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

[Wen Congyang <wency@cn.fujitsu.com>]

On 07/27/2015 06:52 PM, Paolo Bonzini wrote:
> 
> 
> On 27/07/2015 12:44, Wen Congyang wrote:
>>>          * rcu_register_thread() may add nodes to &registry; it will not
>>>          * wake up synchronize_rcu, but that is okay because at least another
>>>          * thread must exit its RCU read-side critical section before
>>>          * synchronize_rcu is done.  The next iteration of the loop will
>>>          * process the new thread or set ->waiting for it.  Hence, this can
>>>          * at worst cause synchronize_rcu() to wait for longer.
>> I don't understand this. The next iteration of the loop will move the new thread's
>> rcu_reader from registry to qsreaders even if we call rcu_read_lock() in the new thread.
>> Because rcu_gp_ongoing() will return false.
> 
> You're right.  This proves that a comment was necessary! :)

Yes, I agree with it.

> 
> Second try:
> 
>      * rcu_register_thread() may add nodes to &registry; it will not
>      * wake up synchronize_rcu, but that is okay because at least another
>      * thread must exit its RCU read-side critical section before
>      * synchronize_rcu is done.  The next iteration of the loop will
>      * move the new thread's rcu_reader from &registry to &qsreaders,
>      * because rcu_gp_ongoing() will return false.

I will update the comment and send it again.

Thanks
Wen Congyang

> 
> Paolo
> .
>