rcu/tree: Skip entry into the page allocator for PREEMPT_RT

To keep the kfree_rcu() code working in purely atomic sections on RT,
such as non-threaded IRQ handlers and raw spinlock sections, avoid
calling into the page allocator which uses sleeping locks on RT.

In fact, even if the  caller is preemptible, the kfree_rcu() code is
not, as the krcp->lock is a raw spinlock.

Calling into the page allocator is optional and avoiding it should be
Ok, especially with the page pre-allocation support in future patches.
Such pre-allocation would further avoid the a need for a dynamically
allocated page in the first place.

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Uladzislau Rezki <urezki@gmail.com>
Co-developed-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c5de5ad..e0425fa 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3202,6 +3202,18 @@ 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);
                }