Re: [PATCH 0/3] 64-bit futexes: Intro

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> IOW, I'm faking it, but I'm making a point. Namely that you can 
> efficiently do read-write lock using *only* 32-bit ops, and without 
> any real kind of limitations on the number of readers and writers.
> 
> So here goes the explanation and the pseudo-code.
> 
>  - have two levels of locking: the contended case, and the uncontended
>    case

i suspect _any_ abstract locking functionality around a data structure 
can be implemented via atomic control over just a single user-space bit.

That bit can be used as a lock and if all access to the state of that 
atomic variable uses it, arbitrary higher-order atomic state transitions 
can be derived from it. The cost would be a bit more instructions in the 
fastpath, but there would still only be a single atomic op (the acquire 
op), as the unlock would be a natural barrier (on x86 at least).

Concurrency (and scheduling) of that lock would still be exactly the 
same as with genuine 64-bit (or even larger) atomic ops, and the 
fastpath would be very close as well.

	Ingo