linux-kernel - Re: [PATCH v2] tty: serial: zs: convert atomic_* to refcount_* APIs for irq

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Date:   Thu, 19 Jan 2023 14:52:36 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Deepak R Varma <drv@...lo.com>
Cc:     "Maciej W. Rozycki" <macro@...am.me.uk>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>
Subject: Re: [PATCH v2] tty: serial: zs: convert atomic_* to refcount_* APIs
 for irq_guard

On Sat, Dec 24, 2022 at 10:25:58PM +0530, Deepak R Varma wrote:
> The refcount_* APIs are designed to address known issues with the
> atomic_t APIs for reference counting. They provide following distinct
> advantages:
>    - protect the reference counters from overflow/underflow
>    - avoid use-after-free errors
>    - provide improved memory ordering guarantee schemes
>    - neater and safer.
> Hence, replace the atomic_* APIs by their equivalent refcount_t
> API functions.
> 
> This patch proposal address the following warnings generated by
> the atomic_as_refcounter.cocci coccinelle script
> atomic_add_return(-1, ...)
> 
> Signed-off-by: Deepak R Varma <drv@...lo.com>
> ---
> 
> Changes in v2:
>    1. Separate the combined change into one variable per patch as
>       suggested by gregkh@...uxfoundation.org
>    2. There was additional feedback on validating the change as it appeared to
>       modify the existing logic. However, I found that the logic does not
>       change with the proposed refcount_* APIs used in this change. Hence that
>       feedback is not applied in this version.
> 
> Please Note:
>    The patch is compile tested using dec_station.defconfig for MIPS architecture.
> 
>  drivers/tty/serial/zs.c | 14 +++++---------
>  drivers/tty/serial/zs.h |  2 +-
>  2 files changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/tty/serial/zs.c b/drivers/tty/serial/zs.c
> index 730c648e32ff..6be9933eff5c 100644
> --- a/drivers/tty/serial/zs.c
> +++ b/drivers/tty/serial/zs.c
> @@ -753,17 +753,15 @@ static int zs_startup(struct uart_port *uport)
>  	struct zs_port *zport = to_zport(uport);
>  	struct zs_scc *scc = zport->scc;
>  	unsigned long flags;
> -	int irq_guard;
>  	int ret;
> 
> -	irq_guard = atomic_add_return(1, &scc->irq_guard);
> -	if (irq_guard == 1) {
> +	refcount_inc(&scc->irq_guard);
> +	if (refcount_read(&scc->irq_guard) == 1) {

This conversion is wrong :(