0x40
cpu_startup_entry+0x29/0x30
start_secondary+0xf8/0x100
common_startup_64+0x13e/0x148
</TASK>


Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
---
 kernel/sched/ext.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index bbd7529fa420..4b1007e06e9a 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -25,7 +25,7 @@ static struct scx_sched __rcu *scx_root;
  * guarantee system safety. Maintain a dedicated task list which contains every
  * task between its fork and eventual free.
  */
-static DEFINE_SPINLOCK(scx_tasks_lock);
+static DEFINE_RAW_SPINLOCK(scx_tasks_lock);
 static LIST_HEAD(scx_tasks);
 
 /* ops enable/disable */
@@ -520,7 +520,7 @@ static void scx_task_iter_start(struct scx_task_iter *iter)
 {
 	memset(iter, 0, sizeof(*iter));
 
-	spin_lock_irq(&scx_tasks_lock);
+	raw_spin_lock_irq(&scx_tasks_lock);
 
 	iter->cursor = (struct sched_ext_entity){ .flags = SCX_TASK_CURSOR };
 	list_add(&iter->cursor.tasks_node, &scx_tasks);
@@ -549,14 +549,14 @@ static void scx_task_iter_unlock(struct scx_task_iter *iter)
 	__scx_task_iter_rq_unlock(iter);
 	if (iter->list_locked) {
 		iter->list_locked = false;
-		spin_unlock_irq(&scx_tasks_lock);
+		raw_spin_unlock_irq(&scx_tasks_lock);
 	}
 }
 
 static void __scx_task_iter_maybe_relock(struct scx_task_iter *iter)
 {
 	if (!iter->list_locked) {
-		spin_lock_irq(&scx_tasks_lock);
+		raw_spin_lock_irq(&scx_tasks_lock);
 		iter->list_locked = true;
 	}
 }
@@ -2983,9 +2983,9 @@ void scx_post_fork(struct task_struct *p)
 		}
 	}
 
-	spin_lock_irq(&scx_tasks_lock);
+	raw_spin_lock_irq(&scx_tasks_lock);
 	list_add_tail(&p->scx.tasks_node, &scx_tasks);
-	spin_unlock_irq(&scx_tasks_lock);
+	raw_spin_unlock_irq(&scx_tasks_lock);
 
 	percpu_up_read(&scx_fork_rwsem);
 }
@@ -3009,9 +3009,9 @@ void sched_ext_dead(struct task_struct *p)
 {
 	unsigned long flags;
 
-	spin_lock_irqsave(&scx_tasks_lock, flags);
+	raw_spin_lock_irqsave(&scx_tasks_lock, flags);
 	list_del_init(&p->scx.tasks_node);
-	spin_unlock_irqrestore(&scx_tasks_lock, flags);
+	raw_spin_unlock_irqrestore(&scx_tasks_lock, flags);
 
 	/*
 	 * @p is off scx_tasks and wholly ours. scx_enable()'s READY -> ENABLED
-- 
2.47.3