linux-kernel - Re: [PATCH printk v4 5/8] printk: nbcon: Add ownership state functions

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Message-ID: <ZQMpCDKsfElXIXI7@alley>
Date:   Thu, 14 Sep 2023 17:38:48 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v4 5/8] printk: nbcon: Add ownership state
 functions

On Fri 2023-09-08 20:56:05, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> Provide functions that are related to the safe handover mechanism
> and allow console drivers to dynamically specify unsafe regions:
> 
>  - nbcon_context_can_proceed()
> 
>    Invoked by a console owner to check whether a handover request
>    is pending or whether the console has been taken over by another
>    context. If a handover request is pending, this function will
>    also perform the handover, thus cancelling its own ownership.
> 
>  - nbcon_context_enter_unsafe()/nbcon_context_exit_unsafe()
> 
>    Invoked by a console owner to denote that the driver is about
>    to enter or leave a critical region where a take over is unsafe.
>    These functions are also cancellation points where loss of
>    ownership can occur.

The last sentence is not completely clear to me. I would write:

     The exit variant is the point where the current owner releases
     the lock for a higher priority context which asked for
     the friendly handover.

>    The unsafe state is stored in the console state and allows a
>    new context to make informed decisions whether to attempt a
>    takeover of such a console. The unsafe state is also available
>    to the driver so that it can make informed decisions about the
>    required actions and possibly take a special emergency path.
> 
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -492,6 +491,115 @@ static void nbcon_context_release(struct nbcon_context *ctxt)
>  	ctxt->pbufs = NULL;
>  }
>  
> +/**
> + * nbcon_context_can_proceed - Check whether ownership can proceed
> + * @ctxt:	The nbcon context from nbcon_context_try_acquire()
> + * @cur:	The current console state
> + *
> + * Return:	True if this context still owns the console. False if
> + *		ownership was handed over or taken.
> + *
> + * Must be invoked after the record was dumped into the assigned buffer
> + * and at appropriate safe places in the driver.

I think that is not longer called explicitly in nbcon_emit_next_record().

I would write something like:

  * Must be invoked when entering the unsafe state to make sure that it still
  * owns the lock. Also must be invoked when exiting the unsafe context
  * to eventually free the lock for a higher priority context which asked
  * for the friendly handover.
  *
  * It can be called inside an unsafe section when the console is just
  * temporary in safe state instead of exiting and entering the unsafe
  * state.
  *
  * Also it can be called in the safe context before doing an expensive
  * safe operation. It does not make sense to do the operation when
  * a higher priority context took the lock.


> + *
> + * When this function returns false then the calling context no longer owns
> + * the console and is no longer allowed to go forward. In this case it must
> + * back out immediately and carefully. The buffer content is also no longer
> + * trusted since it no longer belongs to the calling context.
> + */
> +static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_state *cur)
> +{
> +	unsigned int cpu = smp_processor_id();

With the proposed changes:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr