From: Qian Cai <cai@lca.pw>
To: Petr Mladek <pmladek@suse.com>, Michal Hocko <mhocko@kernel.org>
Cc: akpm@linux-foundation.org, sergey.senozhatsky.work@gmail.com,
rostedt@goodmis.org, peterz@infradead.org, david@redhat.com,
john.ogness@linutronix.de, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()
Date: Mon, 07 Oct 2019 08:34:11 -0400 [thread overview]
Message-ID: <1570451651.5576.285.camel@lca.pw> (raw)
In-Reply-To: <20191007090553.g5cq7qa4tj5yrtaa@pathway.suse.cz>
On Mon, 2019-10-07 at 11:05 +0200, Petr Mladek wrote:
> On Mon 2019-10-07 10:07:42, Michal Hocko wrote:
> > On Fri 04-10-19 18:26:45, Qian Cai wrote:
> > > It is unsafe to call printk() while zone->lock was held, i.e.,
> > >
> > > zone->lock --> console_lock
> > >
> > > because the console could always allocate some memory in different code
> > > paths and form locking chains in an opposite order,
> > >
> > > console_lock --> * --> zone->lock
> > >
> > > As the result, it triggers lockdep splats like below and in different
> > > code paths in this thread [1]. Since has_unmovable_pages() was only used
> > > in set_migratetype_isolate() and is_pageblock_removable_nolock(). Only
> > > the former will set the REPORT_FAILURE flag which will call printk().
> > > Hence, unlock the zone->lock just before the dump_page() there where
> > > when has_unmovable_pages() returns true, there is no need to hold the
> > > lock anyway in the rest of set_migratetype_isolate().
> > >
> > > While at it, remove a problematic printk() in __offline_isolated_pages()
> > > only for debugging as well which will always disable lockdep on debug
> > > kernels.
> >
> > I do not think that removing the printk is the right long term solution.
> > While I do agree that removing the debugging printk __offline_isolated_pages
> > does make sense because it is essentially of a very limited use, this
> > doesn't really solve the underlying problem. There are likely other
> > printks from zone->lock. It would be much more saner to actually
> > disallow consoles to allocate any memory while printk is called from an
> > atomic context.
>
> The current "standard" solution for these situations is to replace
> the problematic printk() with printk_deferred(). It would deffer
> the console handling.
>
> Of course, this is a whack a mole approach. The long term solution
> is to deffer printk() by default. We have finally agreed on this
> few weeks ago on Plumbers conference. It is going to be added
> together with fully lockless log buffer hopefully soon. It will
> be part of upstreaming Real-Time related code.
Does this guarantee that if,
lock(zone->lock)
printk_deferred()
unlock(zone->lock)
that the locks (console_owner and console_sem) in printk_deferred() will always
be processed by the unlock(zone->lock)?
If it is more of timing thing where klogd wakes up, it could still end up with,
zone_lock -> console_owner/console_sem
that causes a deadlock.
next prev parent reply other threads:[~2019-10-07 12:34 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 [this message]
[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
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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