Re: [PATCH v2] mm: memory-failure: use rcu lock instead of tasklist_lock when collect_procs()

[Tong Tiangen <tongtiangen@huawei.com>]

在 2023/8/27 4:28, Matthew Wilcox 写道:
> On Sat, Aug 26, 2023 at 09:46:53AM +0800, Tong Tiangen wrote:
>> " the ``task_struct`` object is freed only after one or more
>> grace periods elapse, with the help of call_rcu(), which is invoked via
>> put_task_struct_rcu_user(). "
>>
>> Combined with the code,when the task exits：
>>
>> release_task()
>> 	__exit_signal()
>> 		__unhash_process()
>> 			list_del_rcu(&p->tasks)
>> 	
>> 	put_task_struct_rcu_user()
>> 		call_rcu(&task->rcu, delayed_put_task_struct);
>> 			
>> delayed_put_task_struct()
>> 	put_task_struct()
>> 		if (refcount_sub_and_test(nr, &t->usage))
>> 			__put_task_struct()
>> 				free_task()
>> 	
>> The code is consistent with the description in the document.
>>
>> According to this understanding, i think for_each_process() under the
>> protection of rcu locl is safe, that is, task_struct in the list will not be
>> destroyed, and get_task_struct() is also safe.
> 
> Aha!  This is different from the usual pattern.  What I'm used to seeing
> is:
> 
> if (refcount_sub_and_test()) {
> 	list_del_rcu();
> 	rcu_free();
> }
> 
> and then on the read side you need a refcount_inc_not_zero(), which we
> didn't have here.  Given this new information you've found, I withdraw
> my objection.  It'd be nice to include some of this analysis in an
> updated changelog (and maybe improved documentation for tasklist?).

OK, commit message and changelog have been updated, and a new patch 
version v3 has been sent.

Thanks,
Tong.

> 
> .