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

[Tong Tiangen]

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.

> .