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Date:   Fri, 4 Feb 2022 13:04:41 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Stephen Brennan <stephen.s.brennan@oracle.com>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-kernel@vger.kernel.org,
        John Ogness <john.ogness@linutronix.de>
Subject: Re: [PATCH v3 4/4] printk: Drop console_sem during panic
Message-ID: <Yfyl2TSdFmn7HqmV@google.com>
References: <20220201185802.98345-1-stephen.s.brennan@oracle.com>
 <20220201185802.98345-5-stephen.s.brennan@oracle.com>
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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?

> +
>  		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);