Re: [PATCH] rcu: Document concurrent quiescent state reporting for offline CPUs

[Joel Fernandes <joelagnelf@nvidia.com>]

On 7/7/2025 10:22 AM, Frederic Weisbecker wrote:
> Le Sun, Jul 06, 2025 at 11:32:08PM -0400, Joel Fernandes a écrit :
>> The synchronization of CPU offlining with GP initialization is confusing
>> to put it mildly (rightfully so as the issue it deals with is complex).
>> Recent discussions brought up a question -- what prevents the
>> rcu_implicit_dyntick_qs() from warning about QS reports for offline
>> CPUs (missing QS reports for offline CPUs causing indefinite hangs).
>>
>> QS reporting for now-offline CPUs should only happen from:
>> - gp_init()
>> - rcutree_cpu_report_dead()
>>
>> Add some documentation on this and refer to it from comments in the code
>> explaining how QS reporting is not missed when these functions are
>> concurrently running.
>>
>> I referred heavily to this post [1] about the need for the ofl_lock.
>> [1] https://lore.kernel.org/all/20180924164443.GF4222@linux.ibm.com/
>>
>> [ Applied paulmck feedback on moving documentation to Requirements.rst ]
>>
>> Link: https://lore.kernel.org/all/01b4d228-9416-43f8-a62e-124b92e8741a@paulmck-laptop/
>> Co-developed-by: Paul E. McKenney <paulmck@kernel.org>
>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> 
> Very nice and welcome!!!
> 
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> 

Thanks, glad you like it. I figured this stuff is better off in the kernel
documentation than rotting in my notes ;-)

And lets continue to keep the discussions going ;-) thanks!

 - Joel