linux-kernel - Re: [PATCH RT] tty: serial: st-asc: Make the locking RT aware

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Date:   Tue, 21 Mar 2017 14:44:37 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     <lionel.debieve@...com>
Cc:     <linux-rt-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <bigeasy@...utronix.de>, <tglx@...utronix.de>,
        Patrice Chotard <patrice.chotard@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCH RT] tty: serial: st-asc: Make the locking RT aware

On Tue, 21 Mar 2017 17:05:40 +0100
<lionel.debieve@...com> wrote:

> From: Lionel Debieve <lionel.debieve@...com>
> 
> The lock is a sleeping lock and local_irq_save() is not the
> standard implementation now. Working for both -RT and non
> RT.

If this is for both RT and non RT then the patch subject should just be
[PATCH] and not [PATCH RT] as the latter tells upstream folks not to
bother.

> 
> Signed-off-by: Lionel Debieve <lionel.debieve@...com>
> ---
>  drivers/tty/serial/st-asc.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/tty/serial/st-asc.c b/drivers/tty/serial/st-asc.c
> index 379e5bd..1815423 100644
> --- a/drivers/tty/serial/st-asc.c
> +++ b/drivers/tty/serial/st-asc.c
> @@ -803,13 +803,12 @@ static void asc_console_write(struct console
> *co, const char *s, unsigned count) int locked = 1;
>  	u32 intenable;
>  
> -	local_irq_save(flags);
>  	if (port->sysrq)
>  		locked = 0; /* asc_interrupt has already claimed the
> lock */ else if (oops_in_progress)
> -		locked = spin_trylock(&port->lock);
> +		locked = spin_trylock_irqsave(&port->lock, flags);
>  	else
> -		spin_lock(&port->lock);
> +		spin_lock_irqsave(&port->lock, flags);
>  
>  	/*
>  	 * Disable interrupts so we don't get the IRQ line bouncing

I'm nervous about the above comment, which in full is:

	/*
	 * Disable interrupts so we don't get the IRQ line bouncing
	 * up and down while interrupts are disabled.
	 */

I'm not sure if disabling interrupts helps on an SMP system. This patch
does change what happens when port->sysrq is set. But I'm not sure we
care.

-- Steve



> @@ -827,8 +826,7 @@ static void asc_console_write(struct console *co,
> const char *s, unsigned count) asc_out(port, ASC_INTEN, intenable);
>  
>  	if (locked)
> -		spin_unlock(&port->lock);
> -	local_irq_restore(flags);
> +		spin_unlock_irqrestore(&port->lock, flags);
>  }
>  
>  static int asc_console_setup(struct console *co, char *options)