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SubjectRe: [PATCH] cpuset: handle race between CPU hotplug and cpuset_hotplug_work
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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+

> ---
> kernel/cpuset.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index c7fd277..b5d2b73 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 in
> + * consequence of race between cpuset_hotplug_work
> + * and cpu hotplug notifier. But we know the top
> + * cpuset's effective_cpus is on its way to be same
> + * with online cpus mask.
> + */
> + cpumask_copy(pmask, cpu_online_mask);
> + return;
> + }
> + }
> cpumask_and(pmask, cs->effective_cpus, cpu_online_mask);
> }
>
>

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