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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <2be969af-53c0-803b-e0b1-eb20d1077dd0@redhat.com>
Date:   Thu, 16 Jan 2020 17:04:41 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>, Qian Cai <cai@....pw>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        pmladek@...e.com, rostedt@...dmis.org, peterz@...radead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v3] mm/hotplug: silence a lockdep splat with
 printk()

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@...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?

I was asking myself for a long time: did anybody actually see this
deadlock in real life?

-- 
Thanks,

David / dhildenb