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From: Qian Cai <cai@lca.pw>
To: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	pmladek@suse.com, rostedt@goodmis.org, peterz@infradead.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next v3] mm/hotplug: silence a lockdep splat with printk()
Date: Thu, 16 Jan 2020 11:27:28 -0500
Message-ID: <A7D86B8F-678F-4159-8796-D358B70BCC79@lca.pw> (raw)
In-Reply-To: <2be969af-53c0-803b-e0b1-eb20d1077dd0@redhat.com>



> On Jan 16, 2020, at 11:04 AM, David Hildenbrand <david@redhat.com> wrote:
> 
> On 16.01.20 16:54, Michal Hocko wrote:
>> On Thu 16-01-20 09:53:13, Qian Cai wrote:
>>> 
>>> 
>>>> On Jan 16, 2020, at 9:28 AM, Michal Hocko <mhocko@kernel.org> wrote:
>>>> 
>>>> On Wed 15-01-20 12:29:16, Qian Cai wrote:
>>>>> It is guaranteed to trigger a lockdep splat if calling printk() with
>>>>> zone->lock held because there are many places (tty, console drivers,
>>>>> debugobjects etc) would allocate some memory with another lock
>>>>> held which is proved to be difficult to fix them all.
>>>> 
>>>> I am still not happy with the above much. What would say about something
>>>> like below instead?
>>>> "
>>>> It is not that hard to trigger lockdep splats by calling printk from
>>>> under zone->lock. Most of them are false positives caused by lock chains
>>>> introduced early in the boot process and they do not cause any real
>>>> problems. There are some console drivers which do allocate from the
>>>> printk context as well and those should be fixed. In any case false
>>>> positives are not that trivial to workaround and it is far from optimal
>>>> to lose lockdep functionality for something that is a non-issue.
>>>> <An example of such a false positive goes here>
>>>> "
>>> 
>>> I feel like I repeated myself too many times. A call trace for one lock dependency
>>> is sometimes from early boot process because lockdep will save the first one it
>>> encountered, but it does not mean the lock dependency will only not happen in
>>> early boot. I spent some time to study those early boot call traces in the given
>>> lockdep splats, and it looks to me the lock dependency is also possible after
>>> the boot.
>> 
>> Then state it explicitly with an example of the trace and explanation
>> that the deadlock is real. If the deadlock is real then it shouldn't be
>> really terribly hard to notice even without lockdep splats which get
>> disabled after the first false positive, right?
> 
> I was asking myself for a long time: did anybody actually see this
> deadlock in real life?

Nobody knows for sure. I think one reason is that not many people will use
memory offiline even if they do, it will mostly not be a continuous activity in
the system. debugobjects make it way easier to reproduce because it allocates
memory in random places, but then it is not all that popular.

  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-15 17:29 Qian Cai
2020-01-16 14:28 ` Michal Hocko
2020-01-16 14:53   ` Qian Cai
2020-01-16 15:54     ` Michal Hocko
2020-01-16 16:04       ` David Hildenbrand
2020-01-16 16:27         ` Qian Cai [this message]
2020-01-16 16:05       ` Qian Cai
2020-01-16 17:43         ` Michal Hocko

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