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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org, mingo@redhat.com,
        peterz@infradead.org,
        Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>,
        Kalle Valo <kvalo@kernel.org>,
        Johannes Berg <johannes@sipsolutions.net>
References: <YrnEvjcG8tmF3dRV@zx2c4.com>
        <20220627145716.641185-1-Jason@zx2c4.com>
Date:   Mon, 27 Jun 2022 14:16:08 -0500
In-Reply-To: <20220627145716.641185-1-Jason@zx2c4.com> (Jason A. Donenfeld's
        message of "Mon, 27 Jun 2022 16:57:16 +0200")
Message-ID: <877d51udc7.fsf@email.froward.int.ebiederm.org>
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Subject: Re: [PATCH v2] signal: break out of wait loops on kthread_stop()
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"Jason A. Donenfeld" <Jason@zx2c4.com> writes:

> I was recently surprised to learn that msleep_interruptible(),
> wait_for_completion_interruptible_timeout(), and related functions
> simply hung when I called kthread_stop() on kthreads using them. The
> solution to fixing the case with msleep_interruptible() was more simply
> to move to schedule_timeout_interruptible(). Why?
>
> The reason is that msleep_interruptible(), and many functions just like
> it, has a loop like this:
>
>         while (timeout && !signal_pending(current))
>                 timeout = schedule_timeout_interruptible(timeout);
>
> The call to kthread_stop() woke up the thread, so schedule_timeout_
> interruptible() returned early, but because signal_pending() returned
> true, it went back into another timeout, which was never woken up.
>
> This wait loop pattern is common to various pieces of code, and I
> suspect that subtle misuse in a kthread that caused a deadlock in the
> code I looked at last week is also found elsewhere.
>
> So this commit causes signal_pending() to return true when
> kthread_stop() is called. This is already what's done for
> TIF_NOTIFY_SIGNAL, for these same purposes of breaking out of wait
> loops, so a similar KTHREAD_SHOULD_STOP check isn't too much
> different.

Semantically this makes a lot of sense.

Bloating up signal_pending which is mainly called in non-kthread
contexts is undesirable.

Instead could you modify kthread_stop to call set_notify_signal().

That is exactly what set_notify_signal is there for.  When you don't
actually have a signal but you want to break out of an interruptible
loop.  My last round of work in the area decoupled set_notify_signal
from any other semantics.


It would be nice to get everything down so that we only need to test
TIF_NOTIFY_SIGNAL in signal_pending.  Unfortunately to do that I need
to do something with task_sigpending, and it hasn't been important
enough to weed through all of those details yet.

Eric


> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Toke Høiland-Jørgensen <toke@redhat.com>
> Cc: Kalle Valo <kvalo@kernel.org>
> Cc: Johannes Berg <johannes@sipsolutions.net>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  include/linux/kthread.h      | 1 +
>  include/linux/sched/signal.h | 9 +++++++++
>  kernel/kthread.c             | 8 ++++++++
>  3 files changed, 18 insertions(+)
>
> diff --git a/include/linux/kthread.h b/include/linux/kthread.h
> index 30e5bec81d2b..7061dde23237 100644
> --- a/include/linux/kthread.h
> +++ b/include/linux/kthread.h
> @@ -87,6 +87,7 @@ void kthread_bind(struct task_struct *k, unsigned int cpu);
>  void kthread_bind_mask(struct task_struct *k, const struct cpumask *mask);
>  int kthread_stop(struct task_struct *k);
>  bool kthread_should_stop(void);
> +bool __kthread_should_stop(struct task_struct *k);
>  bool kthread_should_park(void);
>  bool __kthread_should_park(struct task_struct *k);
>  bool kthread_freezable_should_stop(bool *was_frozen);
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index cafbe03eed01..08700c65b806 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -11,6 +11,7 @@
>  #include <linux/refcount.h>
>  #include <linux/posix-timers.h>
>  #include <linux/mm_types.h>
> +#include <linux/kthread.h>
>  #include <asm/ptrace.h>
>  
>  /*
> @@ -397,6 +398,14 @@ static inline int signal_pending(struct task_struct *p)
>  	 */
>  	if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL)))
>  		return 1;
> +
> +	/*
> +	 * Likewise, KTHREAD_SHOULD_STOP isn't really a signal, but it also
> +	 * requires the same behavior, lest wait loops go forever.
> +	 */
> +	if (unlikely(__kthread_should_stop(p)))
> +		return 1;
> +
>  	return task_sigpending(p);
>  }
>  
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 3c677918d8f2..80f6ba323060 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -145,6 +145,14 @@ void free_kthread_struct(struct task_struct *k)
>  	kfree(kthread);
>  }
>  
> +bool __kthread_should_stop(struct task_struct *k)
> +{
> +	struct kthread *kthread = __to_kthread(k);
> +
> +	return kthread && test_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
> +}
> +EXPORT_SYMBOL_GPL(__kthread_should_stop);
> +
>  /**
>   * kthread_should_stop - should this kthread return now?
>   *