Messages in this thread | | | Date | Tue, 26 Apr 2016 18:31:53 -0400 | From | Rich Felker <> | Subject | Re: [PATCH] futex: fix shared futex operations on nommu |
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On Tue, Apr 26, 2016 at 03:27:10PM -0700, Andrew Morton wrote: > 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.
Indeed, I agree. I'll work on a better patch. At least this sufficed to get discussion started.
> Also, what specifically is the runtime effect of the patch? Does the > futex code presently misbehave on NOMMU when FUTEX_PRIVATE_FLAG is > unset?
Without this patch, all futex ops without FUTEX_PRIVATE_FLAG fail with EFAULT. It's been a while since I tracked down where the EFAULT is generated but it's somewhere in the shared get-key vm logic.
If userspace treats this as an error, the corresponding pthread, etc. functions fail. Otherwise, userspace just spins at 100% cpu retrying FUTEX_WAIT and FUTEX_WAKE "works" fine as a nop against such a wait, etc. (In a sense an always-failing implementation of futex is a working implementation for the basic ops, just a highly suboptimal one.)
Rich
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