X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=kernel%2Frcupdate.c;h=c09605f8d16c9d1c0acfb5322f50ce3aabdfe568;hb=93dcc6ea096c51cc30ad0f6647ed05fead2439bf;hp=0ccd0095ebdc574693556388b8e38edc622eb2a3;hpb=01c1c660f4b8086cad7a62345fd04290f3d82c8f;p=linux-2.6

diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 0ccd0095eb..c09605f8d1 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -56,7 +56,10 @@ static atomic_t rcu_barrier_cpu_count;
 static DEFINE_MUTEX(rcu_barrier_mutex);
 static struct completion rcu_barrier_completion;
 
-/* Because of FASTCALL declaration of complete, we use this wrapper */
+/*
+ * Awaken the corresponding synchronize_rcu() instance now that a
+ * grace period has elapsed.
+ */
 static void wakeme_after_rcu(struct rcu_head  *head)
 {
 	struct rcu_synchronize *rcu;
@@ -115,7 +118,17 @@ void rcu_barrier(void)
 	mutex_lock(&rcu_barrier_mutex);
 	init_completion(&rcu_barrier_completion);
 	atomic_set(&rcu_barrier_cpu_count, 0);
+	/*
+	 * The queueing of callbacks in all CPUs must be atomic with
+	 * respect to RCU, otherwise one CPU may queue a callback,
+	 * wait for a grace period, decrement barrier count and call
+	 * complete(), while other CPUs have not yet queued anything.
+	 * So, we need to make sure that grace periods cannot complete
+	 * until all the callbacks are queued.
+	 */
+	rcu_read_lock();
 	on_each_cpu(rcu_barrier_func, NULL, 0, 1);
+	rcu_read_unlock();
 	wait_for_completion(&rcu_barrier_completion);
 	mutex_unlock(&rcu_barrier_mutex);
 }