Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of get/put_task_struct

[Kui-Feng Lee <sinquersw@gmail.com>]

On 8/21/23 13:03, Oleg Nesterov wrote:
> get_pid_task() makes no sense, the code does put_task_struct() soon after.
> Use find_task_by_pid_ns() instead of find_pid_ns + get_pid_task and kill
> kill put_task_struct(), this allows to do get_task_struct() only once
> before return.
> 
> While at it, kill the unnecessary "if (!pid)" check in the "if (!*tid)"
> block, this matches the next usage of find_pid_ns() + get_pid_task() in
> this function.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>   kernel/bpf/task_iter.c | 12 ++----------
>   1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index 4d1125108014..1589ec3faded 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -42,9 +42,6 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>   	if (!*tid) {
>   		/* The first time, the iterator calls this function. */
>   		pid = find_pid_ns(common->pid, common->ns);
> -		if (!pid)
> -			return NULL;
> -
>   		task = get_pid_task(pid, PIDTYPE_TGID);
>   		if (!task)
>   			return NULL;
> @@ -66,17 +63,12 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>   		return task;
>   	}
>   
> -	pid = find_pid_ns(common->pid_visiting, common->ns);
> -	if (!pid)
> -		return NULL;
> -
> -	task = get_pid_task(pid, PIDTYPE_PID);
> +	task = find_task_by_pid_ns(common->pid_visiting, common->ns);
>   	if (!task)
>   		return NULL;
>   
>   retry:
>   	next_task = next_thread(task);
> -	put_task_struct(task);

It called get_task_struct() against this task to hold a refcount at the
previous time calling this function. When will it release the refcount?

>   
>   	saved_tid = *tid;
>   	*tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);
> @@ -88,7 +80,6 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>   		return NULL;
>   	}
>   
> -	get_task_struct(next_task);
>   	common->pid_visiting = *tid;
>   
>   	if (skip_if_dup_files && task->files == task->group_leader->files) {
> @@ -96,6 +87,7 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>   		goto retry;
>   	}
>   
> +	get_task_struct(next_task);
>   	return next_task;
>   }
>