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

[Wen Congyang]

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?

Thanks
Wen Congyang

> 
> Paolo
>