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
