linux-kernel - Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue

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Date:   Tue, 27 Oct 2020 21:30:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority()

On Tue, Oct 27, 2020 at 07:27:59PM +0000, David Woodhouse wrote:

> > While looking at this I found that weird __add_wait_queue_exclusive()
> > which is used by fs/eventpoll.c and does something similar, except it
> > doesn't keep the FIFO order.
> 
> It does, doesn't it? Except those so-called "exclusive" entries end up
> in FIFO order amongst themselves at the *tail* of the queue, to be
> woken up only after all the other entries before them *haven't* been
> excluded.

__add_wait_queue_exclusive() uses __add_wait_queue() which does
list_add(). It does _not_ add at the tail like normal exclusive users,
and there is exactly _1_ user in tree that does this.

I'm not exactly sure how this happened, but:

  add_wait_queue_exclusive()

and

  __add_wait_queue_exclusive()

are not related :-(

> > The Changelog doesn't state how important this property is to you.
> 
> Because it isn't :)
> 
> The ordering is:
> 
>  { PRIORITY }*  { NON-EXCLUSIVE }* { EXCLUSIVE(sic) }*
> 
> I care that PRIORITY comes before the others, because I want to
> actually exclude the others. Especially the "non-exclusive" ones, which
> the 'exclusive' ones don't actually exclude.
> 
> I absolutely don't care about ordering *within* the set of PRIORITY
> entries, since as I said I expect there to be only one.

Then you could arguably do something like:

	spin_lock_irqsave(&wq_head->lock, flags);
	__add_wait_queue_exclusive(wq_head, wq_entry);
	spin_unlock_irqrestore(&wq_head->lock, flags);

and leave it at that.

But now I'm itching to fix that horrible naming... tomorrow perhaps.