Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace?

[Esben Nielsen <simlo@phys.au.dk>]

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
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


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