linux-kernel - Re: global states: was: Re: [PATCH printk v1 05/18] printk: Add non-BKL console basic infrastructure

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Message-ID: <87v8iz5ww3.fsf@jogness.linutronix.de>
Date:   Fri, 17 Mar 2023 14:35:32 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: global states: was: Re: [PATCH printk v1 05/18] printk: Add
 non-BKL console basic infrastructure

On 2023-03-09, Petr Mladek <pmladek@...e.com> wrote:
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -3472,6 +3492,14 @@ void register_console(struct console *newcon)
>>  	newcon->dropped = 0;
>>  	console_init_seq(newcon, bootcon_registered);
>>  
>> +	if (!(newcon->flags & CON_NO_BKL))
>> +		have_bkl_console = true;
>
> We never clear this value even when the console gets unregistered.

OK. I'll allow it to be cleared on unregister by checking the registered
list.

>> @@ -3515,6 +3543,9 @@ void register_console(struct console *newcon)
>>  			if (con->flags & CON_BOOT)
>>  				unregister_console_locked(con);
>>  		}
>> +
>> +		/* All boot consoles have been unregistered. */
>> +		have_boot_console = false;
>
> The boot consoles can be removed also by printk_late_init().
>
> I would prefer to make this more error-proof and update both
> have_bkl_console and have_boot_console in unregister_console().

OK.

> A solution would be to use a reference counter instead of the boolean.
> I am not sure if it is worth it. But it seems that refcount_read()
> is just simple atomic read, aka READ_ONCE().

Well, we are holding the console_list_lock, so we can just iterate over
the list. Iteration happens later in the series anyway, in order to
create/run the NOBKL threads.

John