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

From	Rich Felker <dalias@libc.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] futex: fix shared futex operations on nommu
Date	2016-04-26 18:30 +0200
Message-ID	<rse26-2io-9@gated-at.bofh.it> (permalink)
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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.

Rich

Thread

[PATCH] futex: fix shared futex operations on nommu Rich Felker <dalias@libc.org> - 2016-04-26 03:10 +0200
  Re: [PATCH] futex: fix shared futex operations on nommu Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2016-04-26 12:00 +0200
    Re: [PATCH] futex: fix shared futex operations on nommu Rich Felker <dalias@libc.org> - 2016-04-26 18:00 +0200
      Re: [PATCH] futex: fix shared futex operations on nommu Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2016-04-26 18:20 +0200
        Re: [PATCH] futex: fix shared futex operations on nommu Rich Felker <dalias@libc.org> - 2016-04-26 18:30 +0200
          Re: [PATCH] futex: fix shared futex operations on nommu Andrew Morton <akpm@linux-foundation.org> - 2016-04-27 00:30 +0200
            Re: [PATCH] futex: fix shared futex operations on nommu Rich Felker <dalias@libc.org> - 2016-04-27 00:40 +0200
  Re: [PATCH] futex: fix shared futex operations on nommu Thomas Gleixner <tglx@linutronix.de> - 2016-04-27 15:50 +0200

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