• Tejun Heo's avatar
    cgroup: don't call migration methods if there are no tasks to migrate · 61046727
    Tejun Heo authored
    Subsystem migration methods shouldn't be called for empty migrations.
    cgroup_migrate_execute() implements this guarantee by bailing early if
    there are no source css_sets.  This used to be correct before
    a79a908f ("cgroup: introduce cgroup namespaces"), but no longer
    since the commit because css_sets can stay pinned without tasks in
    them.
    
    This caused cgroup_migrate_execute() call into cpuset migration
    methods with an empty cgroup_taskset.  cpuset migration methods
    correctly assume that cgroup_taskset_first() never returns NULL;
    however, due to the bug, it can, leading to the following oops.
    
      Unable to handle kernel paging request for data at address 0x00000960
      Faulting instruction address: 0xc0000000001d6868
      Oops: Kernel access of bad area, sig: 11 [#1]
      ...
      CPU: 14 PID: 16947 Comm: kworker/14:0 Tainted: G        W
      4.12.0-rc4-next-20170609 #2
      Workqueue: events cpuset_hotplug_workfn
      task: c00000000ca60580 task.stack: c00000000c728000
      NIP: c0000000001d6868 LR: c0000000001d6858 CTR: c0000000001d6810
      REGS: c00000000c72b720 TRAP: 0300   Tainted: GW (4.12.0-rc4-next-20170609)
      MSR: 8000000000009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 44722422  XER: 20000000
      CFAR: c000000000008710 DAR: 0000000000000960 DSISR: 40000000 SOFTE: 1
      GPR00: c0000000001d6858 c00000000c72b9a0 c000000001536e00 0000000000000000
      GPR04: c00000000c72b9c0 0000000000000000 c00000000c72bad0 c000000766367678
      GPR08: c000000766366d10 c00000000c72b958 c000000001736e00 0000000000000000
      GPR12: c0000000001d6810 c00000000e749300 c000000000123ef8 c000000775af4180
      GPR16: 0000000000000000 0000000000000000 c00000075480e9c0 c00000075480e9e0
      GPR20: c00000075480e8c0 0000000000000001 0000000000000000 c00000000c72ba20
      GPR24: c00000000c72baa0 c00000000c72bac0 c000000001407248 c00000000c72ba20
      GPR28: c00000000141fc80 c00000000c72bac0 c00000000c6bc790 0000000000000000
      NIP [c0000000001d6868] cpuset_can_attach+0x58/0x1b0
      LR [c0000000001d6858] cpuset_can_attach+0x48/0x1b0
      Call Trace:
      [c00000000c72b9a0] [c0000000001d6858] cpuset_can_attach+0x48/0x1b0 (unreliable)
      [c00000000c72ba00] [c0000000001cbe80] cgroup_migrate_execute+0xb0/0x450
      [c00000000c72ba80] [c0000000001d3754] cgroup_transfer_tasks+0x1c4/0x360
      [c00000000c72bba0] [c0000000001d923c] cpuset_hotplug_workfn+0x86c/0xa20
      [c00000000c72bca0] [c00000000011aa44] process_one_work+0x1e4/0x580
      [c00000000c72bd30] [c00000000011ae78] worker_thread+0x98/0x5c0
      [c00000000c72bdc0] [c000000000124058] kthread+0x168/0x1b0
      [c00000000c72be30] [c00000000000b2e8] ret_from_kernel_thread+0x5c/0x74
      Instruction dump:
      f821ffa1 7c7d1b78 60000000 60000000 38810020 7fa3eb78 3f42ffed 4bff4c25
      60000000 3b5a0448 3d420020 eb610020 <e9230960> 7f43d378 e9290000 f92af200
      ---[ end trace dcaaf98fb36d9e64 ]---
    
    This patch fixes the bug by adding an explicit nr_tasks counter to
    cgroup_taskset and skipping calling the migration methods if the
    counter is zero.  While at it, remove the now spurious check on no
    source css_sets.
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    Reported-and-tested-by: default avatarAbdul Haleem <abdhalee@linux.vnet.ibm.com>
    Cc: Roman Gushchin <guro@fb.com>
    Cc: stable@vger.kernel.org # v4.6+
    Fixes: a79a908f ("cgroup: introduce cgroup namespaces")
    Link: http://lkml.kernel.org/r/1497266622.15415.39.camel@abdul.in.ibm.com
    61046727
cgroup.c 139 KB