Re: [Qemu-devel] [PATCH v2 for-2.5] rcu: Allow calling rcu_(un)register_thread() during synchronize_rcu()

[Wen Congyang]

On 07/29/2015 12:14 AM, Paolo Bonzini wrote:
> 
> 
> On 28/07/2015 15:29, Wen Congyang wrote:
>>>>
>>>
>>> If you call it just once, you have the same problem as before.  In fact,
>>> it's worse because instead of having an overflow every 2^31 periods, you
>>> have one every 2 periods.  Instead, by checking that rcu_reader went
>>> through 1 _and_ 3 (or that it was at least once 0, i.e. the thread was
>>> quiescent), you are sure that the thread went through _at least one_
>>> grace period.
>>
>> The overflow is acceptable. We only compare if rcu_reader.ctr is equal than
>> rcu_gp_ctr. If not, we should wait that thread to call rcu_read_unlock().
>> We don't care which is bigger. If no threads calls sync_rcu(), all threads
>> rcu_read.ctr is 0 or rcu_gp_ctr. If one thread calls sync_rcu(), all
>> threads rcu_read.ctr is 0, old_rcu_gp_ctr, or new_rcu_gp_ctr. We only wait 
>> the
>> thread that's rcu_read.ctr is old_rcu_gp_ctr.
> 
> Suppose you have
> 
>     reader                                        writer
>  ---------------------------------------------------------------
>     rcu_read_lock()
>       ctr = atomic_read(&rcu_gp_ctr);
>         ctr = 1
>                                                   synchronize_rcu()
>                                                      rcu_gp_ctr = 3
>       atomic_xchg(&p_rcu_reader->ctr, ctr);
>         rcu_reader.ctr = 1
> 
> We're in the critical section.
> 
>                                                   synchronize_rcu()
>                                                      rcu_gp_ctr = 1
> 
> rcu_gp_ongoing(&p_rcu_reader->ctr) returns false, so synchronize_rcu()
> exits.  But we're still in the critical section.
> 
>     p = atomic_rcu_read(&foo);
>                                                   g_free(p);
> 
> Boom. :)  For more information see
> http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.220.8810

Thanks for your explanation. I understand it now. If we use
rcu_gp_ctr + RCU_GP_CTR for 32-bit host, the problem only exists when we call
synchronize_rcu() 2^31 times while another thread calls rcu_read_lock().

Thanks
Wen Congyang

> 
> Paolo
>