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    SubjectRe: [PATCH] mm: fix lazy vmap purging (use-after-free error)
    On Tue, Feb 24, 2009 at 02:23:19PM +1100, Nick Piggin wrote:
    > 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@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. ;-)
    >
    > Well can you simply return if called if num_online_cpus() == 1, regardless
    > of the state of boot?

    Yep!

    And that is indeed what I do in http://lkml.org/lkml/2009/2/23/305.

    Thanx, Paul

    > > 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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