Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Paul E. McKenney]

On Fri, Apr 14, 2023 at 09:55:28AM -0300, Wander Lairson Costa wrote:
> Due to the possibility of indirectly acquiring sleeping locks, it is
> unsafe to call put_task_struct() in atomic contexts when the kernel is
> compiled with PREEMPT_RT.
> 
> To mitigate this issue, this commit introduces
> put_task_struct_atomic_safe(), which schedules __put_task_struct()
> through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> be a more natural approach, we cannot allocate dynamic memory from
> atomic context in PREEMPT_RT, making the code more complex.
> 
> This implementation ensures safe execution in atomic contexts and
> avoids any potential issues that may arise from using the non-atomic
> version.
> 
> Signed-off-by: Wander Lairson Costa <wander@xxxxxxxxxx>
> Reported-by: Hu Chunyu <chuhu@xxxxxxxxxx>
> Cc: Paul McKenney <paulmck@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
>  kernel/fork.c              |  8 ++++++++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index b597b97b1f8f..5c13b83d7008 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
>  
>  void put_task_struct_rcu_user(struct task_struct *task);
>  
> +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> +
> +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		/*
> +		 * Decrement the refcount explicitly to avoid unnecessarily
> +		 * calling call_rcu.
> +		 */
> +		if (refcount_dec_and_test(&task->usage))
> +			/*
> +			 * under PREEMPT_RT, we can't call put_task_struct
> +			 * in atomic context because it will indirectly
> +			 * acquire sleeping locks.
> +			 * call_rcu() will schedule delayed_put_task_struct_rcu()
> +			 * to be called in process context.
> +			 *
> +			 * __put_task_struct() is called called when
> +			 * refcount_dec_and_test(&t->usage) succeeds.
> +			 *
> +			 * This means that it can't "conflict" with
> +			 * put_task_struct_rcu_user() which abuses ->rcu the same
> +			 * way; rcu_users has a reference so task->usage can't be
> +			 * zero after rcu_users 1 -> 0 transition.
> +			 */
> +			call_rcu(&task->rcu, __delayed_put_task_struct);

This will invoke __delayed_put_task_struct() with softirqs disabled.
Or do softirq-disabled contexts count as non-atomic in PREEMPT_RT?

							Thanx, Paul

> +	} else {
> +		put_task_struct(task);
> +	}
> +}
> +
>  /* Free all architecture-specific resources held by a thread. */
>  void release_thread(struct task_struct *dead_task);
>  
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 0c92f224c68c..9884794fe4b8 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -854,6 +854,14 @@ void __put_task_struct(struct task_struct *tsk)
>  }
>  EXPORT_SYMBOL_GPL(__put_task_struct);
>  
> +void __delayed_put_task_struct(struct rcu_head *rhp)
> +{
> +	struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> +
> +	__put_task_struct(task);
> +}
> +EXPORT_SYMBOL_GPL(__delayed_put_task_struct);
> +
>  void __init __weak arch_task_cache_init(void) { }
>  
>  /*
> -- 
> 2.39.2
>