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

[Paolo Bonzini]

On 28/07/2015 12:33, Wen Congyang wrote:
> On 07/28/2015 06:18 PM, Paolo Bonzini wrote:
>>
>>
>> On 28/07/2015 12:02, Wen Congyang wrote:
>>> I have a question about rcu: while do we call wait_for_readers()
>>> twice for 32-bit host?
>>
>> Because there is a very small but non-zero probability of the counter
>> going up by exactly 2^31 periods (periods are stored in bits 1-31 so you
>> lose one bit) while the thread is sleeping.  This detail of the
>> implementation comes from URCU.
> 
> Yes, so you use rcu_gp_ctr ^ RCU_GP_CTR to instead of rcu_gp_ctr + RCU_GP_CTR.
> The initial value is 1, so rcu_gp_ctr is: 1, 3, 1, 3, ...
> The rcu_gp_ctr will never be 0. I think calling wait_for_readers() once is
> enough.
> 
> Do I miss something?

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.

Paolo