linux-kernel - Re: [PATCH v3 4/4] printk: Drop console

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Date:   Fri, 04 Feb 2022 10:53:48 -0800
From:   Stephen Brennan <stephen.s.brennan@...cle.com>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [PATCH v3 4/4] printk: Drop console_sem during panic

Sergey Senozhatsky <senozhatsky@...omium.org> writes:
> On (22/02/01 10:58), Stephen Brennan wrote:
>> +/*
>> + * Return true when this CPU should unlock console_sem without pushing all
>> + * messages to the console. This reduces the chance that the console is
>> + * locked when the panic CPU tries to use it.
>> + */
>> +static bool abandon_console_lock_in_panic(void)
>> +{
>> +	if (!panic_in_progress())
>> +		return false;
>> +
>> +	/*
>> +	 * We can use raw_smp_processor_id() here because it is impossible for
>> +	 * the task to be migrated to the panic_cpu, or away from it. If
>> +	 * panic_cpu has already been set, and we're not currently executing on
>> +	 * that CPU, then we never will be.
>> +	 */
>> +	return atomic_read(&panic_cpu) != raw_smp_processor_id();
>> +}
>> +
>>  /*
>>   * Can we actually use the console at this time on this cpu?
>>   *
>> @@ -2746,6 +2765,10 @@ void console_unlock(void)
>>  		if (handover)
>>  			return;
>>  
>> +		/* Allow panic_cpu to take over the consoles safely */
>> +		if (abandon_console_lock_in_panic())
>> +			break;
>
> Sorry, why not just `return` like in handover case?

We need to drop console_sem before returning, since the whole benefit
here is to increase the chance that console_sem is unlocked when the
panic_cpu halts this CPU.

in the handover case, there's another cpu waiting, and we're essentially
transferring the console_sem ownership to that cpu, so we explicitly
return and skip the unlocking portion.

Does this need some comments to clarify it?

Admittedly if I had a few more lines of context in the diff, you would
see the console unlock directly after the loop; it's a bit clearer when
you're looking at the function as whole:

2768 		/* Allow panic_cpu to take over the consoles safely */
2769 		if (abandon_console_lock_in_panic())
2770 			break;
2771 
2772 		if (do_cond_resched)
2773 			cond_resched();
2774 	}
2775 
2776 	/* Get consistent value of the next-to-be-used sequence number. */
2777 	next_seq = console_seq;
2778 
2779 	console_locked = 0;
2780 	up_console_sem();

Stephen

>
>> +
>>  		if (do_cond_resched)
>>  			cond_resched();
>>  	}
>> @@ -2763,7 +2786,7 @@ void console_unlock(void)
>>  	 * flush, no worries.
>>  	 */
>>  	retry = prb_read_valid(prb, next_seq, NULL);
>> -	if (retry && console_trylock())
>> +	if (retry && !abandon_console_lock_in_panic() && console_trylock())
>>  		goto again;
>>  }
>>  EXPORT_SYMBOL(console_unlock);