Re: [PATCH] Alternate futex non-page-pinning and COW fix

[Linus Torvalds]

On Thu, 4 Sep 2003, Hugh Dickins wrote:
> 
> Isn't it the case that to use sys_futex (in the way it's intended),
> userspace needs write access to the futex?  FUTEX_WAIT and FUTEX_WAKE
> are used (depending on condition) after decrementing or incrementing
> the futex in userspace.  FUTEX_FD is not such a clear case, but again
> it appears that you'd use it for an async wait after decrementing.
> FUTEX_REQUEUE seems to be a move or remap, doesn't change the picture.

Yes. 

We can certainly just document it as a nonsense op. All I care about is
that it is _consistently_ broken, and that people don't make read-only
MAP_SHARED do something it has never ever done before - differ from a
semantic standpoint.

> The particular case above: if it's !PROT_WRITE MAP_PRIVATE, I'm
> saying that's not an area you can manipulate mutexes in anyway;

However, the thing is, the case really can be a totally writable
MAP_PRIVATE that just hasn't been modified (and thus not COW'ed) _yet_.

But sure, we could just require that futex pages are dirty in this case.

		Linus

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