linux-kernel - Re: [PATCH] printk: Enough to disable preemption in printk deferred context

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Message-ID: <87edog56jv.fsf@jogness.linutronix.de>
Date:   Wed, 19 Apr 2023 15:51:56 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk: Enough to disable preemption in printk deferred
 context

On 2023-04-19, Petr Mladek <pmladek@...e.com> wrote:
> A solution would be to make this more clear in the comment.
> Something like:
>
> /*
>  * The printk_deferred_enter/exit macros are available only as a hack.
>  * They define a per-CPU context where all printk console printing
>  * is deferred because it might cause a deadlock otherwise.
>  *
>  * The API user is responsible for calling the corresponding enter/exit
>  * pair on the same CPU. It is highly recommended to use them only in
>  * a context with interrupts disabled. Otherwise, other unrelated
>  * printk() calls might be deferred when they interrupt/preempt
>  * the deferred code section.
>  */

I an happy with this comment. I saw Michal's follow-up suggestion, but
would prefer this one. It is a more technical desciption of the issue
and clearly recommends that the user should disable interrupts.

If you use this comment:

Reviewed-by: John Ogness <john.ogness@...utronix.de>

> Another solution would be to stay on the "safe" side and keep the
> comment as is or even enforce disabling interrupts by the API.
>
> I would personally just improve the comment. It is good to describe
> the situation correctly. We could always add restrictions when
> there are problems in practice.

Agreed.

John