linux-kernel - [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs

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Message-Id: <f273c7c091b075a7f2107c162f0f3cd6bd9f1bf8.1425735920.git.agordeev@redhat.com>
Date:	Sat,  7 Mar 2015 18:03:36 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Alexander Gordeev <agordeev@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs

Currently a condition when RCU tree is unable to accommodate
the configured number of CPUs is not permitted and causes
a fall back to compile-time values. However, the code has no
means to exceed the RCU tree capacity neither at compile-time
nor in run-time. Therefore, if the condition is met in run-
time then it indicates a serios problem elsewhere and should
be handled with a panic.

Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
---
 kernel/rcu/tree.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 48d640c..7588c7f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3889,16 +3889,19 @@ static void __init rcu_init_geometry(void)
 		rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
 
 	/*
+	 * The tree must be able to accommodate the configured number of CPUs.
+	 * If this limit is exceeded than we have a serious problem elsewhere.
+	 *
 	 * The boot-time rcu_fanout_leaf parameter is only permitted
 	 * to increase the leaf-level fanout, not decrease it.  Of course,
 	 * the leaf-level fanout cannot exceed the number of bits in
-	 * the rcu_node masks.  Finally, the tree must be able to accommodate
-	 * the configured number of CPUs.  Complain and fall back to the
-	 * compile-time values if these limits are exceeded.
+	 * the rcu_node masks.  Complain and fall back to the compile-
+	 * time values if these limits are exceeded.
 	 */
-	if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
-	    rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
-	    n > rcu_capacity[MAX_RCU_LVLS]) {
+	if (n > rcu_capacity[MAX_RCU_LVLS])
+		panic("rcu_init_geometry: rcu_capacity[] is too small");
+	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
+		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {
 		WARN_ON(1);
 		return;
 	}
-- 
1.8.3.1

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