linux-kernel - Re: re. Spurious wakeup on a newly created kthread

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Date:   Tue, 28 Jun 2022 11:51:37 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
        Michal Hocko <mhocko@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: re. Spurious wakeup on a newly created kthread

On Sat 2022-06-25 10:01:35, Linus Torvalds wrote:
> On Fri, Jun 24, 2022 at 10:00 PM Tejun Heo <tj@...nel.org> wrote:
> For example, the reason that quoted patch cannot be right is that this
> code pattern:
> 
>   while (wait_for_completion_interruptible(&worker->ready_to_start))
>     ;
> 
> is not valid kernel code. EVER. There is absolutely no way that can be correct.
> 
> Either that code can take a signal, or it cannot. If it can take a
> signal, it had better react to said signal. If it cannot, it must not
> use an interruptble sleep - since now that loop turned into a
> kernel-side busy-loop.

I agree that the code is ugly and does not make sense.

JFYI, I did it this way because I wanted to calm down the hung task
detector. kthreads ignore signals so that I did not think about
this aspect. I though that I saw this trick somewhere.

Do not ask me why I wanted to calm down the hung task detector.
I quess that I felt overloaded and wished a calm day
or something like this. I agree that it does not make
much sense.

Best Regards,
Petr