linux-kernel - Re: [PATCH -next v3] mm/hotplug: silence a lockdep splat with printk()

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Message-Id: <D469AA91-FF6A-49B8-B894-1FA04C59AA3B@lca.pw>
Date:   Thu, 16 Jan 2020 11:05:07 -0500
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        pmladek@...e.com, rostedt@...dmis.org, peterz@...radead.org,
        david@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v3] mm/hotplug: silence a lockdep splat with
 printk()



> On Jan 16, 2020, at 10:54 AM, Michal Hocko <mhocko@...nel.org> wrote:
> 
> On Thu 16-01-20 09:53:13, Qian Cai wrote:
>> 
>> 
>>> On Jan 16, 2020, at 9:28 AM, Michal Hocko <mhocko@...nel.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.