linux-kernel - Re: [RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic context

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Date: Tue, 16 Jan 2024 08:49:14 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: Leonardo Bras <leobras@...hat.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Thomas Gleixner <tglx@...utronix.de>,
 Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Florian Fainelli <f.fainelli@...il.com>,
 John Ogness <john.ogness@...utronix.de>, Tony Lindgren <tony@...mide.com>,
 Marcelo Tosatti <mtosatti@...hat.com>, Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [RFC PATCH v1 2/2] serial/8250: Avoid getting lock in RT atomic
 context

On 16. 01. 24, 8:32, Leonardo Bras wrote:
> With PREEMPT_RT enabled, a spin_lock_irqsave() becomes a possibly sleeping
> spin_lock(), without preempt_disable() or irq_disable().
> 
> This allows a task T1 to get preempted or interrupted while holding the
> port->lock. If the preempting task T2 need the lock, spin_lock() code
> will schedule T1 back until it finishes using the lock, and then go back to
> T2.
> 
> There is an issue if a T1 holding port->lock is interrupted by an
> IRQ, and this IRQ handler needs to get port->lock for writting (printk):
> spin_lock() code will try to reschedule the interrupt handler, which is in
> atomic context, causing a BUG() for trying to reschedule/sleep in atomic
> context.
> 
> So for the case (PREEMPT_RT && in_atomic()) try to get the lock, and if it
> fails proceed anyway, just like it's done in oops_in_progress case.

Hmm, that appears incorrect to me.

Perhaps we need a raw spin lock? Or maybe I am totally off, as my RT 
knowledge is close to zero.

This needs advices from RT folks...

> Signed-off-by: Leonardo Bras <leobras@...hat.com>
> ---
>   drivers/tty/serial/8250/8250_port.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 8ca061d3bbb92..8480832846319 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -3397,7 +3397,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
>   
>   	touch_nmi_watchdog();
>   
> -	if (oops_in_progress)
> +	if (oops_in_progress || (IS_ENABLED(CONFIG_PREEMPT_RT) && in_atomic())
>   		locked = uart_port_trylock_irqsave(port, &flags);
>   	else
>   		uart_port_lock_irqsave(port, &flags);

-- 
js
suse labs