On Tue, May 31, 2022 at 01:58:31PM +0100, Matthew Wilcox wrote:
> 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?
Looking over this code some more, I think this may harm performance.
ptrace_may_access() itself takes the rcu_read_lock(). So we currently
have:
rcu_read_lock()
rcu_read_lock();
rcu_read_unlock();
rcu_read_unlock();
In at least one RCU configuration, rcu_read_lock() maps to
preempt_disable(). Nested preempt_disable() just bump a counter, while
that counter reaching zero incurs some actual work. So nested
rcu_read_lock() can be better than sequential lock/unlock/lock/unlock.
This needs far better justification.
