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

[Hugh Dickins]

On Wed, 3 Sep 2003, Linus Torvalds wrote:
> 
> If the patches can't be made to work for private mappings, then there's
> something fundamentally wrong with them.

Of course (not).  That's the point, they do work on private mappings, but
the semantics are different on private mappings from on shared mappings:
on private mappings they're private to the mm, on shared mappings they're
shared with other mms (via the shared file).

> So the thing boils down to:
> 
>  - if the futex works on a proper private mapping, then the downgrade is 
>    still proper, and the futex should never care about anything but a real
>    VM_SHARED.

In the usual mm case, yes, deciding by VM_SHARED and
ignoring VM_MAYSHARE turns out to be the right thing to do.

But a futex differs from the usual mm case, that much was clear when
they were invented, but we're still discovering just how they are.

As I've said before, I haven't a clue about the user/glibc end of
futexes, and for all I know a futex on a shared-readonly-cannot-be-
mprotected-for-writing mapping cannot be used as a futex.  If that's
so, then perhaps we should simply prohibit sys_futex on such an area,
and settle this dispute in that way.  Is that the case?

Hugh

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