linux-kernel - Re: [PATCH printk 11/18] printk: Convert console

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Message-ID: <db7bdf7a-597f-398e-9877-01e898733664@gmx.de>
Date:   Fri, 30 Sep 2022 18:53:54 +0200
From:   Helge Deller <deller@....de>
To:     Petr Mladek <pmladek@...e.com>,
        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,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-parisc@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH printk 11/18] printk: Convert console_drivers list to
 hlist

On 9/30/22 16:20, Petr Mladek wrote:
> On Sat 2022-09-24 02:10:47, John Ogness wrote:
>> From: Thomas Gleixner <tglx@...utronix.de>
>>
>> Replace the open coded single linked list with a hlist so a conversion to
>> SRCU protected list walks can reuse the existing primitives.
>>
>> --- a/arch/parisc/kernel/pdc_cons.c
>> +++ b/arch/parisc/kernel/pdc_cons.c
>> @@ -272,15 +267,17 @@ void pdc_console_restart(bool hpmc)
>>   	if (pdc_console_initialized)
>>   		return;
>>
>> -	if (!hpmc && console_drivers)
>> +	if (!hpmc && !hlist_empty(&console_list))
>>   		return;
>>
>>   	/* If we've already seen the output, don't bother to print it again */
>> -	if (console_drivers != NULL)
>> +	if (!hlist_empty(&console_list))
>>   		pdc_cons.flags &= ~CON_PRINTBUFFER;
>>
>> -	while ((console = console_drivers) != NULL)
>> -		unregister_console(console_drivers);
>> +	while (!hlist_empty(&console_list)) {
>> +		unregister_console(READ_ONCE(hlist_entry(console_list.first,
>> +							 struct console, node)));
>
> The READ_ONCE() is in a wrong place. This is why it did not compile.
> It should be:
>
> 		unregister_console(hlist_entry(READ_ONCE(console_list.first),
> 						struct console,
> 						node));
>
> I know that it is all hope for good. But there is also a race between
> the hlist_empty() and hlist_entry().

I wonder if pdc_console() is still needed as it is today.
When this was written, early_console and such didn't worked for parisc
as it should. That's proably why we have this register/unregister in here.

Would it make sense, and would we gain something for this printk-series,
if I'd try to convert pdc_console to a standard earlycon or earlyprintk device?

Helge