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

[Nick Piggin]

On Tuesday 24 February 2009 07:43:59 Paul E. McKenney wrote:
> On Mon, Feb 23, 2009 at 08:33:51PM +0100, Ingo Molnar wrote:
> > * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > On Mon, 23 Feb 2009 08:17:26 -0800 "Paul E. McKenney" 
<paulmck@xxxxxxxxxxxxxxxxxx> 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?
>
> Ouch!!!  Indeed they would.
>
> > More thought is needed.
>
> One fix would be to sprinkle calls to rcu_qsctr_inc() through the
> boot process.  But a much better approach would be for me to make
> synchronize_rcu() check this same flag, and simply return if called
> during early boot.  The rationale for this is that there is but a single
> CPU during early boot, so tinyrcu.c's optimization can be used.  ;-)

Well can you simply return if called if num_online_cpus() == 1, regardless
of the state of boot?

> Out of both paranoia and self defense, I would check num_online_cpus()
> in my proposed call into RCU.  ;-)
>
> Seem reasonable?  And does synchronize_sched() also need the UP-only
> optimization?

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