[PATCH] sched: CPU hotplug race vs. set_cpus_allowed()

[Kirill Korotaev <dev@sw.ru>]

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There is a race between set_cpus_allowed()
and move_task_off_dead_cpu().
__migrate_task() doesn't report any err code, so
task can be left on its runqueue if its cpus_allowed mask
changed so that dest_cpu is not longer a possible target.
Also, chaning cpus_allowed mask requires rq->lock being held.

Changes from original post according to to Con Colivas notes:
- added comment
- cleanup code a bit

Signed-Off-By: Kirill Korotaev <dev@openvz.org>
Acked-By: Ingo Molnar <mingo@elte.hu>

Kirill
P.S. against 2.6.17-mm1

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--- linux-2.6.17-mm1s.orig/kernel/sched.c	2006-06-21 18:53:17.000000000 +0400
+++ linux-2.6.17-mm1.dev/kernel/sched.c	2006-06-26 11:54:19.000000000 +0400
@@ -4857,13 +4857,16 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed);
  *
  * So we race with normal scheduler movements, but that's OK, as long
  * as the task is no longer on this CPU.
+ *
+ * Returns non-zero if task was successfully migrated.
  */
-static void __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
+static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 {
 	runqueue_t *rq_dest, *rq_src;
+	int ret = 0;
 
 	if (unlikely(cpu_is_offline(dest_cpu)))
-		return;
+		return ret;
 
 	rq_src = cpu_rq(src_cpu);
 	rq_dest = cpu_rq(dest_cpu);
@@ -4891,9 +4894,10 @@ static void __migrate_task(struct task_s
 		if (TASK_PREEMPTS_CURR(p, rq_dest))
 			resched_task(rq_dest->curr);
 	}
-
+	ret = 1;
 out:
 	double_rq_unlock(rq_src, rq_dest);
+	return ret;
 }
 
 /*
@@ -4964,10 +4968,13 @@ int sigstop_on_cpu_lost;
 /* Figure out where task on dead CPU should go, use force if neccessary. */
 static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *tsk)
 {
+	runqueue_t *rq;
+	unsigned long flags;
 	int dest_cpu;
 	cpumask_t mask;
 	int force = 0;
 
+restart:
 	/* On same node? */
 	mask = node_to_cpumask(cpu_to_node(dead_cpu));
 	cpus_and(mask, mask, tsk->cpus_allowed);
@@ -4979,8 +4986,10 @@ static void move_task_off_dead_cpu(int d
 
 	/* No more Mr. Nice Guy. */
 	if (dest_cpu == NR_CPUS) {
+		rq = task_rq_lock(tsk, &flags);
 		cpus_setall(tsk->cpus_allowed);
 		dest_cpu = any_online_cpu(tsk->cpus_allowed);
+		task_rq_unlock(rq, &flags);
 
 		/*
 		 * Don't tell them about moving exiting tasks or
@@ -5000,7 +5009,8 @@ static void move_task_off_dead_cpu(int d
 		if (tsk->mm && sigstop_on_cpu_lost)
 			force = 1;
 	}
-	__migrate_task(tsk, dead_cpu, dest_cpu);
+	if (!__migrate_task(tsk, dead_cpu, dest_cpu))
+		goto restart;
 
 	if (force)
 		force_sig_specific(SIGSTOP, tsk);