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

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Date:   Tue, 08 Oct 2019 09:33:41 -0400
From:   Qian Cai <cai@....pw>
To:     Petr Mladek <pmladek@...e.com>
Cc:     sergey.senozhatsky.work@...il.com, rostedt@...dmis.org,
        peterz@...radead.org, mhocko@...nel.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 15:08 +0200, Petr Mladek wrote:
> On Mon 2019-10-07 10:59:10, Qian Cai wrote:
> > It is almost impossible to eliminate all the indirect call chains from
> > console_sem/console_owner_lock to zone->lock because it is too normal that
> > something later needs to allocate some memory dynamically, so as long as it
> > directly call printk() with zone->lock held, it will be in trouble.
> 
> It is not normal the something needs to allocate memory under
> console_sem. Console drivers are supposed to get the message
> out even when the system is in really bad state and it is not
> possible to allocate memory. I consider this a bug in the console
> driver.

Again, it is not directly under console_sem. It is *indirect*.

console_sem --> lockA
lockA --> lockB
lockB --> lockC

Anything allocating memory with lockB or lockC held will form the problematic
lock chain to trigger the lockdep splat with memory offline.