Re: [PATCH v7 1/3] sched/core: warn on call put_task_struct in invalid context

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2023-04-25 08:43:01 [-0300], Wander Lairson Costa wrote:
> Under PREEMPT_RT, spinlocks become sleepable locks. put_task_struct()
> indirectly acquires a spinlock. Therefore, it can't be called in
> atomic/interrupt context in RT kernels.
> 
> To prevent such conditions, add a check for atomic/interrupt context
> before calling put_task_struct().
> 
> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Been only CCed here.

I asked to not special case PREEMPT_RT but doing this (clean up via RCU)
unconditionally. I don't remember that someone said "this is a bad
because $reason".

Lockdep will complain about this on !RT.

The below open codes rtlock_might_resched() with no explanation on why
it works or where it comes from.

The function is named put_task_struct_atomic_safe() yet it behaves it
differently on PREEMPT_RT otherwise it remains put_task_struct().

Not good.

> ---
>  include/linux/sched/task.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 357e0068497c..b597b97b1f8f 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -113,14 +113,28 @@ static inline struct task_struct *get_task_struct(struct task_struct *t)
>  
>  extern void __put_task_struct(struct task_struct *t);
>  
> +#define PUT_TASK_RESCHED_OFFSETS \
> +	(rcu_preempt_depth() << MIGHT_RESCHED_RCU_SHIFT)
> +
> +#define __put_task_might_resched() \
> +	__might_resched(__FILE__, __LINE__, PUT_TASK_RESCHED_OFFSETS)
> +
> +#define put_task_might_resched()			\
> +	do {						\
> +		if (IS_ENABLED(CONFIG_PREEMPT_RT))	\
> +			__put_task_might_resched();	\
> +	} while (0)
> +
>  static inline void put_task_struct(struct task_struct *t)
>  {
> +	put_task_might_resched();
>  	if (refcount_dec_and_test(&t->usage))
>  		__put_task_struct(t);
>  }
>  
>  static inline void put_task_struct_many(struct task_struct *t, int nr)
>  {
> +	put_task_might_resched();
>  	if (refcount_sub_and_test(nr, &t->usage))
>  		__put_task_struct(t);
>  }
> -- 
> 2.40.0
> 

Sebastian