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

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.

Signed-off-by: Kirill Korotaev <dev@openvz.org>
Acked-By: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 14 insertions, 4 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 235c421631d..678335a8b39 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4412,13 +4412,16 @@ 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);
@@ -4446,9 +4449,10 @@ static void __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
                 if (TASK_PREEMPTS_CURR(p, rq_dest))
                         resched_task(rq_dest->curr);
         }
-
+        ret = 1;
 out:
         double_rq_unlock(rq_src, rq_dest);
+        return ret;
 }
 
 /*
@@ -4518,9 +4522,12 @@ wait_to_die:
 /* 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;
 
+restart:
         /* On same node? */
         mask = node_to_cpumask(cpu_to_node(dead_cpu));
         cpus_and(mask, mask, tsk->cpus_allowed);
@@ -4532,8 +4539,10 @@ static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *tsk)
 
         /* 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
@@ -4545,7 +4554,8 @@ static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *tsk)
                                "longer affine to cpu%d\n",
                                tsk->pid, tsk->comm, dead_cpu);
         }
-        __migrate_task(tsk, dead_cpu, dest_cpu);
+        if (!__migrate_task(tsk, dead_cpu, dest_cpu))
+                goto restart;
 }
 
 /*