linux-kernel - Re: [PATCH] futex: fix shared futex operations on nommu

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Date:	Tue, 26 Apr 2016 12:27:39 -0400
From:	Rich Felker <dalias@...c.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Greg Ungerer <gerg@...inux.org>,
	Steven Miao <realmz6@...il.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH] futex: fix shared futex operations on nommu

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