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

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Date:   Wed, 16 Feb 2022 12:27:06 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        John Ogness <john.ogness@...utronix.de>,
        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 (22/02/15 10:29), Petr Mladek wrote:
> > printk has been using enter/exit naming for a while now (starting with
> > nmi enter/exit, then printk_safe enter/exit and soon direct enter/exit);
> > so may be we can follow suit here and use printk_cpu_sync_enter() and
> > printk_cpu_sync_exit()?
> 
> Interesting idea.
> 
> Honestly, I do not like it much.

  :) OK

> The existing enter/exit API is not blocking but "cpu_sync" API is.
> 
> 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
> 
> 
> "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.

I don't mind "get/put".

The thing that looks nice in enter/exit is that enter/exit don't
expose any implementation details: is there a lock behind or not,
is there a ref-counter or not, and so on.