Re: sched_setscheduler() vs idle_balance() race

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Thu, 2015-05-28 at 15:53 +0200, Peter Zijlstra wrote:
> On Thu, May 28, 2015 at 09:43:52AM +0200, Mike Galbraith wrote:
> > Hi Peter,
> > 
> > I'm not seeing what prevents pull_task() from yanking a task out from
> > under __sched_setscheduler().  A box sprinkling smoldering 3.0 kernel
> > wreckage all over my bugzilla mbox isn't seeing it either ;-)
> 
> Say, how easy can that thing be reproduced?
> 
> The below is compile tested only, but it might just work if I didn't
> miss anything :-)

Seems trying to make the target invisible to balancing created a new
race: dequeue target, do stuff that may drop rq->lock while it's
dequeued, target sneaks into schedule(), dequeues itself (#2), boom.

On my desktop box I meet..

crash> bt
PID: 6281   TASK: ffff880401950000  CPU: 5   COMMAND: "massive_intr_x"
 #0 [ffff8800da9d79c0] machine_kexec at ffffffff8103c428
 #1 [ffff8800da9d7a20] crash_kexec at ffffffff810c98e5
 #2 [ffff8800da9d7af0] oops_end at ffffffff81006418
 #3 [ffff8800da9d7b20] no_context at ffffffff815b4296
 #4 [ffff8800da9d7b80] __bad_area_nosemaphore at ffffffff815b4353
 #5 [ffff8800da9d7bd0] bad_area at ffffffff815b4691
 #6 [ffff8800da9d7c00] __do_page_fault at ffffffff81044eba
 #7 [ffff8800da9d7c70] do_page_fault at ffffffff8104500c
 #8 [ffff8800da9d7c80] page_fault at ffffffff815c10b2
    [exception RIP: set_next_entity+28]
    RIP: ffffffff81080bac  RSP: ffff8800da9d7d38  RFLAGS: 00010092
    RAX: 0000000000000000  RBX: 0000000000000000  RCX: 00000000044aa200
    RDX: 0000000000000000  RSI: 0000000000000000  RDI: ffff88041ed55968
    RBP: ffff8800da9d7d78   R8: 0000000000000000   R9: 0000000000000000
    R10: 0000000000000000  R11: 0000000000000246  R12: ffff88041ed55968
    R13: 0000000000015900  R14: 0000000000000005  R15: ffff88041ed55900
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #9 [ffff8800da9d7d80] pick_next_task_fair at ffffffff81084071
#10 [ffff8800da9d7df0] __schedule at ffffffff815bb507
#11 [ffff8800da9d7e40] schedule at ffffffff815bbcb7
#12 [ffff8800da9d7e60] do_nanosleep at ffffffff815be615
#13 [ffff8800da9d7ea0] hrtimer_nanosleep at ffffffff810acc96
#14 [ffff8800da9d7f20] sys_nanosleep at ffffffff810acdb6
#15 [ffff8800da9d7f50] system_call_fastpath at ffffffff815bf61b
    RIP: 00007fbde0eb3130  RSP: 00007ffcb4a0a7e8  RFLAGS: 00000246
    RAX: ffffffffffffffda  RBX: ffff8800da9d7f90  RCX: 00007fbde0eb3130
    RDX: 0000000055695905  RSI: 0000000000000000  RDI: 00007ffcb4a0a800
    RBP: 00007ffcb4a0a7e0   R8: 0000000000000000   R9: 00007ffcb4a0a740
    R10: 00007ffcb4a0a5b0  R11: 0000000000000246  R12: 0000000000000010
    R13: ffffffff815bf5f2  R14: ffffffffffffff10  R15: 00007ffcb4a0a800
    ORIG_RAX: 0000000000000023  CS: 0033  SS: 002b
crash> struct -x rq ffff88041ed55900
struct rq {
  lock = {
    raw_lock = {
      {
        head_tail = 0xb0ae, 
        tickets = {
          head = 0xae, 
          tail = 0xb0
        }
      }
    }
  }, 
  nr_running = 0xffffffff, 
  cpu_load = {0x3ff, 0x3ea, 0x35a, 0x29b, 0x225}, 
  last_load_update_tick = 0xffffa827, 
  nohz_stamp = 0x0, 
  nohz_flags = 0x0, 
  load = {
    weight = 0xfffffffffffffc00, 
    inv_weight = 0x0
  }, 
  nr_load_updates = 0xb644, 
  nr_switches = 0x5b5b0, 
  cfs = {
    load = {
      weight = 0xfffffffffffffc00, 
      inv_weight = 0x0
    }, 
    nr_running = 0xffffffff,
    h_nr_running = 0xffffffff, 
    exec_clock = 0xc9070a1c3, 
    min_vruntime = 0x70cee14b9, 
    tasks_timeline = {
      rb_node = 0x0
    }, 
    rb_leftmost = 0x0, 
    curr = 0x0, 
    next = 0x0, 
    last = 0x0, 
    skip = 0x0,