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From: Qian Cai <cai@lca.pw>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	pmladek@suse.com, rostedt@goodmis.org, peterz@infradead.org,
	david@redhat.com, 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 11:05:07 -0500
Message-ID: <D469AA91-FF6A-49B8-B894-1FA04C59AA3B@lca.pw> (raw)
In-Reply-To: <20200116155434.GB19428@dhcp22.suse.cz>



> On Jan 16, 2020, at 10:54 AM, Michal Hocko <mhocko@kernel.org> 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?

A deadlock could be really hard to trigger though which needs a perfect
timing between multiple threads.

  parent 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
2020-01-16 16:05       ` Qian Cai [this message]
2020-01-16 17:43         ` Michal Hocko

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