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

[Paolo Bonzini <pbonzini@redhat.com>]

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

Paolo