linux-kernel - Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

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Date:   Tue, 08 Oct 2019 15:35:24 -0400
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Petr Mladek <pmladek@...e.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        akpm@...ux-foundation.org, sergey.senozhatsky.work@...il.com,
        rostedt@...dmis.org, peterz@...radead.org, linux-mm@...ck.org,
        john.ogness@...utronix.de, david@...hat.com,
        linux-kernel@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

On Tue, 2019-10-08 at 21:17 +0200, Michal Hocko wrote:
> On Tue 08-10-19 15:06:13, Qian Cai wrote:
> > On Tue, 2019-10-08 at 20:35 +0200, Michal Hocko wrote:
> 
> [...]
> > > I fully agree that this class of lockdep splats are annoying especially
> > > when they make the lockdep unusable but please discuss this with lockdep
> > > maintainers and try to find some solution rather than go and try to
> > > workaround the problem all over the place. If there are places that
> > > would result in a cleaner code then go for it but please do not make the
> > > code worse just because of a non existent problem flagged by a false
> > > positive.
> > 
> > It makes me wonder what make you think it is a false positive for sure.
> 
> Because this is an early init code? Because if it were a real deadlock

No, that alone does not prove it is a false positive. There are many places
could generate that lock dependency but lockdep will always save the earliest
one.

> then your system wouldn't boot to get run with the real userspace
> (remember there is zone->lock spinlock involved and that means that you
> would hit hard lock after few seconds - but I feel I am repeating
> myself).