linux-kernel - Re: [PATCH] ubi: check kthread_should_stop() after the setting of task state

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Date:   Tue, 4 Aug 2020 00:11:31 +0200
From:   Richard Weinberger <richard.weinberger@...il.com>
To:     Zhihao Cheng <chengzhihao1@...wei.com>
Cc:     linux-mtd@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        Richard Weinberger <richard@....at>,
        "zhangyi (F)" <yi.zhang@...wei.com>
Subject: Re: [PATCH] ubi: check kthread_should_stop() after the setting of
 task state

On Mon, Aug 3, 2020 at 4:01 AM Zhihao Cheng <chengzhihao1@...wei.com> wrote:
> > Hmm, I see the problem but I fear this patch does not cure the race completely.
> > It just lowers the chance to hit it.
> > What if KTHREAD_SHOULD_STOP is set right after you checked for it?
>
> The patch can handle this case. ubi_thread will exit at
> kthread_should_stop() in next iteration.

How can it reach the next iteration?
Maybe I didn't fully get your explanation.

As far as I understand the problem correctly, the following happens:
1. ubi_thread is running and the program counter is somewhere between
"if (kthread_should_stop())"
and schedule()
2. While detaching kthread_stop() is called
3. Since the program counter in the thread is right before schedule(),
it does not check KTHREAD_SHOULD_STOP
and blindly calls into schedule()
4. The thread goes to sleep and nothing wakes it anymore -> endless wait.

Is this correct so far?

Your solution is putting another check for KTHREAD_SHOULD_STOP before
schedule().
I argue that this will just reduce the chance to hit the race window
because it can still happen
that kthread_stop() is being called right after the second check and
again before schedule().
Then we end up with the same situation.

-- 
Thanks,
//richard