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@xxxxxxxxxxxxxx> > > Cc: Li Zefan <lizefan@xxxxxxxxxx> > > Cc: cgroups@xxxxxxxxxxxxxxx > > Cc: linux-kernel@xxxxxxxxxxxxxxx > > Thanks for fixing this! > > Acked-by: Zefan Li <lizefan@xxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> # 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 -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
