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

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Message-ID: <1570541032.5576.297.camel@lca.pw>
Date:   Tue, 08 Oct 2019 09:23:52 -0400
From:   Qian Cai <cai@....pw>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        sergey.senozhatsky.work@...il.com, peterz@...radead.org,
        linux-mm@...ck.org, john.ogness@...utronix.de,
        akpm@...ux-foundation.org, david@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

On Tue, 2019-10-08 at 09:13 -0400, Steven Rostedt wrote:
> On Tue, 8 Oct 2019 10:15:10 +0200
> Petr Mladek <pmladek@...e.com> wrote:
> 
> > There are basically three possibilities:
> > 
> > 1. Do crazy exercises with locks all around the kernel to
> >    avoid the deadlocks. It is usually not worth it. And
> >    it is a "whack a mole" approach.
> > 
> > 2. Use printk_deferred() in problematic code paths. It is
> >    a "whack a mole" approach as well. And we would end up
> >    with printk_deferred() used almost everywhere.
> > 
> > 3. Always deffer the console handling in printk(). This would
> >    help also to avoid soft lockups. Several people pushed
> >    against this last few years because it might reduce
> >    the chance to see the message in case of system crash.
> > 
> > As I said, there has finally been agreement to always do
> > the offload few weeks ago. John Ogness is working on it.
> > So we might have the systematic solution for these deadlocks
> > rather sooner than later.
> 
> Another solution is to add the printk_deferred() in these places that
> cause lockdep splats, and when John's work is done, it would be easy to
> grep for them and remove them as they would no longer be needed.
> 
> This way we don't play whack-a-mole forever (only until we have a
> proper solution) and everyone is happy that we no longer have these
> false positive or I-don't-care lockdep splats which hide real lockdep
> splats because lockdep shuts off as soon as it discovers its first
> splat.

I feel like that is what I trying to do, but there seems a lot of resistances
with that approach where pragmatism met with perfectionism.