On Tue, Jan 10, 2023 at 01:17:54PM +0530, Deepak R Varma wrote:
> 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;
> }
> }
>
This feels odd to me, why not just use a normal lock instead?
thanks,
greg k-h
