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Message-ID: <e0c71c5c-09e6-d94e-6db3-3acf3ee502d6@gmail.com>
Date: Mon, 21 Aug 2023 10:55:58 -0700
From: Kui-Feng Lee <sinquersw@...il.com>
To: Oleg Nesterov <oleg@...hat.com>, Yonghong Song <yhs@...com>,
Kui-Feng Lee <kuifeng@...com>,
Andrii Nakryiko <andrii@...nel.org>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
Martin KaFai Lau <martin.lau@...nel.org>,
bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of
next_thread()
On 8/21/23 08:09, Oleg Nesterov wrote:
> 1. find_pid_ns() + get_pid_task() under rcu_read_lock() guarantees that we
> can safely iterate the task->thread_group list. Even if this task exits
> right after get_pid_task() (or goto retry) and pid_alive() returns 0 >
> Kill the unnecessary pid_alive() check.
This function will return next_task holding a refcount, and release the
refcount until the next time calling the same function. Meanwhile,
the returned task A may be killed, and its next task B may be
killed after A as well, before calling this function again.
However, even task B is destroyed (free), A's next is still pointing to
task B. When this function is called again for the same iterator,
it doesn't promise that B is still there.
Does that make sense to you?
>
> 2. next_thread() simply can't return NULL, kill the bogus "if (!next_task)"
> check.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
> kernel/bpf/task_iter.c | 7 -------
> 1 file changed, 7 deletions(-)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index c4ab9d6cdbe9..4d1125108014 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -75,15 +75,8 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
> return NULL;
>
> retry:
> - if (!pid_alive(task)) {
> - put_task_struct(task);
> - return NULL;
> - }
> -
> next_task = next_thread(task);
> put_task_struct(task);
> - if (!next_task)
> - return NULL;
>
> saved_tid = *tid;
> *tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);
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