lkml.org 
[lkml]   [2009]   [Feb]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] mm: fix lazy vmap purging (use-after-free error)
    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


    \
     
     \ /
      Last update: 2009-02-23 21:47    [W:0.025 / U:0.108 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site