linux-kernel - Re: [patch V2 3/7] futex: Add op for hash preallocation

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Message-ID: <20160519193642.GA1872@f23x64.localdomain>
Date:	Thu, 19 May 2016 12:36:42 -0700
From:	Darren Hart <dvhart@...radead.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 3/7] futex: Add op for hash preallocation

On Thu, May 19, 2016 at 02:28:49PM +0200, Peter Zijlstra wrote:
> On Sat, May 07, 2016 at 10:47:38AM +0200, Thomas Gleixner wrote:
> > On Fri, 6 May 2016, Darren Hart wrote:
> 
> > > So this seems like it could be tricky for the user as system libraries, like
> > > glibc, make use of futexes. Can we guarantee that "sys_futex" is not called by
> > > the time main() is called?
> > 
> > To the extent of my testing I never observed that the hash was automatically
> > created when I called futex(PREALLOC) right away in main. But yes, that might
> > need some thought.
> 
> I suspect that even if glibc uses futexes before main(), they will not
> be contended, because, last time I checked, the C runtime environment is
> very much single threaded unless explicitly made not so by the program.
> 
> In any case (re)hashing if the hash is empty is 'easy', if there's already state,
> not so much.

It certainly would be nice to be able to resize the hash if it's empty.

-- 
Darren Hart
Intel Open Source Technology Center