Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 14 May 2019 13:35:38 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     minyard@acm.org, linux-rt-users@vger.kernel.org,
        Corey Minyard <cminyard@mvista.com>,
        linux-kernel@vger.kernel.org, tglx@linutronix.de,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH RT v2] Fix a lockup in wait_for_completion() and friends
Message-ID: <20190514113538.GL2589@hirez.programming.kicks-ass.net>
References: <20190508205728.25557-1-minyard@acm.org>
 <20190509161925.kul66w54wpjcinuc@linutronix.de>
 <20190514084356.GJ2589@hirez.programming.kicks-ass.net>
 <20190514091219.nesriqe7qplk3476@linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190514091219.nesriqe7qplk3476@linutronix.de>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.