Message-ID: <1429201365.6795.51.camel@stgolabs.net>
Subject: Re: [PATCH 2/3 v2] futex: avoid double wake up in futex_wake() on
 -RT
From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Darren Hart <darren@dvhart.com>, Steven Rostedt <rostedt@goodmis.org>,
        fredrik.markstrom@windriver.com,
        Manfred Spraul <manfred@colorfullife.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
Date: Thu, 16 Apr 2015 09:22:45 -0700
In-Reply-To: <20150416155408.GU27490@worktop.programming.kicks-ass.net>
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	 <1429160982.6795.35.camel@stgolabs.net>
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On Thu, 2015-04-16 at 17:54 +0200, Peter Zijlstra wrote:
> On Thu, Apr 16, 2015 at 07:42:55AM -0700, Davidlohr Bueso wrote:
> > On Thu, 2015-04-16 at 12:16 +0200, Peter Zijlstra wrote:
> > > But if we decide we want to go do this, I'd propose we reintroduce this
> > > delayed wake list thing again.
> > 
> > Given that futexes aren't the only potential users, I definitely agree.
> > Lemme cleanup the patches and I'll resend. Now, one thing I wonder about
> > is if we should bother making it a delayed list a plist instead -- as
> > not all users would consider rt-tasks like futexes do.
> 
> plist don't work and should not be used for tasks anymore. I suppose I
> should go rip them out of futexes too. If you want to make the thing
> priority aware we should probably abstract the rb-tree from rtmutex.c

Hmm yeah I noticed that, but a tree for this thing seems like an
overkill imho. I mean, at least wrt futexes, I don't think there would
ever be a nr_wake - within a single futex call as we use the list on the
stack - value large enough to even remotely justify the data structure.

Oh and that reminds me, we should update the rtmutex docs, it still
mentions plist -- but replacing those nice ascii art lists with trees is
evil ;)

Thanks,
Davidlohr

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