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

    Thank you,
    ./drv


    Thank you,
    ./drv

    >
    > thanks,
    > --
    > js
    > suse labs
    >


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