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; } }