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    SubjectRe: [PATCH v2 04/11] memcg: disable softirq in lock_page_cgroup()
    From
    On Sat, Oct 16, 2010 at 6:14 AM, Greg Thelen <gthelen@google.com> wrote:
    > If pages are being migrated from a memcg, then updates to that
    > memcg's page statistics are protected by grabbing a bit spin lock
    > using lock_page_cgroup().  In an upcoming commit memcg dirty page
    > accounting will be updating memcg page accounting (specifically:
    > num writeback pages) from softirq.  Avoid a deadlocking nested
    > spin lock attempt by disabling softirq on the local processor
    > when grabbing the page_cgroup bit_spin_lock in lock_page_cgroup().
    > This avoids the following deadlock:
    > statistic
    >      CPU 0             CPU 1
    >                    inc_file_mapped
    >                    rcu_read_lock
    >  start move
    >  synchronize_rcu
    >                    lock_page_cgroup
    >                      softirq
    >                      test_clear_page_writeback
    >                      mem_cgroup_dec_page_stat(NR_WRITEBACK)
    >                      rcu_read_lock
    >                      lock_page_cgroup   /* deadlock */
    >                      unlock_page_cgroup
    >                      rcu_read_unlock
    >                    unlock_page_cgroup
    >                    rcu_read_unlock
    >
    > By disabling softirq in lock_page_cgroup, nested calls are avoided.
    > The softirq would be delayed until after inc_file_mapped enables
    > softirq when calling unlock_page_cgroup().
    >
    > The normal, fast path, of memcg page stat updates typically
    > does not need to call lock_page_cgroup(), so this change does
    > not affect the performance of the common case page accounting.
    >
    > Signed-off-by: Andrea Righi <arighi@develer.com>
    > Signed-off-by: Greg Thelen <gthelen@google.com>
    > ---
    >  include/linux/page_cgroup.h |    6 ++++++
    >  1 files changed, 6 insertions(+), 0 deletions(-)
    >
    > diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
    > index b59c298..0585546 100644
    > --- a/include/linux/page_cgroup.h
    > +++ b/include/linux/page_cgroup.h
    > @@ -3,6 +3,8 @@
    >
    >  #ifdef CONFIG_CGROUP_MEM_RES_CTLR
    >  #include <linux/bit_spinlock.h>
    > +#include <linux/hardirq.h>
    > +
    >  /*
    >  * Page Cgroup can be considered as an extended mem_map.
    >  * A page_cgroup page is associated with every page descriptor. The
    > @@ -119,12 +121,16 @@ static inline enum zone_type page_cgroup_zid(struct page_cgroup *pc)
    >
    >  static inline void lock_page_cgroup(struct page_cgroup *pc)
    >  {
    > +       /* This routine is only deadlock safe from softirq or lower. */
    > +       VM_BUG_ON(in_irq());
    > +       local_bh_disable();
    >        bit_spin_lock(PCG_LOCK, &pc->flags);
    >  }
    >
    >  static inline void unlock_page_cgroup(struct page_cgroup *pc)
    >  {
    >        bit_spin_unlock(PCG_LOCK, &pc->flags);
    > +       local_bh_enable();
    >  }
    >
    >  #else /* CONFIG_CGROUP_MEM_RES_CTLR */
    > --
    > 1.7.1
    >
    >

    Please, see recent Kame's patch.
    http://lkml.org/lkml/2010/10/15/54

    --
    Kind regards,
    Minchan Kim
    --
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