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

[Linus Torvalds]

On Fri, 30 May 2008, Linus Torvalds wrote:
> 
> Also note how you can use 64-bit atomic ops to do that all in user space, 
> without actually requiring 64-bit futex support - as long as the bits that 
> matter for waking (like "was there more than one pending writer") fit in 
> the one 32-bit word.

So here's an example of this:

 - make all the readers/writers actually update a 64-bit word (by using 
   cmpxchg8b in user space to actually get the locks)

 - but organize things so that a reader only needs to look at the high 32 
   bits to actually make it's wakeup-decision, and a sleeping writer only 
   needs to look at the low 32 bits. How? Make the low bits of the words 
   contain the "contention status".

Is it possible? I dunno. I personally suspect it is. I also suspect you 
didn't even try.

			Linus
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