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

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Date:	Sat, 7 May 2016 13:40:57 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Darren Hart <dvhart@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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, 7 May 2016, Thomas Gleixner wrote:
> On Fri, 6 May 2016, Darren Hart wrote:
> > > Note, that this call must be issued before the first futex operation in the
> > > process because that would automatically allocate the default sized hash.
> > 
> > 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.

Thinking more about it. If a process is single threaded and it definitely is
up to the point where it reaches main(), there is nothing which might cause a
sys_futex() call except something which would use shared futexes in the depth
of init code. I doubt that this happens, and if it does, then it's some non
standard feature^Whackery which I do not care about at all.

Thanks,

	tglx