linux-kernel - Re: [PATCH 2/3 v2] futex: avoid double wake up in futex

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Message-ID: <1429201365.6795.51.camel@stgolabs.net>
Date:	Thu, 16 Apr 2015 09:22:45 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Darren Hart <darren@...art.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	fredrik.markstrom@...driver.com,
	Manfred Spraul <manfred@...orfullife.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH 2/3 v2] futex: avoid double wake up in futex_wake() on
 -RT

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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