linux-kernel - Re: [PATCH 1/9] macintosh/via-macii: Access autopoll

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Date:   Sun, 9 Aug 2020 12:01:38 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Finn Thain <fthain@...egraphics.com.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Joshua Thompson <funaho@...ai.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org,
        Laurent Vivier <lvivier@...hat.com>,
        Mark Cave-Ayland <mark.cave-ayland@...nde.co.uk>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/9] macintosh/via-macii: Access autopoll_devs when
 inside lock

Hi,

On Sun, Jun 28, 2020 at 02:23:12PM +1000, Finn Thain wrote:
> The interrupt handler should be excluded when accessing the autopoll_devs
> variable.
> 

I am quite baffled by this patch. Other than adding an unnecessary lock /
unlock sequence, accessing a variable (which is derived from another
variable) from inside or outside a lock does not make a difference.
If autopoll_devs = devs & 0xfffe is 0 inside the lock, it will just
as much be 0 outside the lock, and vice versa.

Can you explain this in some more detail ? Not that is matters much since
the change already made it into mainline, but I would like to understand
what if anything I am missing here.

Thanks,
Guenter

> Fixes: d95fd5fce88f0 ("m68k: Mac II ADB fixes") # v5.0+
> Tested-by: Stan Johnson <userm57@...oo.com>
> Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
> ---
>  drivers/macintosh/via-macii.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/macintosh/via-macii.c b/drivers/macintosh/via-macii.c
> index ac824d7b2dcfc..6aa903529570d 100644
> --- a/drivers/macintosh/via-macii.c
> +++ b/drivers/macintosh/via-macii.c
> @@ -270,15 +270,12 @@ static int macii_autopoll(int devs)
>  	unsigned long flags;
>  	int err = 0;
>  
> +	local_irq_save(flags);
> +
>  	/* bit 1 == device 1, and so on. */
>  	autopoll_devs = devs & 0xFFFE;
>  
> -	if (!autopoll_devs)
> -		return 0;
> -
> -	local_irq_save(flags);
> -
> -	if (current_req == NULL) {
> +	if (autopoll_devs && !current_req) {
>  		/* Send a Talk Reg 0. The controller will repeatedly transmit
>  		 * this as long as it is idle.
>  		 */