From: Qian Cai <cai@lca.pw>
To: Petr Mladek <pmladek@suse.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
sergey.senozhatsky.work@gmail.com, peterz@infradead.org,
Michal Hocko <mhocko@kernel.org>,
linux-mm@kvack.org, john.ogness@linutronix.de,
akpm@linux-foundation.org, david@redhat.com,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()
Date: Tue, 08 Oct 2019 10:03:01 -0400 [thread overview]
Message-ID: <1570543381.5576.301.camel@lca.pw> (raw)
In-Reply-To: <20191008134256.5ti6rjkvadn5b5q4@pathway.suse.cz>
On Tue, 2019-10-08 at 15:42 +0200, Petr Mladek wrote:
> On Tue 2019-10-08 09:23:52, Qian Cai wrote:
> > On Tue, 2019-10-08 at 09:13 -0400, Steven Rostedt wrote:
> > > On Tue, 8 Oct 2019 10:15:10 +0200
> > > Petr Mladek <pmladek@suse.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.
>
> No, the resistance was against complicated code changes (games with
> locks) and against removing useful messages. Such changes might cause
> more harm than good.
I don't think there is "removing useful messages" in this patch. That one
printk() in __offline_isolated_pages() basically as Michal mentioned it is that
useful, but could be converted to printk_deferred() if anyone objected.
It is more complicated to convert dump_page() to use printk_deferred().
>
> I am not -mm maintainer so I could not guarantee that a patch
> using printk_deferred() will get accepted. But it will have much
> bigger chance than the original patch.
>
> Anyway, printk_deferred() is a lost war. It is temporary solution
> for one particular scenario. But as you said, there might be many
> others. The long term solution is the printk rework.
>
> Best Regards,
> Petr
next prev parent reply other threads:[~2019-10-08 14:03 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-04 22:26 [PATCH v2] mm/page_isolation: fix a deadlock with printk() Qian Cai
2019-10-07 8:07 ` Michal Hocko
2019-10-07 9:05 ` Petr Mladek
2019-10-07 11:33 ` Michal Hocko
2019-10-07 12:34 ` Qian Cai
[not found] ` <FB72D947-A0F9-43E7-80D9-D7ACE33849C7@lca.pw>
2019-10-07 11:37 ` Michal Hocko
2019-10-07 12:11 ` Qian Cai
2019-10-07 12:43 ` Michal Hocko
2019-10-07 13:07 ` Qian Cai
2019-10-07 14:10 ` Petr Mladek
2019-10-07 14:30 ` Petr Mladek
2019-10-07 14:49 ` Michal Hocko
2019-10-08 7:43 ` Petr Mladek
2019-10-08 8:27 ` Michal Hocko
2019-10-08 12:56 ` Christian Borntraeger
2019-10-08 16:08 ` Qian Cai
2019-10-08 18:35 ` Michal Hocko
2019-10-08 19:06 ` Qian Cai
2019-10-08 19:17 ` Michal Hocko
2019-10-08 19:35 ` Qian Cai
2019-10-09 11:49 ` Petr Mladek
2019-10-09 13:06 ` Qian Cai
2019-10-09 13:27 ` Michal Hocko
2019-10-09 13:43 ` Qian Cai
2019-10-09 13:51 ` Michal Hocko
2019-10-09 14:19 ` Qian Cai
2019-10-09 14:34 ` Michal Hocko
2019-10-09 15:08 ` Qian Cai
2019-10-09 16:23 ` Michal Hocko
2019-10-10 9:01 ` Qian Cai
2019-10-10 10:59 ` Michal Hocko
2019-10-10 13:11 ` Qian Cai
2019-10-10 14:18 ` Michal Hocko
2019-10-10 14:47 ` Qian Cai
2019-10-10 17:30 ` Michal Hocko
2019-10-10 17:48 ` Qian Cai
2019-10-10 18:06 ` Michal Hocko
2019-10-10 18:59 ` David Hildenbrand
2019-10-09 14:24 ` Petr Mladek
2019-10-09 14:46 ` Qian Cai
2019-10-10 7:57 ` Petr Mladek
2019-10-09 11:39 ` Petr Mladek
2019-10-09 13:56 ` Peter Oberparleiter
2019-10-09 14:26 ` Michal Hocko
2019-10-10 5:12 ` Sergey Senozhatsky
2019-10-10 7:40 ` Michal Hocko
2019-10-10 8:16 ` Sergey Senozhatsky
2019-10-10 8:37 ` Michal Hocko
2019-10-10 8:21 ` Petr Mladek
2019-10-10 8:39 ` Sergey Senozhatsky
2019-10-10 11:11 ` Petr Mladek
2019-10-09 15:25 ` Qian Cai
2019-10-07 14:59 ` Qian Cai
2019-10-07 15:12 ` Michal Hocko
2019-10-07 15:33 ` Qian Cai
2019-10-08 8:15 ` Petr Mladek
2019-10-08 9:32 ` Qian Cai
2019-10-08 13:13 ` Steven Rostedt
2019-10-08 13:23 ` Qian Cai
2019-10-08 13:33 ` Steven Rostedt
2019-10-08 13:42 ` Petr Mladek
2019-10-08 13:48 ` Michal Hocko
2019-10-08 14:03 ` Qian Cai [this message]
2019-10-08 14:08 ` Michal Hocko
2019-10-08 8:40 ` Michal Hocko
2019-10-08 10:04 ` Qian Cai
2019-10-08 10:39 ` Michal Hocko
2019-10-08 12:00 ` Qian Cai
2019-10-08 12:39 ` Michal Hocko
2019-10-08 13:06 ` Qian Cai
2019-10-08 13:37 ` Michal Hocko
2019-10-08 13:08 ` Petr Mladek
2019-10-08 13:33 ` Qian Cai
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