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sched: Provide rt_mutex specific scheduler helpers

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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
This commit is contained in:
Peter Zijlstra 2023-09-08 18:22:51 +02:00
parent de1474b46d
commit 6b596e62ed
3 changed files with 39 additions and 4 deletions
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3
include/linux/sched.h
View file

@ -911,6 +911,9 @@ 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;

4
include/linux/sched/rt.h
View file

@ -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.
*/

36
kernel/sched/core.c
View file

@ -6723,9 +6723,6 @@ 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.
@ -6783,7 +6780,12 @@ 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);
}
@ -7044,6 +7046,32 @@ 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)