linux-kernel - Re: [PATCH 1/2] sched/wait: abort_exclusive_wait() should pass TASK_NORMAL to wake

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Message-ID: <20160901180902.GD10153@twins.programming.kicks-ass.net>
Date:   Thu, 1 Sep 2016 20:09:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Al Viro <viro@...IV.linux.org.uk>,
        Bart Van Assche <bvanassche@....org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Neil Brown <neilb@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/wait: abort_exclusive_wait() should pass
 TASK_NORMAL to wake_up()

On Thu, Sep 01, 2016 at 07:26:58PM +0200, Oleg Nesterov wrote:
> On 09/01, Peter Zijlstra wrote:
> >
> > So mixing INTERRUPTIBLE and UNINTERRUPTIBLE and then not using
> > TASK_NORMAL for wakeups is a mis-feature/abuse of waitqueues IMO.
> 
> Heh, agreed. When I was doing this fix I suddenly realize that I do
> not understand why do we have, say, wake_up_interruptible().
> 
> I mean, I can't imagine the "real" use-case when you actually want
> to wake up only the INTERRUPTIBLE tasks and leave the UNINTERRUPTIBLE
> sleeping. Exclusive or not.
> 
> It seems that wake_up_interruptible() is mostly used simply because
> the caller knows that UNINTERRUPTIBLE waiters are not possible, this
> is often the case.

I suspect the same.

> Actually, I think that TASK_NORMAL should be used even if wq mixes
> UNINTERRUPTIBLE and KILLABLE waiters. The fact that TASK_KILLABLE
> includes TASK_UNINTERRUPTIBLE is just "implementation detail" even
> if I do not think this will be ever changed.

I think its a fairly fundamental thing, its part of the semantics of
TASK_KILLABLE. Namely its UNINTERRUPTIBLE, except you can interrupt it
with fatal. A TASK_NORMAL wake should very much wake a TASK_KILLABLE
sleep, and for that to happen they need to share a bit, namely
UNINTERRUPTIBLE.

But I agree with you, all waitqueue wakeups _should_ simply be
TASK_NORMAL. Like said, I don't see it ever makes sense to play games
with it.

Now, let me try and get back to making sense of your abort abortion ;-)