On Mon, Apr 20, 2020 at 11:38:35AM -0400, Joel Fernandes (Google) wrote: > To keep kfree_rcu() path working on raw non-preemptible sections, > prevent the optional entry into the allocator as it uses sleeping locks. > In fact, even if the caller of kfree_rcu() is preemptible, this path > still is not, as krcp->lock is a raw spinlock as done in previous > patches. With additional page pre-allocation in the works, hitting this > return is going to be much less likely soon so just prevent it for now > so that PREEMPT_RT does not break. Note that page allocation here is an > optimization and skipping it still makes kfree_rcu() work. > > Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > Co-developed-by: Uladzislau Rezki <urezki@xxxxxxxxx> > Signed-off-by: Uladzislau Rezki <urezki@xxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > kernel/rcu/tree.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index cf68d3d9f5b81..cd61649e1b001 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3092,6 +3092,18 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, > if (!bnode) { > WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE); > > + /* > + * To keep this path working on raw non-preemptible > + * sections, prevent the optional entry into the > + * allocator as it uses sleeping locks. In fact, even > + * if the caller of kfree_rcu() is preemptible, this > + * path still is not, as krcp->lock is a raw spinlock. > + * With additional page pre-allocation in the works, > + * hitting this return is going to be much less likely. > + */ > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) > + return false; > + > bnode = (struct kfree_rcu_bulk_data *) > __get_free_page(GFP_NOWAIT | __GFP_NOWARN); > } This will not be correctly working by just reverting everything to the "rcu_head path" for CONFIG_PREEMPT_RT kernel. We need to preallocate at least once. I can add caching on top of this series to address it. -- Vlad Rezki