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

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Date:   Tue, 10 Jan 2023 13:17:54 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
        "ishkamiel@...il.com" <ishkamiel@...il.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dwindsor@...il.com" <dwindsor@...il.com>
Subject: Re: [PATCH v4 1/2] tty: serial: dz: convert atomic_* to refcount_*
 APIs for map_guard

On Tue, Jan 10, 2023 at 07:27:44AM +0000, Reshetova, Elena wrote:
>  
> > On Tue, Jan 03, 2023 at 09:59:52AM +0100, Jiri Slaby wrote:
> > > 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?
> > 
> > Hello Jiri,
> > I found this [1] commit which introduced similar transformation in a
> > neighbouring driver. Can you please comment how is this different from the
> > current patch proposal?
> > 
> > [1] commit ID: 22a33651a56f ("convert sbd_duart.map_guard from atomic_t to
> > refcount_t")
> > 
> > On a side note, I have not been able to find an exact 1:1 map to the
> > atomic_add_result API. I am wondering should we have one?
> 

Hello Elena,

> In past we have decided not to provide this API for refcount_t
> because for truly correctly behaving reference counters it should not be needed
> (vs atomics that cover a broader range of use cases). 

So, there is no FAA refcount wrapper? I think this is a pretty common need.
Please correct me if I am wrong.

> Can you use !refcount_inc_not_zero in the above case?

I actually did try that but was not sure if truly addresses the objection.
Please attached and let me know if you have a feedback on the alternate
approach.

Thank you,
./drv


> 
> Best Regards,
> Elena.

View attachment "code_diff" of type "text/plain" (1420 bytes)