linux-kernel - Re: [PATCH 7/8] usb, cdc-acm: optimize barrier usage for Rmw atomic bitops

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Date:   Thu, 30 Jan 2020 11:06:17 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     oneukum@...e.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 7/8] usb, cdc-acm: optimize barrier usage for Rmw atomic
 bitops

On Wed, Jan 29, 2020 at 10:15:45AM -0800, Davidlohr Bueso wrote:
> We can avoid the unnecessary barrier on non LL/SC architectures,
> such as x86. Instead, use the smp_mb__after_atomic() call.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>

Reviewed-by: Johan Hovold <johan@...nel.org>

> ---
>  drivers/usb/class/cdc-acm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
> index 62f4fb9b362f..0bc4b558f387 100644
> --- a/drivers/usb/class/cdc-acm.c
> +++ b/drivers/usb/class/cdc-acm.c
> @@ -842,7 +842,7 @@ static void acm_tty_unthrottle(struct tty_struct *tty)
>  	clear_bit(ACM_THROTTLED, &acm->flags);
>  
>  	/* Matches the smp_mb__after_atomic() in acm_read_bulk_callback(). */
> -	smp_mb();
> +	smp_mb__after_atomic();
>  
>  	acm_submit_read_urbs(acm, GFP_KERNEL);
>  }

We have another barrier in USB serial which can also be relaxed this
way. I'll post a patch shortly.

Johan