[tip: core/rcu] rcu/tree: Skip entry into the page allocator for PREEMPT_RT

[tip-bot2 for Joel Fernandes (Google)]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     4d2919411867848fab78c7cb13139e17ad8b85bc
Gitweb:        https://git.kernel.org/tip/4d2919411867848fab78c7cb13139e17ad8b85bc
Author:        Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
AuthorDate:    Mon, 25 May 2020 23:47:46 +02:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 29 Jun 2020 11:59:25 -07:00

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@xxxxxxxxxxxxx>
Reviewed-by: Uladzislau Rezki <urezki@xxxxxxxxx>
Co-developed-by: Uladzislau Rezki <urezki@xxxxxxxxx>
Signed-off-by: Uladzislau Rezki <urezki@xxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c5de5ad..e0425fa 100644
--- 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);
 		}