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

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Date:   Thu, 16 Jan 2020 16:54:34 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     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,
        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 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?

-- 
Michal Hocko
SUSE Labs