Messages in this thread | | | Date | Thu, 26 Sep 2013 18:58:40 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() |
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Peter,
Sorry. Unlikely I will be able to read this patch today. So let me ask another potentially wrong question without any thinking.
On 09/26, Peter Zijlstra wrote: > > +void __get_online_cpus(void) > +{ > +again: > + /* See __srcu_read_lock() */ > + __this_cpu_inc(__cpuhp_refcount); > + smp_mb(); /* A matches B, E */ > + __this_cpu_inc(cpuhp_seq); > + > + if (unlikely(__cpuhp_state == readers_block)) {
OK. Either we should see state = BLOCK or the writer should notice the change in __cpuhp_refcount/seq. (altough I'd like to recheck this cpuhp_seq logic ;)
> + atomic_inc(&cpuhp_waitcount); > + __put_online_cpus();
OK, this does wake(cpuhp_writer).
> void cpu_hotplug_begin(void) > { > ... > + /* > + * Notify new readers to block; up until now, and thus throughout the > + * longish synchronize_sched() above, new readers could still come in. > + */ > + __cpuhp_state = readers_block; > + > + smp_mb(); /* E matches A */ > + > + /* > + * If they don't see our writer of readers_block to __cpuhp_state, > + * then we are guaranteed to see their __cpuhp_refcount increment, and > + * therefore will wait for them. > + */ > + > + /* Wait for all now active readers to complete. */ > + wait_event(cpuhp_writer, cpuhp_readers_active_check());
But. doesn't this mean that we need __wait_event() here as well?
Isn't it possible that the reader sees BLOCK but the writer does _not_ see the change in __cpuhp_refcount/cpuhp_seq? Those mb's guarantee "either", not "both".
Don't we need to ensure that we can't check cpuhp_readers_active_check() after wake(cpuhp_writer) was already called by the reader and before we take the same lock?
Oleg.
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