linux-kernel - Re: [PATCH printk v1 01/13] printk: rename cpulock functions

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Date:   Thu, 17 Feb 2022 15:40:36 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marco Elver <elver@...gle.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Alexander Potapenko <glider@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH printk v1 01/13] printk: rename cpulock functions

On 2022-02-15, Petr Mladek <pmladek@...e.com> wrote:
> This patch is about how to make it more obvious that this
> API has to be used carefully. I see the following main risks when
> using this API:
>
>      + it might cause deadlocks, especially in panic
>
>      + it is supposed to be tail lock (no other locks allowed in this
>        context)
>
>      + it is re-entrant
>
>      + it does not prevent parallel (nested) access on the same CPU

I can add more information about the risks to the function documentation
(include/linux/printk.h).

> "get/put" are more acceptable for me. They create at lest some feeling
> that it tries to get something and it might take some time. Even
> thought many "get/put" APIs are not blocking.

OK. So I will keep the naming for the series v2.

> BTW: The API disables IRQ. So the nested access is limited to two
> levels: normal/IRQ and nested NMI contexts.

And there are reentrant call chains. For example, we already have:

nmi_cpu_backtrace()
    printk_cpu_sync_get_irqsave(flags);
    dump_stack();
        printk_cpu_sync_get_irqsave(flags);
        __dump_stack(log_lvl);
        printk_cpu_sync_put_irqrestore(flags);
    printk_cpu_sync_put_irqrestore(flags);

John