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    SubjectRe: [PATCH -rt] memcg: use migrate_disable()/migrate_enable( ) in memcg_check_events()
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    On Wed, 2011-11-16 at 17:16 +0800, Yong Zhang wrote:
    > Looking at commit 4799401f [memcg: Fix race condition in
    > memcg_check_events() with this_cpu usage], we just want
    > to disable migration. So use the right API in -rt. This
    > will cure below warning.
    >

    > mm/memcontrol.c | 4 ++--
    > 1 files changed, 2 insertions(+), 2 deletions(-)
    >
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index 6aff93c..afa1954 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -722,7 +722,7 @@ static void __mem_cgroup_target_update(struct mem_cgroup *memcg, int target)
    > */
    > static void memcg_check_events(struct mem_cgroup *memcg, struct page *page)
    > {
    > - preempt_disable();
    > + migrate_disable();

    No this won't work. Not even for -rt. If we disable migration but not
    preemption, then two tasks can take this path. And the checks in
    __memcg_event_check() will be corrupted because nothing is protecting
    the updates from two tasks going into the same path.

    Perhaps a local_lock would work.

    -- Steve

    > /* threshold event is triggered in finer grain than soft limit */
    > if (unlikely(__memcg_event_check(memcg, MEM_CGROUP_TARGET_THRESH))) {
    > mem_cgroup_threshold(memcg);
    > @@ -742,7 +742,7 @@ static void memcg_check_events(struct mem_cgroup *memcg, struct page *page)
    > }
    > #endif
    > }
    > - preempt_enable();
    > + migrate_enable();
    > }
    >
    > static struct mem_cgroup *mem_cgroup_from_cont(struct cgroup *cont)




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