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

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Date:	Mon, 06 Jun 2011 19:05:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
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.

On Mon, 2011-06-06 at 18:42 +0200, Eric Dumazet wrote:
> 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 ?

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).


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