linux-kernel - Re: [PATCH RT v2] Fix a lockup in wait_for

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Message-ID: <20190514152521.2gz6w2bahhgkxav7@linutronix.de>
Date:   Tue, 14 May 2019 17:25:21 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     minyard@....org, linux-rt-users@...r.kernel.org,
        Corey Minyard <cminyard@...sta.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH RT v2] Fix a lockup in wait_for_completion() and friends

On 2019-05-14 13:35:38 [+0200], Peter Zijlstra wrote:
> On Tue, May 14, 2019 at 11:12:19AM +0200, Sebastian Andrzej Siewior wrote:
> > On 2019-05-14 10:43:56 [+0200], Peter Zijlstra wrote:
> > > Now.. that will fix it, but I think it is also wrong.
> > > 
> > > The problem being that it violates FIFO, something that might be more
> > > important on -RT than elsewhere.
> > 
> > Wouldn't -RT be more about waking the task with the highest priority
> > instead the one that waited the longest?
> 
> Possibly, but that's a far larger patch. Also, even with that
> completions do not avoid inversions and thus don't really make nice RT
> primitives anyway.

See. So it does not really matter if a particular waiter ends up at the
end of the queue.
Anyway, I don't really think we need this but if you want it, let me add
it.
What about the other question I had regarding completion_done()?

Sebastian