linux-kernel - Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)

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Message-ID: <20090223193351.GA11921@elte.hu>
Date:	Mon, 23 Feb 2009 20:33:51 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	paulmck@...ux.vnet.ibm.com, Nick Piggin <nickpiggin@...oo.com.au>,
	Vegard Nossum <vegard.nossum@...il.com>, stable@...nel.org,
	Nick Piggin <npiggin@...e.de>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Mon, 23 Feb 2009 08:17:26 -0800 "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Tue, Feb 24, 2009 at 12:29:36AM +1100, Nick Piggin wrote:
> > > On Monday 23 February 2009 16:17:09 Paul E. McKenney wrote:
> > > 
> > > > The boot CPU runs in the context of its idle thread during boot-up.
> > > > During this time, idle_cpu(0) will always return nonzero, which will
> > > > fool Classic and Hierarchical RCU into deciding that a large chunk of
> > > > the boot-up sequence is a big long quiescent state.  This in turn causes
> > > > RCU to prematurely end grace periods during this time.
> > > >
> > > > This patch creates a new global variable that is set to 1 just before
> > > > the boot CPU first enters the scheduler, after which the idle task
> > > > really is idle.
> > > 
> > > Nice work all (btw. if this patch goes in rather than using system_state,
> > > then please make the variable __read_mostly).
> > 
> > Hmmm...  I misread this and made system_state be __read_mostly.  Let
> > me know if this is bad, easy to fix if needed.
> 
> Please don't use system_state.  The whole thing is just bad 
> design. It's a global variable, breaks encapsulation, creates 
> interactions etc. CS-101 stuff.

ok, i've removed the patch - Paul, would you mind to re-send 
your original flag solution, with it marked __read_mostly and 
with the extern declarations put into a suitable header file?

Paul, incidentally, this very minute i tracked down that the 
patch is also causing boot lockups in -tip testing. I havent yet 
fully debugged it, but a question comes immediately: if there's 
no grace periods during bootup, wont rcu_sync() & friends just 
hang indefinitely?

More thought is needed.

	Ingo
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