linux-kernel - Re: [PATCH printk v1 11/13] printk: reimplement console

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Message-ID: <87ley73ka2.fsf@jogness.linutronix.de>
Date:   Fri, 18 Feb 2022 23:09:01 +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>
Subject: Re: [PATCH printk v1 11/13] printk: reimplement console_lock for
 proper kthread support

On 2022-02-18, John Ogness <john.ogness@...utronix.de> wrote:
>> This is another complexity. An easier solution would be to do:
>>
>> console_trylock()
>> {
>> 	if (down_trylock(&console_sem))
>> 		return failed;
>>
>> 	for_each_console(con) {
>> 		if (mutext_trylock(&con->lock))
>> 			goto fail;
>> 	}
>>
>> 	// sucess
>>
>> fail:
>> 	for_each_console(con_fail) {
>> 		if (con_fail == con)
>> 			break;
>> 		mutex_unlock(&con->lock);
>> }
>
> No, this cannot work. Semaphores are the _only_ synchonization object in
> the kernel that is allowed to perform a trylock from any context
> (because they don't have owners).

Sorry, I need to clarify this bold statement.

Semaphores are the only synchonization object in the kernel that is
allowed to perform a trylock from any context and does not disable
preemption upon locking. (This is important because console lock holders
expect preemption to be unaffected when taking the lock.)

John