2022-12-24 17:44:03

by Deepak R Varma

Subject: [PATCH v2] tty: serial: zs: convert atomic_* to refcount_* APIs for irq_guard

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.
Hence, replace the atomic_* APIs by their equivalent refcount_t
API functions.

This patch proposal address the following warnings generated by
the atomic_as_refcounter.cocci coccinelle script
atomic_add_return(-1, ...)

Signed-off-by: Deepak R Varma <[email protected]>
---

Changes in v2:
1. Separate the combined change into one variable per patch as
suggested by [email protected]
2. There was additional feedback on validating the change as it appeared to
modify the existing logic. However, I found that the logic does not
change with the proposed refcount_* APIs used in this change. Hence that
feedback is not applied in this version.

Please Note:
The patch is compile tested using dec_station.defconfig for MIPS architecture.

drivers/tty/serial/zs.c | 14 +++++---------
drivers/tty/serial/zs.h | 2 +-
2 files changed, 6 insertions(+), 10 deletions(-)

diff --git a/drivers/tty/serial/zs.c b/drivers/tty/serial/zs.c
index 730c648e32ff..6be9933eff5c 100644
--- a/drivers/tty/serial/zs.c
+++ b/drivers/tty/serial/zs.c
@@ -753,17 +753,15 @@ static int zs_startup(struct uart_port *uport)
struct zs_port *zport = to_zport(uport);
struct zs_scc *scc = zport->scc;
unsigned long flags;
- int irq_guard;
int ret;

- irq_guard = atomic_add_return(1, &scc->irq_guard);
- if (irq_guard == 1) {
+ refcount_inc(&scc->irq_guard);
+ if (refcount_read(&scc->irq_guard) == 1) {
ret = request_irq(zport->port.irq, zs_interrupt,
IRQF_SHARED, "scc", scc);
if (ret) {
- atomic_add(-1, &scc->irq_guard);
- printk(KERN_ERR "zs: can't get irq %d\n",
- zport->port.irq);
+ refcount_dec(&scc->irq_guard);
+ printk(KERN_ERR "zs: can't get irq %d\n", zport->port.irq);
return ret;
}
}
@@ -806,7 +804,6 @@ static void zs_shutdown(struct uart_port *uport)
struct zs_port *zport = to_zport(uport);
struct zs_scc *scc = zport->scc;
unsigned long flags;
- int irq_guard;

spin_lock_irqsave(&scc->zlock, flags);

@@ -816,8 +813,7 @@ static void zs_shutdown(struct uart_port *uport)

spin_unlock_irqrestore(&scc->zlock, flags);

- irq_guard = atomic_add_return(-1, &scc->irq_guard);
- if (!irq_guard)
+ if (refcount_dec_and_test(&scc->irq_guard))
free_irq(zport->port.irq, scc);
}

diff --git a/drivers/tty/serial/zs.h b/drivers/tty/serial/zs.h
index 26ef8eafa1c1..bd97b73d7e16 100644
--- a/drivers/tty/serial/zs.h
+++ b/drivers/tty/serial/zs.h
@@ -40,7 +40,7 @@ struct zs_port {
struct zs_scc {
struct zs_port zport[2];
spinlock_t zlock;
- atomic_t irq_guard;
+ refcount_t irq_guard;
int initialised;
};

--
2.34.1

2023-01-20 05:30:53

by Greg KH

Subject: Re: [PATCH v2] tty: serial: zs: convert atomic_* to refcount_* APIs for irq_guard

On Sat, Dec 24, 2022 at 10:25:58PM +0530, 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.
> Hence, replace the atomic_* APIs by their equivalent refcount_t
> API functions.
>
> This patch proposal address the following warnings generated by
> the atomic_as_refcounter.cocci coccinelle script
> atomic_add_return(-1, ...)
>
> Signed-off-by: Deepak R Varma <[email protected]>
> ---
>
> Changes in v2:
> 1. Separate the combined change into one variable per patch as
> suggested by [email protected]
> 2. There was additional feedback on validating the change as it appeared to
> modify the existing logic. However, I found that the logic does not
> change with the proposed refcount_* APIs used in this change. Hence that
> feedback is not applied in this version.
>
> Please Note:
> The patch is compile tested using dec_station.defconfig for MIPS architecture.
>
> drivers/tty/serial/zs.c | 14 +++++---------
> drivers/tty/serial/zs.h | 2 +-
> 2 files changed, 6 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/tty/serial/zs.c b/drivers/tty/serial/zs.c
> index 730c648e32ff..6be9933eff5c 100644
> --- a/drivers/tty/serial/zs.c
> +++ b/drivers/tty/serial/zs.c
> @@ -753,17 +753,15 @@ static int zs_startup(struct uart_port *uport)
> struct zs_port *zport = to_zport(uport);
> struct zs_scc *scc = zport->scc;
> unsigned long flags;
> - int irq_guard;
> int ret;
>
> - irq_guard = atomic_add_return(1, &scc->irq_guard);
> - if (irq_guard == 1) {
> + refcount_inc(&scc->irq_guard);
> + if (refcount_read(&scc->irq_guard) == 1) {

This conversion is wrong :(