On 02/11/2013 01:20 AM, Oleg Nesterov wrote: > On 02/11, Srivatsa S. Bhat wrote: >> >> On 02/10/2013 11:36 PM, Oleg Nesterov wrote: >>>>> +static void announce_writer_inactive(struct percpu_rwlock *pcpu_rwlock) >>>>> +{ >>>>> + unsigned int cpu; >>>>> + >>>>> + drop_writer_signal(pcpu_rwlock, smp_processor_id()); >>>> >>>> Why do we drop ourselves twice? More to the point, why is it important to >>>> drop ourselves first? >>> >>> And don't we need mb() _before_ we clear ->writer_signal ? >>> >> >> Oh, right! Or, how about moving announce_writer_inactive() to _after_ >> write_unlock()? > > Not sure this will help... but, either way it seems we have another > problem... > > percpu_rwlock tries to be "generic". This means we should "ignore" its > usage in hotplug, and _write_lock should not race with _write_unlock. > Yes, good point! > IOW. Suppose that _write_unlock clears ->writer_signal. We need to ensure > that this can't race with another write which wants to set this flag. > Perhaps it should be counter as well, and it should be protected by > the same ->global_rwlock, but _write_lock() should drop it before > sync_all_readers() and then take it again? Hmm, or we could just add an extra mb() like you suggested, and keep it simple... > >>>>> +static inline void sync_reader(struct percpu_rwlock *pcpu_rwlock, >>>>> + unsigned int cpu) >>>>> +{ >>>>> + smp_rmb(); /* Paired with smp_[w]mb() in percpu_read_[un]lock() */ >>>> >>>> As I understand it, the purpose of this memory barrier is to ensure >>>> that the stores in drop_writer_signal() happen before the reads from >>>> ->reader_refcnt in reader_uses_percpu_refcnt(), thus preventing the >>>> race between a new reader attempting to use the fastpath and this writer >>>> acquiring the lock. Unless I am confused, this must be smp_mb() rather >>>> than smp_rmb(). >>> >>> And note that before sync_reader() we call announce_writer_active() which >>> already adds mb() before sync_all_readers/sync_reader, so this rmb() looks >>> unneeded. >>> >> >> My intention was to help the writer see the ->reader_refcnt drop to zero >> ASAP; hence I used smp_wmb() at reader and smp_rmb() here at the writer. > > Hmm, interesting... Not sure, but can't really comment. However I can > answer your next question: > Paul told me in another mail that I was expecting too much out of memory barriers, like increasing the speed of electrons and what not ;-) [ It would have been cool though, if it had such magical powers :P ] >> Please correct me if my understanding of memory barriers is wrong here.. > > Who? Me??? No we have paulmck for that ;) > Haha ;-) Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html