path: root/kernel
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authorJoonwoo Park <joonwoop@codeaurora.org>2016-09-11 21:14:58 -0700
committerTejun Heo <tj@kernel.org>2016-09-13 11:26:27 -0400
commit28b89b9e6f7b6c8fef7b3af39828722bca20cfee (patch)
tree2981360b9d628fb7c2454e1c2042b3eb89d9c99e /kernel
parent568ac888215c7fb2fabe8ea739b00ec3c1f5d440 (diff)
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>
Diffstat (limited to 'kernel')
1 files changed, 14 insertions, 3 deletions
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index c27e53326bef..c08585ad996b 100644
--- a/kernel/cpuset.c
+++ b/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);