Re: [PATCH] backport: sched/rtmutex/deadline: Fix a PI crash for deadline tasks

[Peter Zijlstra]

On Tue, Nov 06, 2018 at 01:47:21PM +0100, Henrik Austad wrote:
> From: Xunlei Pang <xlpang@xxxxxxxxxx>
> 
> On some of our systems, we notice this error popping up on occasion,
> completely hanging the system.
> 
>    [<ffffffc0000ee398>] enqueue_task_dl+0x1f0/0x420
>    [<ffffffc0000d0f14>] activate_task+0x7c/0x90
>    [<ffffffc0000edbdc>] push_dl_task+0x164/0x1c8
>    [<ffffffc0000edc60>] push_dl_tasks+0x20/0x30
>    [<ffffffc0000cc00c>] __balance_callback+0x44/0x68
>    [<ffffffc000d2c018>] __schedule+0x6f0/0x728
>    [<ffffffc000d2c278>] schedule+0x78/0x98
>    [<ffffffc000d2e76c>] __rt_mutex_slowlock+0x9c/0x108
>    [<ffffffc000d2e9d0>] rt_mutex_slowlock+0xd8/0x198
>    [<ffffffc0000f7f28>] rt_mutex_timed_futex_lock+0x30/0x40
>    [<ffffffc00012c1a8>] futex_lock_pi+0x200/0x3b0
>    [<ffffffc00012cf84>] do_futex+0x1c4/0x550
> 
> It runs an 4.4 kernel on an arm64 rig. The signature looks suspciously
> similar to what Xuneli Pang observed in his crash, and with this fix, my
> issue goes away (my system has survivied approx 1500 reboots and a few
> nasty tests so far)
> 
> Alongside this patch in the tree, there are a few other bits and pieces
> pertaining to futex, rtmutex and kernel/sched/, but those patches
> creates
> weird crashes that I have not been able to dissect yet. Once (if) I have
> been able to figure those out (and test), they will be sent later.
> 
> I am sure other users of LTS that also use sched_deadline will run into
> this issue, so I think it is a good candidate for 4.4-stable. Possibly
> also
> to 4.9 and 4.14, but I have not had time to test for those versions.

But this patch relies on:

  2a1c60299406 ("rtmutex: Deboost before waking up the top waiter")

for pointer stability, but that patch in turn relies on the whole
FUTEX_UNLOCK_PI patch set:

 $ git log --oneline 499f5aca2cdd5e958b27e2655e7e7f82524f46b1..56222b212e8edb1cf51f5dd73ff645809b082b40

  56222b212e8e futex: Drop hb->lock before enqueueing on the rtmutex
  bebe5b514345 futex: Futex_unlock_pi() determinism
  cfafcd117da0 futex: Rework futex_lock_pi() to use rt_mutex_*_proxy_lock()
  38d589f2fd08 futex,rt_mutex: Restructure rt_mutex_finish_proxy_lock()
  50809358dd71 futex,rt_mutex: Introduce rt_mutex_init_waiter()
  16ffa12d7425 futex: Pull rt_mutex_futex_unlock() out from under hb->lock
  73d786bd043e futex: Rework inconsistent rt_mutex/futex_q state
  bf92cf3a5100 futex: Cleanup refcounting
  734009e96d19 futex: Change locking rules
  5293c2efda37 futex,rt_mutex: Provide futex specific rt_mutex API
  fffa954fb528 futex: Remove rt_mutex_deadlock_account_*()
  1b367ece0d7e futex: Use smp_store_release() in mark_wake_futex()

and possibly some follow-up fixes on that (I have vague memories of
that).

As is, just the one patch you propose isn't correct :/

Yes, that was a ginormous amount of work to fix a seemingly simple splat
:-(