linux-kernel - Re: [PATCH] futex: Ensure get_futex_key

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Date:	Mon, 20 Oct 2014 11:11:40 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Matteo Franchin <Matteo.Franchin@....com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a
 barrier

On Sat, 18 Oct 2014, Davidlohr Bueso wrote:
> On Sat, 2014-10-18 at 13:50 -0700, Linus Torvalds wrote:
> > On Sat, Oct 18, 2014 at 12:58 PM, Davidlohr Bueso <dave@...olabs.net> wrote:
> > >
> > > And [get/put]_futex_keys() shouldn't even be called for private futexes.
> > > The following patch had some very minor testing on a 60 core box last
> > > night, but passes both Darren's and perf's tests. So I *think* this is
> > > right, but lack of sleep and I overall just don't trust them futexes!
> > 
> > Hmm. I don't see the advantage of making the code more complex in
> > order to avoid the functions that are no-ops for the !fshared case?
> > 
> > IOW, as far as I can tell, this patch doesn't actually really *change*
> > anything. Am I missing something?
> 
> Right, all we do is avoid a NOP, but I don't see how this patch makes
> the code more complex. In fact, the whole idea is to make it easier to
> read and makes the key referencing differences between shared and
> private futexes crystal clear, hoping to mitigate future bugs. 

I tend to disagree. The current code is symetric versus get/drop and
you make it unsymetric by avoiding the drop call with a pointless
extra conditional in all call sites.

I really had to look twice to figure out that the patch is correct,
but I really cannot see any value and definitely have a hard time how
this makes the code clearer and would prevent future bugs.

I rather keep it symetric and document the NOP property for private
futexes in both get and drop.

Thanks,

	tglx


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