Re: [PATCH v4 10/19] sched: Fix migrate_disable() vs set_cpus_allowed_ptr()

[Qian Cai <cai@redhat.com>]

On Fri, 2020-10-23 at 12:12 +0200, Peter Zijlstra wrote:
> Concurrent migrate_disable() and set_cpus_allowed_ptr() has
> interesting features. We rely on set_cpus_allowed_ptr() to not return
> until the task runs inside the provided mask. This expectation is
> exported to userspace.
> 
> This means that any set_cpus_allowed_ptr() caller must wait until
> migrate_enable() allows migrations.
> 
> At the same time, we don't want migrate_enable() to schedule, due to
> patterns like:
> 
> 	preempt_disable();
> 	migrate_disable();
> 	...
> 	migrate_enable();
> 	preempt_enable();
> 
> And:
> 
> 	raw_spin_lock(&B);
> 	spin_unlock(&A);
> 
> this means that when migrate_enable() must restore the affinity
> mask, it cannot wait for completion thereof. Luck will have it that
> that is exactly the case where there is a pending
> set_cpus_allowed_ptr(), so let that provide storage for the async stop
> machine.
> 
> Much thanks to Valentin who used TLA+ most effective and found lots of
> 'interesting' cases.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/sched.h |    1 
>  kernel/sched/core.c   |  234 +++++++++++++++++++++++++++++++++++++++++++-----
> --
>  2 files changed, 205 insertions(+), 30 deletions(-)

Some syscall fuzzing from an unprivileged user starts to trigger this below
since this commit first appeared in the linux-next today. Does it ring any
bells?

[12065.065837][ T1310] INFO: task trinity-c30:91730 blocked for more than 368 seconds.
[12065.073524][ T1310]       Tainted: G             L    5.10.0-rc3-next-20201112 #2
[12065.081076][ T1310] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[12065.089648][ T1310] task:trinity-c30     state:D stack:26576 pid:91730 ppid: 82688 flags:0x00000000
[12065.098818][ T1310] Call trace:
[12065.101987][ T1310]  __switch_to+0xf0/0x1a8
[12065.106227][ T1310]  __schedule+0x6ec/0x1708
[12065.110505][ T1310]  schedule+0x1bc/0x3b0
[12065.114562][ T1310]  schedule_timeout+0x3c4/0x4c0
[12065.119275][ T1310]  wait_for_completion+0x13c/0x248
[12065.124257][ T1310]  affine_move_task+0x410/0x688
(inlined by) affine_move_task at kernel/sched/core.c:2261
[12065.129013][ T1310]  __set_cpus_allowed_ptr+0x1b4/0x370
[12065.134248][ T1310]  sched_setaffinity+0x4f0/0x7e8
[12065.139088][ T1310]  __arm64_sys_sched_setaffinity+0x1f4/0x2a0
[12065.144972][ T1310]  do_el0_svc+0x124/0x228
[12065.149165][ T1310]  el0_sync_handler+0x208/0x384
[12065.153876][ T1310]  el0_sync+0x140/0x180
[12065.157971][ T1310] 
[12065.157971][ T1310] Showing all locks held in the system:
[12065.166401][ T1310] 1 lock held by khungtaskd/1310:
[12065.171288][ T1310]  #0: ffff800018d0cb40 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire.constprop.56+0x0/0x38
[12065.182210][ T1310] 4 locks held by trinity-main/82688:
[12065.187515][ T1310] 2 locks held by kworker/u513:3/82813:
[12065.192922][ T1310]  #0: ffff000000419d38 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x69c/0x18c8
[12065.203890][ T1310]  #1: ffff0000122bfd40 ((work_completion)(&buf->work)){+.+.}-{0:0}, at: __update_idle_core+0xa8/0x460
[12065.214916][ T1310] 1 lock held by trinity-c35/137168:
[12065.220061][ T1310]  #0: ffff0087ce767898 (&tty->ldisc_sem){++++}-{0:0}, at: ldsem_down_read+0x3c/0x48
[12065.229483][ T1310] 3 locks held by trinity-c61/137611:
[12065.234757][ T1310] 1 lock held by trinity-c7/137630:
[12065.239828][ T1310] 1 lock held by trinity-c57/137714:
[12065.242612][T137611] futex_wake_op: trinity-c61 tries to shift op by 1008; fix this program
[12065.245012][ T1310] 1 lock held by trinity-c52/137771:
[12065.258538][ T1310] 2 locks held by trinity-c42/137835:
[12065.263783][ T1310] 4 locks held by trinity-c22/137868:
[12065.269051][ T1310]  #0: ffff000e78503798 (&rq->lock){-.-.}-{2:2}, at: newidle_balance+0x92c/0xd78
[12065.278155][ T1310]  #1: ffff0087ce767930 (&tty->atomic_write_lock){+.+.}-{3:3}, at: tty_write_lock+0x30/0x58
[12065.288317][ T1310]  #2: ffff800018d0cb40 (rcu_read_lock){....}-{1:2}, at: __mutex_lock+0x24c/0x1310
[12065.297592][ T1310]  #3: ffff800018d0cb40 (rcu_read_lock){....}-{1:2}, at: lock_page_memcg+0x98/0x240
[12065.307026][ T1310] 2 locks held by trinity-c34/137896:
[12065.312266][ T1310]  #0: ffff000e78463798 (&rq->lock){-.-.}-{2:2}, at: __schedule+0x22c/0x1708
[12065.321023][ T1310]  #1: ffff800018d0cb40 (rcu_read_lock){....}-{1:2}, at: __update_idle_core+0xa8/0x460
[12065.330663][ T1310] 2 locks held by trinity-c43/137909:
[12065.335996][ T1310] 1 lock held by trinity-c24/137910:
[12065.341164][ T1310] 1 lock held by trinity-c1/137954:
[12065.346272][ T1310] 1 lock held by trinity-c49/138020:
[12065.351425][ T1310] 1 lock held by trinity-c10/138021:
[12065.356649][ T1310] 1 lock held by trinity-c32/138039:
[12065.361813][ T1310] 4 locks held by trinity-c36/138042:
[12065.367129][ T1310] 2 locks held by trinity-c14/138061:
[12065.372378][ T1310] 2 locks held by trinity-c38/138070:
[12065.377688][ T1310] 1 lock held by trinity-c50/138074:
[12065.382885][ T1310] 1 lock held by trinity-c12/138085:
[12065.388186][ T1310] 1 lock held by trinity-c4/138087:
[12065.393272][ T1310] 3 locks held by trinity-c6/138091:
[12065.398492][ T1310] 2 locks held by trinity-c48/138095:
[12065.403757][ T1310] 2 locks held by trinity-c62/138097:
[12065.409045][ T1310] 2 locks held by trinity-main/138107:
[12065.414441][ T1310] 1 lock held by modprobe/138108:
[12065.419351][ T1310] 
[12065.421560][ T1310] =============================================
[12065.421560][ T1310]