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

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Message-Id: <20090223115926.ed03e954.akpm@linux-foundation.org>
Date:	Mon, 23 Feb 2009 11:59:26 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	nickpiggin@...oo.com.au, vegard.nossum@...il.com, mingo@...e.hu,
	stable@...nel.org, npiggin@...e.de, penberg@...helsinki.fi,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)

On Mon, 23 Feb 2009 11:30:57 -0800
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Mon, Feb 23, 2009 at 11:10:09AM -0800, Andrew Morton 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.  Would it help if I wrapped an accessor function around system_state?

That doesn't help the core problem: system_state creates interactions
between unrelated subsystems.  And the number of interactions grows
exponentially with the number of subsystems which use system_state.

> I do need some sort of global state if I am to solve this problem.  ;-)

As I suggested before, please add a new state variable which is private
to your subsystem.  That way it remains isolated from other subsystems.


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