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    SubjectRe: [PATCH] cpuset: handle race between CPU hotplug and cpuset_hotplug_work
    On Mon, Sep 12, 2016 at 10:48:31AM +0800, Zefan Li wrote:
    > Cc: Tejun
    >
    > On 2016/9/9 8:41, Joonwoo Park wrote:
    > > Discrepancy between cpu_online_mask and cpuset's effective CPU masks on
    > > cpuset hierarchy is inevitable since cpuset defers updating of
    > > effective CPU masks with workqueue while nothing prevents system from
    > > doing CPU hotplug. For that reason guarantee_online_cpus() walks up
    > > the cpuset hierarchy until it finds intersection under the assumption
    > > that top cpuset's effective CPU mask intersects with cpu_online_mask
    > > even under such race.
    > >
    > > However a sequence of CPU hotplugs can open a time window which is none
    > > of effective CPUs in the top cpuset intersects with cpu_online_mask.
    > >
    > > For example when there are 4 possible CPUs 0-3 where 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 online
    > > CPUs eventually. Hence fall back to online CPU mask when there is no
    > > intersection between top cpuset's effective_cpus and online CPU mask.
    > >
    > > Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
    > > Cc: Li Zefan <lizefan@huawei.com>
    > > Cc: cgroups@vger.kernel.org
    > > Cc: linux-kernel@vger.kernel.org
    >
    > Thanks for fixing this!
    >
    > Acked-by: Zefan Li <lizefan@huawei.com>
    > Cc: <stable@vger.kernel.org> # 3.17+
    >

    Thanks for reviewing.

    Shortly I will send v2 which has few grammar error fixes in the
    changelog.
    No code change has made.

    Joonwoo

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