linux-kernel - Re: [PATCH, take4] FUTEX : new PRIVATE futexes

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Date:	Wed, 11 Apr 2007 11:35:25 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Ulrich Drepper <drepper@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...e.de>,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	"Shai Fultheim (Shai@...lex86.org)" <shai@...lex86.org>,
	pravin b shelar <pravin.shelar@...softinc.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH, take4] FUTEX : new PRIVATE futexes

On Wed, 11 Apr 2007 19:23:26 +1000
Nick Piggin <nickpiggin@...oo.com.au> wrote:


> But... that isn't there in mainline. Why is it in -mm? At any rate, that makes
> it a no brainer to change.

Seems to be related to lguest. Ask Rusty :)

> 
> > 
> > As this external thing certainly is not doing the check itself, to be on the safe side we should enforce it in get_futex_key(). I agree with you : If we want to maximize performance, we could say : The check *must* be done by the caller.
> 
> Well we _control_ the API, so let's make it as clean and performant as possible
> from the start.

Take a look at do_futex().
Adding checks in callers just increase code size. I tried this got only bad results.
This would speedup only the slow path (ie when some user code want to give us non aligned addrs)
A single factorized check is cleaner and not slower, since we reduce icache pressure.

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