• Joonwoo Park's avatar
    cpuset: handle race between CPU hotplug and cpuset_hotplug_work · 28b89b9e
    Joonwoo Park authored
    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: default avatarJoonwoo Park <joonwoop@codeaurora.org>
    Acked-by: default avatarLi 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: default avatarTejun Heo <tj@kernel.org>
    28b89b9e
cpuset.c 76.5 KB