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

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Date:   Tue, 15 Feb 2022 10:29:19 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     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 Mon 2022-02-14 15:49:08, Sergey Senozhatsky wrote:
> On (22/02/11 13:44), Petr Mladek wrote:
> > On Mon 2022-02-07 20:49:11, John Ogness wrote:
> > > Since the printk cpulock is CPU-reentrant and since it is used
> > > in all contexts, its usage must be carefully considered and
> > > most likely will require programming locklessly. To avoid
> > > mistaking the printk cpulock as a typical lock, rename it to
> > > cpu_sync. The main functions then become:
> > > 
> > >     printk_cpu_sync_get_irqsave(flags);
> > >     printk_cpu_sync_put_irqrestore(flags);
> > 
> > It is possible that I will understand the motivation later when
> > reading the entire patchset. But my initial reaction is confusion ;-)
> > 
> > From mo POV, it is a lock. It tries to get exclusive access and
> > has to wait until the current owner releases it.
> 
> 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. 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.

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

Best Regards,
Petr