linux-kernel - Re: [PATCH next v1 2/3] printk: remove safe buffers

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Date:   Mon, 29 Mar 2021 12:04:38 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Alistair Popple <alistair@...ple.id.au>,
        Jordan Niethe <jniethe5@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Cédric Le Goater <clg@...d.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, Yue Hu <huyue2@...ong.com>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Rafael Aquini <aquini@...hat.com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        "Guilherme G. Piccoli" <gpiccoli@...onical.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linuxppc-dev@...ts.ozlabs.org, kexec@...ts.infradead.org
Subject: Re: [PATCH next v1 2/3] printk: remove safe buffers

On Fri 2021-03-26 12:12:37, John Ogness wrote:
> On 2021-03-23, Petr Mladek <pmladek@...e.com> wrote:
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> -
> >>  	if (seq != prb_next_seq(&printk_rb_static)) {
> >>  		pr_err("dropped %llu messages\n",
> >>  		       prb_next_seq(&printk_rb_static) - seq);
> >> @@ -2666,7 +2631,6 @@ void console_unlock(void)
> >>  		size_t ext_len = 0;
> >>  		size_t len;
> >>  
> >> -		printk_safe_enter_irqsave(flags);
> >>  skip:
> >>  		if (!prb_read_valid(prb, console_seq, &r))
> >>  			break;
> >> @@ -2711,6 +2675,8 @@ void console_unlock(void)
> >>  				printk_time);
> >>  		console_seq++;
> >>  
> >> +		printk_safe_enter_irqsave(flags);
> >
> > What is the purpose of the printk_safe context here, please?
> 
> console_lock_spinning_enable() needs to be called with interrupts
> disabled. I should have just used local_irq_save().
> 
> I could add local_irq_save() to console_lock_spinning_enable() and
> restore them at the end of console_lock_spinning_disable_and_check(),
> but then I would need to add a @flags argument to both functions. I
> think it is simpler to just do the disable/enable from the caller,
> console_unlock().

I see. I have missed it that all this code have to be called with
interrupts disabled.

OK, it is a must-to-have because of the spinning. But I wonder if some
console drivers rely on the fact that the write() callback is
called with interrupts disabled.

IMHO, it would be a bug when any write() callback expects that
callers disabled the interrupts.

Do you plan to remove the console-spinning stuff after offloading
consoles to the kthreads?

Will you call console write() callback with irq enabled from
the kthread?

Anyway, we should at least add a comment why the interrupts are
disabled.


> BTW, I could not find any sane way of disabling interrupts via a
> raw_spin_lock_irqsave() of @console_owner_lock because of the how it is
> used with lockdep. In particular for
> console_lock_spinning_disable_and_check().

I see. IMHO, we would need to explicitly call local_irq_save()/restore()
if we moved them to console_lock_spinning_enable()/disable_and_check().
I mean to do:


static void console_lock_spinning_enable(unsigned long *flags)
{
	local_irq_save(*flags);

	raw_spin_lock(&console_owner_lock);
	console_owner = current;
	raw_spin_unlock(&console_owner_lock);

	/* The waiter may spin on us after setting console_owner */
	spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
}

...

Best Regards,
Petr