linux-kernel - Re: [RFC patch 4/7] futex: Add support for attached futexes

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Date:	Sat, 2 Apr 2016 20:01:29 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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: [RFC patch 4/7] futex: Add support for attached futexes

On Sat, 2 Apr 2016, Peter Zijlstra wrote:
> On Sat, Apr 02, 2016 at 11:09:18AM -0000, Thomas Gleixner wrote:
> > +/**
> > + * hash_local_futex - Return the hash bucket in the task local cache
> > + * @uaddr:	The user space address of the futex
> > + * @prime:	The prime number for the modulo operation
> > + *
> > + * That's a primitive hash function, but it turned out to be the most
> > + * efficient one for the task local cache as we don't have anything to
> > + * mix in like we have for the global hash.
> > + */
> > +static inline unsigned int
> > +hash_local_futex(void __user *uaddr, unsigned int prime)
> > +{
> > +	return ((unsigned long) uaddr) % prime;
> > +}
> 
> > +static unsigned int hash_prime(unsigned int size)
> > +{
> > +	switch(size) {
> > +	case   16:
> > +	default:   return 13;
> > +	case   32: return 31;
> > +	case   64: return 61;
> > +	case  128: return 127;
> > +	case  256: return 251;
> > +	case  512: return 509;
> > +	case 1024: return 1021;
> > +	case 2048: return 2039;
> > +	case 4096: return 4093;
> > +	}
> > +}
> 
> One wonders what's wrong with include/linux/hash.h ? It appears to me
> hash_ptr() is pretty much what you want, no?

Nothing is wrong with that. Just my futex fried brain not being able to see
the obvious:)

Thanks,

	tglx