sched: Provide rt_mutex specific scheduler helpers

JIRA: https://issues.redhat.com/browse/RHEL-28616

commit 6b596e62ed9f90c4a97e68ae1f7b1af5beeb3c05
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Fri, 8 Sep 2023 18:22:51 +0200

    sched: Provide rt_mutex specific scheduler helpers

    With PREEMPT_RT there is a rt_mutex recursion problem where
    sched_submit_work() can use an rtlock (aka spinlock_t). More
    specifically what happens is:

      mutex_lock() /* really rt_mutex */
        ...
          __rt_mutex_slowlock_locked()
            task_blocks_on_rt_mutex()
              // enqueue current task as waiter
              // do PI chain walk
            rt_mutex_slowlock_block()
              schedule()
                sched_submit_work()
                  ...
                  spin_lock() /* really rtlock */
                    ...
                      __rt_mutex_slowlock_locked()
                        task_blocks_on_rt_mutex()
                          // enqueue current task as waiter *AGAIN*
                          // *CONFUSION*

    Fix this by making rt_mutex do the sched_submit_work() early, before
    it enqueues itself as a waiter -- before it even knows *if* it will
    wait.

    [[ basically Thomas' patch but with different naming and a few asserts
       added ]]

    Originally-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20230908162254.999499-5-bigeasy@linutronix.de

Signed-off-by: Waiman Long <longman@redhat.com>

include/linux/sched.h

@@ -921,6 +921,9 @@ struct task_struct {
 	 * ->sched_remote_wakeup gets used, so it can be in this word.
 	 */
 	unsigned			sched_remote_wakeup:1;
+#ifdef CONFIG_RT_MUTEXES
+	unsigned			sched_rt_mutex:1;
+#endif
 
 	/* Bit to tell LSMs we're in execve(): */
 	unsigned			in_execve:1;

include/linux/sched/rt.h

@@ -30,6 +30,10 @@ static inline bool task_is_realtime(struct task_struct *tsk)
 }
 
 #ifdef CONFIG_RT_MUTEXES
+extern void rt_mutex_pre_schedule(void);
+extern void rt_mutex_schedule(void);
+extern void rt_mutex_post_schedule(void);
+
 /*
  * Must hold either p->pi_lock or task_rq(p)->lock.
  */

kernel/sched/core.c

@@ -6763,9 +6763,6 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	static DEFINE_WAIT_OVERRIDE_MAP(sched_map, LD_WAIT_CONFIG);
 	unsigned int task_flags;
 
-	if (task_is_running(tsk))
-		return;
-
 	/*
 	 * Establish LD_WAIT_CONFIG context to ensure none of the code called
 	 * will use a blocking primitive -- which would lead to recursion.
@@ -6823,7 +6820,12 @@ asmlinkage __visible void __sched schedule(void)
 {
 	struct task_struct *tsk = current;
 
-	sched_submit_work(tsk);
+#ifdef CONFIG_RT_MUTEXES
+	lockdep_assert(!tsk->sched_rt_mutex);
+#endif
+
+	if (!task_is_running(tsk))
+		sched_submit_work(tsk);
 	__schedule_loop(SM_NONE);
 	sched_update_worker(tsk);
 }
@@ -7113,6 +7115,32 @@ static void __setscheduler_prio(struct task_struct *p, int prio)
 
 #ifdef CONFIG_RT_MUTEXES
 
+/*
+ * Would be more useful with typeof()/auto_type but they don't mix with
+ * bit-fields. Since it's a local thing, use int. Keep the generic sounding
+ * name such that if someone were to implement this function we get to compare
+ * notes.
+ */
+#define fetch_and_set(x, v) ({ int _x = (x); (x) = (v); _x; })
+
+void rt_mutex_pre_schedule(void)
+{
+	lockdep_assert(!fetch_and_set(current->sched_rt_mutex, 1));
+	sched_submit_work(current);
+}
+
+void rt_mutex_schedule(void)
+{
+	lockdep_assert(current->sched_rt_mutex);
+	__schedule_loop(SM_NONE);
+}
+
+void rt_mutex_post_schedule(void)
+{
+	sched_update_worker(current);
+	lockdep_assert(fetch_and_set(current->sched_rt_mutex, 0));
+}
+
 static inline int __rt_effective_prio(struct task_struct *pi_task, int prio)
 {
 	if (pi_task)