Messages in this thread | | | Date | Tue, 24 Sep 2013 19:02:22 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() |
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On 09/24, Peter Zijlstra wrote: > > On Tue, Sep 24, 2013 at 09:49:00AM -0700, Paul E. McKenney wrote: > > > > void cpu_hotplug_done(void) > > > > { > > > > + /* Signal the writer is done */ > > > > + cpuhp_writer = 0; > > > > + wake_up_all(&cpuhp_wq); > > > > + > > > > + /* Wait for any pending readers to be running */ > > > > + cpuhp_writer_wait(!atomic_read(&cpuhp_waitcount)); > > > > + cpuhp_writer_task = NULL; > > > > > > We also need to ensure that the next reader should see all changes > > > done by the writer, iow this lacks "realease" semantics. > > > > Good point -- I was expecting wake_up_all() to provide the release > > semantics, but code could be reordered into __wake_up()'s critical > > section, especially in the case where there was nothing to wake > > up, but where there were new readers starting concurrently with > > cpu_hotplug_done(). > > Doh, indeed. I missed this in Oleg's email, but yes I made that same > assumption about wake_up_all().
Well, I think this is even worse... No matter what the writer does, the new reader needs mb() after it checks !__cpuhp_writer. Or we need another synchronize_sched() in cpu_hotplug_done(). This is what percpu_rw_semaphore() does (to remind, this can be turned into call_rcu).
Oleg.
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