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

[Waiman Long <longman@redhat.com>]

On 4/14/23 08:55, 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@redhat.com>
> Reported-by: Hu Chunyu <chuhu@redhat.com>
> Cc: Paul McKenney <paulmck@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> ---
>   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()
delayed_put_task_struct_rcu()?
> +			 * to be called in process context.
> +			 *
> +			 * __put_task_struct() is called called when
"called called"?
> +			 * 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.

Note that put_task_struct_rcu_user() isn't the only user of task->rcu. 
delayed_free_task() in kernel/fork.c also uses it, though it is only 
called in the error case. Still you may need to take a look to make sure 
that there is no conflict.

Cheers,
Longman