Re: [PATCH v4 1/2] tty: serial: dz: convert atomic_* to refcount_* APIs for map_guard

[Deepak R Varma <drv@xxxxxxxxx>]

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.

############## ORIGINAL CODE ##################################
-       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");
                        return -EBUSY;
                }
        }

############## INITIAL APPROACH ##################################
+       refcount_inc(&mux->map_guard);
+       if (refcount_read(&mux->map_guard) == 1) {
+               if (!request_mem_region(uport->mapbase, dec_kn_slot_size, "dz")) {
+                       refcount_dec(&mux->map_guard);
+                       printk(KERN_ERR "dz: Unable to reserve MMIO resource\n");
                        return -EBUSY;
                }
        }

############## ALTERNATE APPROACH ##################################

+       if (!refcount_inc_not_zero(&mux->map_guard)) {
+               refcount_inc(&mux->map_guard);
+               if (!request_mem_region(uport->mapbase, dec_kn_slot_size, "dz")) {
+                       refcount_dec(&mux->map_guard);
+                       printk(KERN_ERR "dz: Unable to reserve MMIO resource\n");
                        return -EBUSY;
                }
        }