On Wed, Jun 27, 2018 at 07:52:23PM +0900, Tetsuo Handa wrote: > On 2018/06/27 8:50, Paul E. McKenney wrote: > > On Wed, Jun 27, 2018 at 05:10:48AM +0900, Tetsuo Handa wrote: > >> 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? > > You are pointing out that "number of rsp->call() is called" > "number of > rcu_oom_callback() is called" can happen if concurrently called, aren't you? Yes. Reusing an rcu_head before invocation of the earlier use is very bad indeed. ;-) > Then, you are not missing anything. You will need to use something equivalent > to oom_lock even if you can convert rcu_oom_notify() to use shrinkers. What should I look at to work out whether it makes sense to convert rcu_oom_notify() to shrinkers, and if so, how to go about it? Or are you simply asking me to serialize rcu_oom_notify()? (Which is of course not difficult, so please just let me know.) Thanx, Paul
