linux-kernel - Re: Change in functionality of futex() system call.

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Date:	Mon, 6 Jun 2011 13:27:07 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	David Oliver <david@...advisors.com>,
	linux-kernel@...r.kernel.org,
	Shawn Bohrer <sbohrer@...advisors.com>,
	Zachary Vonler <zvonler@...advisors.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <dvhart@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Change in functionality of futex() system call.

On Mon, Jun 06, 2011 at 07:11:37PM +0200, Eric Dumazet wrote:
> Le lundi 06 juin 2011 à 19:05 +0200, Peter Zijlstra a écrit :
> 
> > Dunno, using futexes on private file maps is stupid imo, its just asking
> > for trouble, ro private file maps are even worse. Forcing the COW is the
> > only sane answer in that it gives consistent results and 'breaks' silly
> > expectations early instead of sometimes.
> > 
> > Anyway, that's not really the issue here, as David uses MAP_SHARED (as
> > one should if one is interested in the shared value).
> 
> Sure, but maybe another guy is 'stupid' and uses MAP_PRIVATE on its
> read-only mappings. With old kernels this was working, and we were not
> doing the COW.

That sounds like a bug in both the kernel and userspace. I would expect
a MAP_PRIVATE not be seen by any other process regardless. That's the
definition of PRIVATE.

 From: http://www.gnu.org/s/hello/manual/libc/Memory_002dmapped-I_002fO.html

MAP_PRIVATE
    This specifies that writes to the region should never be written back
    to the attached file. Instead, a copy is made for the process, and the
    region will be swapped normally if memory runs low. No other process
    will see the changes. 

-- Steve
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