Re: [PATCH] futex: fix shared futex operations on nommu

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 26 Apr 2016 12:27:39 -0400 Rich Felker <dalias@libc.org> wrote:

> On Tue, Apr 26, 2016 at 06:11:07PM +0200, Sebastian Andrzej Siewior wrote:
> > * Rich Felker | 2016-04-26 11:53:44 [-0400]:
> > 
> > >The whole shared futex logic is meaningless for nommu. Perhaps I
> > >should have written a better message, though.
> > >
> > >With MMU, shared futex keys need to identify the physical backing for
> > >a memory address because it may be mapped at different addresses in
> > >different processes (or even multiple times in the same process).
> > >Without MMU this cannot happen. You only have physical addresses. So
> > >the "private futex" behavior of using the virtual address as the key
> > >is always correct (for both shared and private cases) on nommu
> > >systems.
> > 
> > So using a shared futex on NOMMU does work but it would be more
> > efficient to always use a private futex instead.
> > Is this what you are saying?
> 
> No. What I'm saying is that the current code paths for shared futex
> are mmu-specific. They neither work (due to different mm internals, I
> think) nor make sense (due to lack of virtual addresses that map to
> the same physical address) on nommu.
> 
> The private futex code paths are correct for either private or shared
> futexes on nommu. This is both the natural theoretical prediction, and
> confirmed by testing the patch.

It is apparent from Sebastian's questioning that a code comment will be
needed, please.

Also, what specifically is the runtime effect of the patch?  Does the
futex code presently misbehave on NOMMU when FUTEX_PRIVATE_FLAG is
unset?