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

    Vegard, I would like to still use your patch in vmalloc.c as well. It
    solves a possible use-after-free with preemptible RCU, and also helps with
    a patch I have to conditionally disable lazy vmap unmapping (for Xen).

    We _could_ disable RCU there instead to solve the preemptible RCU bug, but
    your patch I think is less overhead.

    So with appropriate changelog update, please also resend your patch
    (with Acked-by: Nick Piggin <npiggin@suse.de>)

    Thanks,
    Nick

    > Located-by: Vegard Nossum <vegard.nossum@gmail.com>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > ---
    >
    > init/main.c | 3 +++
    > kernel/rcuclassic.c | 4 +++-
    > kernel/rcutree.c | 4 +++-
    > 3 files changed, 9 insertions(+), 2 deletions(-)
    >
    > diff --git a/init/main.c b/init/main.c
    > index 8442094..51f4b71 100644
    > --- a/init/main.c
    > +++ b/init/main.c
    > @@ -121,6 +121,8 @@ static char *static_command_line;
    > static char *execute_command;
    > static char *ramdisk_execute_command;
    >
    > +int idle_task_is_really_idle; /* set to 1 late in boot. */
    > +
    > #ifdef CONFIG_SMP
    > /* Setup configured maximum number of CPUs to activate */
    > unsigned int __initdata setup_max_cpus = NR_CPUS;
    > @@ -463,6 +465,7 @@ static noinline void __init_refok rest_init(void)
    > * at least once to get things moving:
    > */
    > init_idle_bootup_task(current);
    > + idle_task_is_really_idle = 1;
    > preempt_enable_no_resched();
    > schedule();
    > preempt_disable();
    > diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
    > index bd5a900..a758fa6 100644
    > --- a/kernel/rcuclassic.c
    > +++ b/kernel/rcuclassic.c
    > @@ -678,8 +678,10 @@ int rcu_needs_cpu(int cpu)
    > */
    > void rcu_check_callbacks(int cpu, int user)
    > {
    > + extern int idle_task_is_really_idle;
    > +
    > if (user ||
    > - (idle_cpu(cpu) && !in_softirq() &&
    > + (idle_cpu(cpu) && idle_task_is_really_idle && !in_softirq() &&
    > hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
    >
    > /*
    > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > index b2fd602..e996d85 100644
    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    > @@ -947,8 +947,10 @@ static void rcu_do_batch(struct rcu_data *rdp)
    > */
    > void rcu_check_callbacks(int cpu, int user)
    > {
    > + extern int idle_task_is_really_idle;
    > +
    > if (user ||
    > - (idle_cpu(cpu) && !in_softirq() &&
    > + (idle_cpu(cpu) && idle_task_is_really_idle && !in_softirq() &&
    > hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
    >
    > /*
    > --
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