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

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Date:	Mon, 06 Jun 2011 18:42:44 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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.

Le lundi 06 juin 2011 à 18:29 +0200, Peter Zijlstra a écrit :
> On Mon, 2011-06-06 at 18:22 +0200, Eric Dumazet wrote:
> > Le lundi 06 juin 2011 à 18:16 +0200, Peter Zijlstra a écrit :
> > 
> > > Hmm, wouldn't that still be susceptible to the zero-page thing if: we
> > > create a writable private file map of a sparse file, touch a page and
> > > then remap the thing RO?
> > > 
> > > 
> > > 
> > 
> > Also I am not sure how MAP_PRIVATE could be affected. If we still try a
> > RW gup()... It will allocate a page for us, instead of still pointing to
> > shared one.
> > 
> > On previous kernel, the application using read-only mapping could use
> > MAP_PRIVATE or MAP_SHARED with same 'behavior'
> 
> But by not forcing the COW you get different behaviour depending on when
> you call FUTEX_WAIT, surely that's not correct either?


As long as the current process never writes to the page holding the
futex, the page stay shared. Behavior should be same with PRIVATE or
SHARED ?

In David Oliver case, this is needed : He wants to catch a change in a
file/memory region written by another process.



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