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

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Date:	Thu, 19 May 2016 14:28:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Darren Hart <dvhart@...radead.org>,
	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 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.