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

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Message-ID: <20160426161107.GF8295@linutronix.de>
Date:	Tue, 26 Apr 2016 18:11:07 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Rich Felker <dalias@...c.org>
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

* 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?

>Rich

Sebastian