Commit 4d291941 authored May 25, 2020 by Joel Fernandes (Google) Committed by Paul E. McKenney Jun 29, 2020

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>

parent 8ac88f71

kernel/rcu/tree.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -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);
		}

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