cpuset: handle race between CPU hotplug and cpuset_hotplug_work

A discrepancy between cpu_online_mask and cpuset's effective_cpus
mask is inevitable during hotplug since cpuset defers updating of
effective_cpus mask using a workqueue, during which time nothing
prevents the system from more hotplug operations.  For that reason
guarantee_online_cpus() walks up the cpuset hierarchy until it finds
an intersection under the assumption that top cpuset's effective_cpus
mask intersects with cpu_online_mask even with such a race occurring.

However a sequence of CPU hotplugs can open a time window, during which
none of the effective CPUs in the top cpuset intersect with
cpu_online_mask.

For example when there are 4 possible CPUs 0-3 and only CPU0 is online:

  ========================  ===========================
   cpu_online_mask           top_cpuset.effective_cpus
  ========================  ===========================
   echo 1 > cpu2/online.
   CPU hotplug notifier woke up hotplug work but not yet scheduled.
      [0,2]                     [0]

   echo 0 > cpu0/online.
   The workqueue is still runnable.
      [2]                       [0]
  ========================  ===========================

  Now there is no intersection between cpu_online_mask and
  top_cpuset.effective_cpus.  Thus invoking sys_sched_setaffinity() at
  this moment can cause following:

   Unable to handle kernel NULL pointer dereference at virtual address 000000d0
   ------------[ cut here ]------------
   Kernel BUG at ffffffc0001389b0 [verbose debug info unavailable]
   Internal error: Oops - BUG: 96000005 [#1] PREEMPT SMP
   Modules linked in:
   CPU: 2 PID: 1420 Comm: taskset Tainted: G        W       4.4.8+ #98
   task: ffffffc06a5c4880 ti: ffffffc06e124000 task.ti: ffffffc06e124000
   PC is at guarantee_online_cpus+0x2c/0x58
   LR is at cpuset_cpus_allowed+0x4c/0x6c
   <snip>
   Process taskset (pid: 1420, stack limit = 0xffffffc06e124020)
   Call trace:
   [<ffffffc0001389b0>] guarantee_online_cpus+0x2c/0x58
   [<ffffffc00013b208>] cpuset_cpus_allowed+0x4c/0x6c
   [<ffffffc0000d61f0>] sched_setaffinity+0xc0/0x1ac
   [<ffffffc0000d6374>] SyS_sched_setaffinity+0x98/0xac
   [<ffffffc000085cb0>] el0_svc_naked+0x24/0x28

The top cpuset's effective_cpus are guaranteed to be identical to
cpu_online_mask eventually.  Hence fall back to cpu_online_mask when
there is no intersection between top cpuset's effective_cpus and
cpu_online_mask.
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: cgroups@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: <stable@vger.kernel.org> # 3.17+
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cpuset.c

@@ -325,8 +325,7 @@ static struct file_system_type cpuset_fs_type = {
 /*
  * Return in pmask the portion of a cpusets's cpus_allowed that
  * are online.  If none are online, walk up the cpuset hierarchy
- * until we find one that does have some online cpus.  The top
- * cpuset always has some cpus online.
+ * until we find one that does have some online cpus.
  *
  * One way or another, we guarantee to return some non-empty subset
  * of cpu_online_mask.
@@ -335,8 +334,20 @@ static struct file_system_type cpuset_fs_type = {
  */
 static void guarantee_online_cpus(struct cpuset *cs, struct cpumask *pmask)
 {
-	while (!cpumask_intersects(cs->effective_cpus, cpu_online_mask))
+	while (!cpumask_intersects(cs->effective_cpus, cpu_online_mask)) {
 		cs = parent_cs(cs);
+		if (unlikely(!cs)) {
+			/*
+			 * The top cpuset doesn't have any online cpu as a
+			 * consequence of a race between cpuset_hotplug_work
+			 * and cpu hotplug notifier.  But we know the top
+			 * cpuset's effective_cpus is on its way to to be
+			 * identical to cpu_online_mask.
+			 */
+			cpumask_copy(pmask, cpu_online_mask);
+			return;
+		}
+	}
 	cpumask_and(pmask, cs->effective_cpus, cpu_online_mask);
 }