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

[Rusty Russell <rusty@rustcorp.com.au>]

In message <20030907132010.GB19977@mail.jlokier.co.uk> you write:
> I don't see a problem, as long as it is documented.  Anybody grokking
> the old futex code would expect futexes to move with mappings.

BTW, I don't know of anyone *doing* this, but IMHO it's not worth a
single line of kernel code, since if you don't adjust your futex
addresses when you mremap, the try_down_futex will segv after the poll
or whatever.  As a programmer, I would *expect* to have to reset the
futexes (along with every other pointer into the map) when mremap
happens: after all, I told the kernel to watch the old address.  If it
still works, great, but I'd not expect it.

> > Yes.  Invalidate is nice because it catches a programmer mistake.  But
> > why not solve the problem by just holding an mm reference, too?
> 
> That would work.  An mm isn't that huge once everything's been
> unmapped by exit.  Alternatively, mm-private futexes can be woken when
> the mm is destroyed.
> 
> I just implemented the latter, but come to think of it a reference to
> a dead mm is light enough not to bother with a list of "futexes
> attached to mm to destroy on exit".
> 
> So I'll throw that away and provide a patch which just takes a reference.
> (Also, takes inode references).

Agreed.  You can get a page per 2 fds trivially anyway with pipes, so
staying within that bound is fairly safe.

Cheers!
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.