Date:   Fri, 25 Aug 2017 18:42:34 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>,
        Byungchul Park <byungchul.park@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: WARNING: possible circular locking dependency detected

On 2017-08-25 12:03:04 [+0200], Borislav Petkov wrote:
> Hey,
Hi Borislav,

> tglx says I have something for ya :-)
:)
> ======================================================
> WARNING: possible circular locking dependency detected
> 4.13.0-rc6+ #1 Not tainted
> ------------------------------------------------------
> watchdog/3/27 is trying to acquire lock:
>  (cpu_hotplug_lock.rw_sem){++++}, at: [<ffffffff8100c489>] release_ds_buffers+0x29/0xd0
> 
> but now in release context of a crosslock acquired at the following:
>  ((complete)&self->parked){+.+.}, at: [<ffffffff810895f6>] kthread_park+0x46/0x60
> 
> which lock already depends on the new lock.
> 
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #2 ((complete)&self->parked){+.+.}:
>        __lock_acquire+0x10af/0x1110
>        lock_acquire+0xea/0x1f0
>        wait_for_completion+0x3b/0x130
>        kthread_park+0x46/0x60
>        __smpboot_create_thread.part.5+0x7d/0xf0
>        smpboot_register_percpu_thread_cpumask+0xa2/0x100
>        spawn_ksoftirqd+0x3b/0x45
>        do_one_initcall+0x52/0x198
>        kernel_init_freeable+0x6f/0x1a1
>        kernel_init+0xe/0x100
>        ret_from_fork+0x2a/0x40
> 
> -> #1 (smpboot_threads_lock){+.+.}:
>        __lock_acquire+0x10af/0x1110
>        lock_acquire+0xea/0x1f0
>        __mutex_lock+0x6c/0x940
>        mutex_lock_nested+0x1b/0x20
>        smpboot_register_percpu_thread_cpumask+0x42/0x100
>        spawn_ksoftirqd+0x3b/0x45
>        do_one_initcall+0x52/0x198
>        kernel_init_freeable+0x6f/0x1a1
>        kernel_init+0xe/0x100
>        ret_from_fork+0x2a/0x40
> 
> -> #0 (cpu_hotplug_lock.rw_sem){++++}:
>        cpus_read_lock+0x2a/0x90
>        release_ds_buffers+0x29/0xd0
>        x86_release_hardware+0x8f/0xa0
>        hw_perf_event_destroy+0xe/0x20
>        _free_event+0xa7/0x250
> 
> other info that might help us debug this:
> 
> Chain exists of:
>   cpu_hotplug_lock.rw_sem --> smpboot_threads_lock --> (complete)&self->parked
> 
>  Possible unsafe locking scenario by crosslock:
> 
>        CPU0                    CPU1
>        ----                    ----
>   lock(smpboot_threads_lock);
>   lock((complete)&self->parked);
>                                lock(cpu_hotplug_lock.rw_sem);
>                                unlock((complete)&self->parked);
> 
>  *** DEADLOCK ***
> 
> 1 lock held by watchdog/3/27:
>  #0:  (&x->wait){....}, at: [<ffffffff810b115d>] complete+0x1d/0x60
> 
> stack backtrace:
> CPU: 3 PID: 27 Comm: watchdog/3 Not tainted 4.13.0-rc6+ #1
> Hardware name: LENOVO 2320CTO/2320CTO, BIOS G2ET86WW (2.06 ) 11/13/2012
> Call Trace:
>  dump_stack+0x86/0xcf
>  print_circular_bug+0x1fa/0x2f0
>  check_prev_add+0x3be/0x700
>  ? __lock_acquire+0x4c6/0x1110
>  ? trace_event_raw_event_lock+0xf0/0xf0
>  lock_commit_crosslock+0x40d/0x590
>  ? lock_commit_crosslock+0x40d/0x590
>  complete+0x29/0x60
>  __kthread_parkme+0x54/0x80
>  kthread_parkme+0x24/0x40
>  smpboot_thread_fn+0x95/0x230
>  kthread+0x147/0x180
>  ? sort_range+0x30/0x30
>  ? kthread_create_on_node+0x40/0x40
>  ret_from_fork+0x2a/0x40

I don't really get that one. I *think* it complains about the
cpu_hotplug_lock.rw_sem which was held while waiting for the completion
->parked (this is smpboot_register_percpu_thread() -> kthread_park() ->
wait_for_completion()) *and* then other case where kthread_park() waits
for the thread (while holding cpu_hotplug_lock.rw_sem) and out of
nowhere release_ds_buffers() comes on the CPU and asks for the
cpu_hotplug_lock.rw_sem. So something like this happens:

            <...>-1     [002] ....     1.050786: kthread_park: watchdog/3
            <...>-1     [002] ....     1.050794: kthread_park: wait… <- wait_for_completion()
            <...>-27    [003] ....     1.050844: release_ds_buffers: before
            <...>-27    [003] ....     1.050857: release_ds_buffers: after
            <...>-27    [003] ....     1.050905: release_ds_buffers: released
            <...>-27    [003] ....     1.050908: __kthread_parkme: do complete <- complete()
            <...>-27    [003] d..1     1.050916: print_circular_bug:
            <...>-1     [002] ....     1.459364: kthread_park: wait… done

We can change the timing by adding a printk to kthread_parkme() and then
lockdep won't see release_ds_buffers() within the wait_for_completion() section:

            <...>-1     [003] ....     1.065596: kthread_park: watchdog/3
            <...>-27    [003] ....     1.065657: release_ds_buffers: before
            <...>-27    [003] ....     1.065669: release_ds_buffers: after
            <...>-27    [003] ....     1.065718: release_ds_buffers: released
            <...>-27    [003] ....     1.072390: __kthread_parkme: do complete <- complete
            <...>-1     [003] ....     1.072402: kthread_park: wait… <- wait_for_completion()
            <...>-1     [003] ....     1.072404: kthread_park: wait… done

different timing, no complains.
I don't think that this is a bug because we can have multiple readers of
the rwsem and *if* a someone is holding a write lock then
release_ds_buffers() would be simply deferred until smbboot_.* and the
write-lock holder are done.
If my interpretation of the situation is correct, how do we teach this
lockdep.

Sebastian

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