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    SubjectRe: [PATCH 3.3] memcg: fix deadlock by inverting lrucare nesting
    On Tue, Feb 28, 2012 at 09:25:02PM -0800, Hugh Dickins wrote:
    > We have forgotten the rules of lock nesting: the irq-safe ones must be
    > taken inside the non-irq-safe ones, otherwise we are open to deadlock:
    >
    > CPU0 CPU1
    > ---- ----
    > lock(&(&pc->lock)->rlock);
    > local_irq_disable();
    > lock(&(&zone->lru_lock)->rlock);
    > lock(&(&pc->lock)->rlock);
    > <Interrupt>
    > lock(&(&zone->lru_lock)->rlock);
    >
    > To check a different locking issue, I happened to add a spin_lock to
    > memcg's bit_spin_lock in lock_page_cgroup(), and lockdep very quickly
    > complained about __mem_cgroup_commit_charge_lrucare() (on CPU1 above).
    >
    > So delete __mem_cgroup_commit_charge_lrucare(), passing a bool lrucare
    > to __mem_cgroup_commit_charge() instead, taking zone->lru_lock under
    > lock_page_cgroup() in the lrucare case.
    >
    > The original was using spin_lock_irqsave, but we'd be in more trouble
    > if it were ever called at interrupt time: unconditional _irq is enough.
    > And ClearPageLRU before del from lru, SetPageLRU before add to lru: no
    > strong reason, but that is the ordering used consistently elsewhere.
    >
    > Signed-off-by: Hugh Dickins <hughd@google.com>

    Acked-by: Johannes Weiner <hannes@cmpxchg.org>


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