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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Feb 23, 2009 at 08:33:51PM +0100, Ingo Molnar wrote:
> 
> * Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > On Mon, 23 Feb 2009 08:17:26 -0800 "Paul E. McKenney" <paulmck@linux.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?

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.  ;-)

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?

							Thanx, Paul