linux-kernel - Re: printk() + memory offline deadlock (WAS Re: page_alloc.shuffle=1 + CONFIG_PROVE

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Date:   Tue, 24 Sep 2019 10:28:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Qian Cai <cai@....pw>,
        Catalin Marinas <catalin.marinas@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Will Deacon <will@...nel.org>, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-kernel@...ts.infradead.org,
        Theodore Ts'o <tytso@....edu>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: printk() + memory offline deadlock (WAS Re: page_alloc.shuffle=1
 + CONFIG_PROVE_LOCKING=y = arm64 hang)

On (09/23/19 14:58), Petr Mladek wrote:
> 
> If I understand it correctly then this is the re-appearing problem.
> The only systematic solution with the current approach is to
> take port->lock in printk_safe/printk_deferred context.

It probably is.
We have a number of reverse paths. TTY invokes MM under port->lock,
UART invokes TTY under port->lock, MM invokes UART under zone->lock,
UART invokes sched under port->lock, shced invokes UART, UART invokes
UART under port->lock (!), and so on.

> But this is a massive change that almost nobody wants. Instead,
> we want the changes that were discussed on Plumbers.
>
> Now, the question is what to do with existing kernels. There were
> several lockdep reports. And I am a bit lost. Did anyone seen
> real deadlocks or just the lockdep reports?

I think so. Qian Cai mentioned "a hang" in one of his reports
(was it unseeded random()?). I'll send a formal patch maybe,
since there were no objections.

> To be clear. I would feel more comfortable when there are no
> deadlocks. But I also do not want to invest too much time
> into old kernels. All these problems were there for ages.
> We could finally see them because lockdep was enabled in printk()
> thanks to printk_safe.

True.
Everyone was so much happier when printk() used to do

       lockdep_off();
       call_console_drivers();
       lockdep_on();

Now we can have lockdep and RCU checks enabled, yet somehow
printk_safe is still "a terrible thing" :)

	-ss