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    SubjectRe: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer.
    On Wed, Jun 27, 2018 at 05:10:48AM +0900, Tetsuo Handa wrote:
    > On 2018/06/27 2:03, Paul E. McKenney wrote:
    > > There are a lot of ways it could be made concurrency safe. If you need
    > > me to do this, please do let me know.
    > >
    > > That said, the way it is now written, if you invoke rcu_oom_notify()
    > > twice in a row, the second invocation will wait until the memory from
    > > the first invocation is freed. What do you want me to do if you invoke
    > > me concurrently?
    > >
    > > 1. One invocation "wins", waits for the earlier callbacks to
    > > complete, then encourages any subsequent callbacks to be
    > > processed more quickly. The other invocations return
    > > immediately without doing anything.
    > >
    > > 2. The invocations serialize, with each invocation waiting for
    > > the callbacks from previous invocation (in mutex_lock() order
    > > or some such), and then starting a new round.
    > >
    > > 3. Something else?
    > >
    > > Thanx, Paul
    >
    > As far as I can see,
    >
    > - atomic_set(&oom_callback_count, 1);
    > + atomic_inc(&oom_callback_count);
    >
    > should be sufficient.

    I don't see how that helps. For example, suppose that two tasks
    invoked rcu_oom_notify() at about the same time. Then they could
    both see oom_callback_count equal to zero, both atomically increment
    oom_callback_count, then both do the IPI invoking rcu_oom_notify_cpu()
    on each online CPU.

    So far, so good. But rcu_oom_notify_cpu() enqueues a per-CPU RCU
    callback, and enqueuing the same callback twice in quick succession
    would fatally tangle RCU's callback lists.

    What am I missing here?

    Thanx, Paul

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