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Subject: Re: [PATCH] task_work: return -EBUSY when adding same work
To:     yaozhenguo <yaozhenguo1@gmail.com>, oleg@redhat.comm
Cc:     linux-kernel@vger.kernel.org, yaozhenguo@jd.com
References: <20210709122712.42844-1-yaozhenguo1@gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <872612b5-b9c6-43aa-a167-1c204d0f1c5a@kernel.dk>
Date:   Fri, 9 Jul 2021 08:18:27 -0600
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On 7/9/21 6:27 AM, yaozhenguo wrote:
> when same work is added to task->task_works list one by one,
> the list becomes endless loop. So return -EBUSY when this
> situation happen.
> 
> Signed-off-by: yaozhenguo <yaozhenguo1@gmail.com>
> ---
>  kernel/task_work.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index 1698fbe..5061ebf 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -27,7 +27,7 @@
>   * list is LIFO.
>   *
>   * RETURNS:
> - * 0 if succeeds or -ESRCH.
> + * 0 if succeeds or -ESRCH, -EBUSY.
>   */
>  int task_work_add(struct task_struct *task, struct callback_head *work,
>  		  enum task_work_notify_mode notify)
> @@ -41,6 +41,8 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>  		head = READ_ONCE(task->task_works);
>  		if (unlikely(head == &work_exited))
>  			return -ESRCH;
> +		if (unlikely(head == work))
> +			return -EBUSY;
>  		work->next = head;
>  	} while (cmpxchg(&task->task_works, head, work) != head);

I don't think there's anything conceptually wrong with this patch, but
it makes me think that you hit this condition. It's really a bug in the
caller, of course, is a WARN_ON_ONCE() warranted here? And who was the
caller?

-- 
Jens Axboe