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Date:   Tue, 3 Jan 2023 09:59:52 +0100
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Subject: Re: [PATCH v4 1/2] tty: serial: dz: convert atomic_* to refcount_*
 APIs for map_guard
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To:     Deepak R Varma <drv@mailo.com>,
        "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Saurabh Singh Sengar <ssengar@microsoft.com>,
        Praveen Kumar <kumarpraveen@linux.microsoft.com>
References: <cover.1671898144.git.drv@mailo.com>
 <d85c7441b96ce387d9010142efc3469d53b6aedc.1671898144.git.drv@mailo.com>
From:   Jiri Slaby <jirislaby@kernel.org>
In-Reply-To: <d85c7441b96ce387d9010142efc3469d53b6aedc.1671898144.git.drv@mailo.com>
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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