linux-kernel - Re: [PATCH printk v4 03/39] printk: Prepare for SRCU console list protection

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Date:   Tue, 15 Nov 2022 11:50:32 +0100
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, Miguel Ojeda <ojeda@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH printk v4 03/39] printk: Prepare for SRCU console list
 protection

On Mon 2022-11-14 17:34:56, John Ogness wrote:
> Provide an NMI-safe SRCU protected variant to walk the console list.
> 
> Note that all console fields are now set before adding the console
> to the list to avoid the console becoming visible by SCRU readers
> before being fully initialized.
> 
> This is a preparatory change for a new console infrastructure which
> operates independent of the console BKL.
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3254,7 +3303,7 @@ int unregister_console(struct console *console)
>  		return -ENODEV;
>  	}
>  
> -	hlist_del_init(&console->node);
> +	hlist_del_init_rcu(&console->node);

This should be hlist_del_rcu() here so that a list walker that it just
processing this console could see the rest of the list.

It seems that hlist_unhashed() checks only node.pprev pointer
so that we even do not need to initialize it.

We discussed this in v3 in the patch implementing
console_force_preferred_locked(), see
https://lore.kernel.org/r/Y20aBwNWT19YDeib@alley.
I forgot that unregister_console() might have the same problem.

Best Regards,
Petr

>  
>  	/*
>  	 * <HISTORICAL>
> @@ -3269,6 +3318,14 @@ int unregister_console(struct console *console)
>  		console_first()->flags |= CON_CONSDEV;
>  
>  	console_unlock();
> +
> +	/*
> +	 * Ensure that all SRCU list walks have completed. All contexts
> +	 * must not be able to see this console in the list so that any
> +	 * exit/cleanup routines can be performed safely.
> +	 */
> +	synchronize_srcu(&console_srcu);
> +
>  	console_sysfs_notify();
>  
>  	if (console->exit)
> -- 
> 2.30.2