Re: [PATCH] mm: memory-failure: use rcu lock instead of tasklist_lock when collect_procs()

[Matthew Wilcox]

On Mon, Aug 21, 2023 at 10:25:34AM +0800, Tong Tiangen wrote:
> +++ b/mm/memory-failure.c
> @@ -546,24 +546,26 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
>   * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO)
>   * on behalf of the thread group. Return task_struct of the (first found)
>   * dedicated thread if found, and return NULL otherwise.
> - *
> - * We already hold read_lock(&tasklist_lock) in the caller, so we don't
> - * have to call rcu_read_lock/unlock() in this function.
>   */
>  static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
>  {
>  	struct task_struct *t;
>  
> +	rcu_read_lock();
>  	for_each_thread(tsk, t) {
>  		if (t->flags & PF_MCE_PROCESS) {
>  			if (t->flags & PF_MCE_EARLY)
> -				return t;
> +				goto found;
>  		} else {
>  			if (sysctl_memory_failure_early_kill)
> -				return t;
> +				goto found;
>  		}
>  	}
> -	return NULL;
> +
> +	t = NULL;
> +found:
> +	rcu_read_unlock();
> +	return t;
>  }

I don't understand why you need to modify find_early_kill_thread() at
all.  It's still true that the caller holds _a_ lock protecting it; the
comment needs to be updated to reflect that it might be the RCU lock
or the tasklist_lock (or did you change all callers?), but there's no
need for this function to take the RCU lock itself, afaics?