[PATCH 4/6] sched: Provide rt_mutex specific scheduler helpers

Peter Zijlstra
2023-08-15T11:18:06Z

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>
---
include/linux/sched.h | 3 +++
include/linux/sched/rt.h | 4 ++++
kernel/sched/core.c | 36 ++++++++++++++++++++++++++++++++----
3 files changed, 39 insertions(+), 4 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -906,6 +906,9 @@ 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;
--- a/include/linux/sched/rt.h
+++ b/include/linux/sched/rt.h
@@ -30,6 +30,10 @@ static inline bool task_is_realtime(stru
}

#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.
*/
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6738,9 +6738,6 @@ static inline void sched_submit_work(str
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.
@@ -6798,7 +6795,12 @@ asmlinkage __visible void __sched schedu
{
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);
}
@@ -7059,6 +7061,32 @@ static void __setscheduler_prio(struct t

#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)