Re: [PATCH v9] kernel/fork: beware of __put_task_struct calling context

[Oleg Nesterov <oleg@redhat.com>]

On 05/16, Wander Lairson Costa wrote:
>
>  static inline void put_task_struct(struct task_struct *t)
>  {
> -	if (refcount_dec_and_test(&t->usage))
> +	if (!refcount_dec_and_test(&t->usage))
> +		return;
> +
> +	/*
> +	 * 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 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.
> +	 *
> +	 * delayed_free_task() also uses ->rcu, but it is only called
> +	 * when it fails to fork a process. Therefore, there is no
> +	 * way it can conflict with put_task_struct().
> +	 */
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> +		call_rcu(&t->rcu, __put_task_struct_rcu_cb);
> +	else
>  		__put_task_struct(t);
>  }

LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...

Again, I'll try to investigate when I have time although I am not sure I can really help.

Perhaps you too can try to do this ? ;)

Oleg.