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>
28b89b9e6f