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Date: Fri, 25 Aug 2017 16:47:55 +0200
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Borislav Petkov <bp@alien8.de>,
        Byungchul Park <byungchul.park@lge.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: WARNING: possible circular locking dependency detected
Message-ID: <20170825144755.ms2h2j2xe6gznnqi@linutronix.de>
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On 2017-08-25 12:03:04 [+0200], Borislav Petkov wrote:
> ======================================================
> WARNING: possible circular locking dependency detected
> 4.13.0-rc6+ #1 Not tainted
> ------------------------------------------------------

While looking at this, I stumbled upon another one also enabled by
"completion annotation" in the TIP:

| ======================================================
| WARNING: possible circular locking dependency detected
| 4.13.0-rc6-00758-gd80d4177391f-dirty #112 Not tainted
| ------------------------------------------------------
| cpu-off.sh/426 is trying to acquire lock:
|  ((complete)&st->done){+.+.}, at: [<ffffffff810cb344>] takedown_cpu+0x84/0xf0
|
| but task is already holding lock:
|  (sparse_irq_lock){+.+.}, at: [<ffffffff811220f2>] irq_lock_sparse+0x12/0x20
|
| which lock already depends on the new lock.
|
| the existing dependency chain (in reverse order) is:
|
| -> #1 (sparse_irq_lock){+.+.}:
|        __mutex_lock+0x88/0x9a0
|        mutex_lock_nested+0x16/0x20
|        irq_lock_sparse+0x12/0x20
|        irq_affinity_online_cpu+0x13/0xd0
|        cpuhp_invoke_callback+0x4a/0x130
|
| -> #0 ((complete)&st->done){+.+.}:
|        check_prev_add+0x351/0x700
|        __lock_acquire+0x114a/0x1220
|        lock_acquire+0x47/0x70
|        wait_for_completion+0x5c/0x180
|        takedown_cpu+0x84/0xf0
|        cpuhp_invoke_callback+0x4a/0x130
|        cpuhp_down_callbacks+0x3d/0x80
…
|
| other info that might help us debug this:
|
|  Possible unsafe locking scenario:
|        CPU0                    CPU1
|        ----                    ----
|   lock(sparse_irq_lock);
|                                lock((complete)&st->done);
|                                lock(sparse_irq_lock);
|   lock((complete)&st->done);
|
|  *** DEADLOCK ***

We hold the sparse_irq_lock lock while waiting for the completion in the
CPU-down case and in the CPU-up case we acquire the sparse_irq_lock lock
while the other CPU is waiting for the completion.
This is not an issue if my interpretation of lockdep here is correct.

How do we annotate this?

Sebastian