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

From: John Ogness <john.ogness@linutronix.de>
To: Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Marco Elver <elver@google.com>,
	Stephen Boyd <swboyd@chromium.org>,
	Alexander Potapenko <glider@google.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH printk v1 01/13] printk: rename cpulock functions
Date: Thu, 17 Feb 2022 15:40:36 +0106	[thread overview]
Message-ID: <87fsoh8sub.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <Ygtyb4WFVlv/earX@alley>

On 2022-02-15, Petr Mladek <pmladek@suse.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