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

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Message-ID: <20200115083718.GV19428@dhcp22.suse.cz>
Date:   Wed, 15 Jan 2020 09:37:18 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        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] mm/hotplug: silence a lockdep splat with printk()

On Tue 14-01-20 16:40:49, Qian Cai wrote:
> 
> 
> > On Jan 14, 2020, at 4:02 PM, Michal Hocko <mhocko@...nel.org> wrote:
> > 
> > Yeah, that was a long discussion with a lot of lockdep false positives.
> > I believe I have made it clear that the console code shouldn't depend on
> > memory allocation because that is just too fragile. If that is not
> > possible for some reason then it has to be mentioned in the changelog.
> > I really do not want us to add kludges to the MM code just because of
> > printk deficiencies unless that is absolutely inevitable.
> 
> I don’t know how to convince you, but both random number generator
> and printk() maintainers agreed to get ride of printk() with
> zone->lock held as you can see in the approved commit mentioned in
> this patch description because it is a whac-a-mole to fix other
> places.

I really do not understand this argument. It is quite a specific path in
the console code which cannot allocate any memory or use locks which
depend on the allocation via a lock chain, right? So how come this is a
whack a mole?

Also any console that really needs GFP_ATOMIC to write something out is
just inherently broken so it should better be fixed rather than
worked around.
-- 
Michal Hocko
SUSE Labs