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Date:   Wed, 13 May 2020 15:11:17 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andriin@...com>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <ast@...com>, <daniel@...earbox.net>
CC:     <andrii.nakryiko@...il.com>, <kernel-team@...com>
Subject: Re: [PATCH bpf-next] bpf: fix bpf_iter's task iterator logic



On 5/13/20 2:20 PM, Andrii Nakryiko wrote:
> task_seq_get_next might stop prematurely if get_pid_task() fails to get
> task_struct. Failure to do so doesn't mean that there are no more tasks with
> higher pids. Procfs's iteration algorithm (see next_tgid in fs/proc/base.c)
> does a retry in such case. After this fix, instead of stopping prematurely
> after about 300 tasks on my server, bpf_iter program now returns >4000, which
> sounds much closer to reality.
> 
> Cc: Yonghong Song <yhs@...com>
> Fixes: eaaacd23910f ("bpf: Add task and task/file iterator targets")
> Signed-off-by: Andrii Nakryiko <andriin@...com>

Thanks for the fix. We did this retry logic for bpf_map which is
idr based logic too. But forgot to check for task which has the
same issue.

Acked-by: Yonghong Song <yhs@...com>

> ---
>   kernel/bpf/task_iter.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index a9b7264dda08..e1836def6738 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -27,9 +27,15 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
>   	struct pid *pid;
>   
>   	rcu_read_lock();
> +retry:
>   	pid = idr_get_next(&ns->idr, tid);
> -	if (pid)
> +	if (pid) {
>   		task = get_pid_task(pid, PIDTYPE_PID);
> +		if (!task) {
> +			*tid++;
> +			goto retry;
> +		}
> +	}
>   	rcu_read_unlock();
>   
>   	return task;
> 

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