On Tue, 26 Apr 2016 12:27:39 -0400 Rich Felker <dal...@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?
