Re: [mm PATCH 3/6] RCU: Fix barriers

[Dipankar Sarma <dipankar@in.ibm.com>]

Fix rcu_barrier() to work properly in preemptive kernel environment.
Also, the ordering of callback must be preserved while moving
callbacks to another CPU during CPU hotplug.

Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
---



diff -puN kernel/rcuclassic.c~rcu-fix-barriers kernel/rcuclassic.c
--- linux-2.6.20-rc3-mm1-rcu/kernel/rcuclassic.c~rcu-fix-barriers	2007-01-15 15:36:47.000000000 +0530
+++ linux-2.6.20-rc3-mm1-rcu-dipankar/kernel/rcuclassic.c	2007-01-15 15:36:47.000000000 +0530
@@ -350,9 +350,9 @@ static void __rcu_offline_cpu(struct rcu
 	if (rcp->cur != rcp->completed)
 		cpu_quiet(rdp->cpu, rcp);
 	spin_unlock_bh(&rcp->lock);
+	rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
 	rcu_move_batch(this_rdp, rdp->curlist, rdp->curtail);
 	rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail);
-	rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
 }
 
 static void rcu_offline_cpu(int cpu)
diff -puN kernel/rcupdate.c~rcu-fix-barriers kernel/rcupdate.c
--- linux-2.6.20-rc3-mm1-rcu/kernel/rcupdate.c~rcu-fix-barriers	2007-01-15 15:36:47.000000000 +0530
+++ linux-2.6.20-rc3-mm1-rcu-dipankar/kernel/rcupdate.c	2007-01-15 15:36:47.000000000 +0530
@@ -117,7 +117,18 @@ 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
+	 * haven't yet queued anything. So, we need to make sure
+	 * that no grace period happens 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);
 }

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