Date: Thu, 16 Feb 2006 21:17:18 +0100 (MET)
From: Esben Nielsen <simlo@phys.au.dk>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Daniel Walker <dwalker@mvista.com>, Ingo Molnar <mingo@elte.hu>,
       linux-kernel@vger.kernel.org, Ulrich Drepper <drepper@redhat.com>,
       Thomas Gleixner <tglx@linutronix.de>, Andrew Morton <akpm@osdl.org>
Subject: Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace?
In-Reply-To: <Pine.OSF.4.05.10602162057040.20911-100000@da410>
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On Thu, 16 Feb 2006, Esben Nielsen wrote:

> On Thu, 16 Feb 2006, Arjan van de Ven wrote:
> 
> > On Thu, 2006-02-16 at 20:06 +0100, Esben Nielsen wrote:
> > > I just jump into a thread somewhere to ask my question :-)
> > > 
> > > Why does the list have to be in userspace?
> > 
> > because it's faster ;)
> > 
> >
> Faster??? 
> As I see it, extra manipulations have to be done even in the non-congested
> case: Every time the lock is taken the locking thread has to add the lock
> to a the list, and reversely remove the lock from the list. I.e.
> instructions are _added_ to the fast path where you stay purely in
> userspace.
> 
> I am ofcourse comparing to a solution where you do a syscall on everytime
Correction:
"I am ofcourse NOT comparing"
...
> you do a lock. What I am asking about is whethere it wouldn't be
> enough to maintain the list at the FUTEX_WAIT/FUTEX_WAKE operation - i.e.
> the slow path where you have to go into the kernel.
> 
> Esben
> 
Esben

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