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Date:   Thu, 10 Oct 2019 09:40:49 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     Peter Oberparleiter <oberpar@linux.ibm.com>, Qian Cai <cai@lca.pw>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Petr Mladek <pmladek@suse.com>, akpm@linux-foundation.org,
        rostedt@goodmis.org, peterz@infradead.org, linux-mm@kvack.org,
        john.ogness@linutronix.de, david@redhat.com,
        linux-kernel@vger.kernel.org,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()
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On Thu 10-10-19 14:12:01, Sergey Senozhatsky wrote:
> On (10/09/19 16:26), Michal Hocko wrote:
> > On Wed 09-10-19 15:56:32, Peter Oberparleiter wrote:
> > [...]
> > > A generic solution would be preferable from my point of view though,
> > > because otherwise each console driver owner would need to ensure that any
> > > lock taken in their console.write implementation is never held while
> > > memory is allocated/released.
> >
> > Considering that console.write is called from essentially arbitrary code
> > path IIUC then all the locks used in this path should be pretty much
> > tail locks or console internal ones without external dependencies.
> 
> That's a good expectation, but I guess it's not always the case.
> 
> One example might be NET console - net subsystem locks, net device
> drivers locks, maybe even some MM locks (skb allocations?).

I am not familiar with the netconsole code TBH. If there is absolutely
no way around that then we might have to bite a bullet and consider some
of MM locks a land of no printk. I have already said that in this
thread. I am mostly pushing back on "let's just go the simplest way"
approach.
 
> But even more "commonly used" consoles sometimes break that
> expectation. E.g. 8250
> 
> serial8250_console_write()
>  serial8250_modem_status()
>   wake_up_interruptible()

By that expectation you mean they are using external locks or that they
really _need_ to allocate. Because if you are pointing to
wake_up_interruptible and therefore the rq then this is a well known
thing and I was under impression even documented but I can only see
LOGLEVEL_SCHED that is arguably a very obscure way to document the fact.

-- 
Michal Hocko
SUSE Labs