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From:   "Uladzislau Rezki (Sony)" <urezki@gmail.com>
To:     LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Matthew Wilcox <willy@infradead.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        RCU <rcu@vger.kernel.org>, Uladzislau Rezki <urezki@gmail.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: [PATCH 02/24] rcu/tree: Skip entry into the page allocator for PREEMPT_RT
Date:   Tue, 28 Apr 2020 22:58:41 +0200
Message-Id: <20200428205903.61704-3-urezki@gmail.com>
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References: <20200428205903.61704-1-urezki@gmail.com>
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From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

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@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>
---
 kernel/rcu/tree.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cf68d3d9f5b8..cd61649e1b00 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);
 		}
-- 
2.20.1