linux-kernel - Re: [PATCH v4 1/2] tty: serial: dz: convert atomic_* to refcount_* APIs for map

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Date:   Tue, 3 Jan 2023 09:59:52 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Deepak R Varma <drv@...lo.com>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>
Subject: Re: [PATCH v4 1/2] tty: serial: dz: convert atomic_* to refcount_*
 APIs for map_guard

On 26. 12. 22, 7:21, 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.

Really? (see below)

> --- a/drivers/tty/serial/dz.c
> +++ b/drivers/tty/serial/dz.c
...
> @@ -687,23 +686,19 @@ static int dz_map_port(struct uart_port *uport)
>   static int dz_request_port(struct uart_port *uport)
>   {
>   	struct dz_mux *mux = to_dport(uport)->mux;
> -	int map_guard;
>   	int ret;
> 
> -	map_guard = atomic_add_return(1, &mux->map_guard);
> -	if (map_guard == 1) {
> -		if (!request_mem_region(uport->mapbase, dec_kn_slot_size,
> -					"dz")) {
> -			atomic_add(-1, &mux->map_guard);
> -			printk(KERN_ERR
> -			       "dz: Unable to reserve MMIO resource\n");
> +	refcount_inc(&mux->map_guard);
> +	if (refcount_read(&mux->map_guard) == 1) {

This is now racy, right?

thanks,
-- 
js
suse labs