Re: [PATCH v4 1/4] mm: reduce the rcu lock duration

[Matthew Wilcox]

On Mon, May 30, 2022 at 07:30:13PM +0800, Miaohe Lin wrote:
> Commit 3268c63eded4 ("mm: fix move/migrate_pages() race on task struct")
> extends the period of the rcu_read_lock until after the permissions checks
> are done to prevent the task pointed to from changing from under us. But
> the task_struct refcount is also taken at that time, the reference to task
> is guaranteed to be stable. So it's unnecessary to extend the period of
> the rcu_read_lock. Release the rcu lock after task refcount is successfully
> grabbed to reduce the rcu holding time.

But why bother?  You know the RCU read lock isn't a "real" lock, right?