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drivers/uart: stm32: fix dead lock on poll_out

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A dead lock could happen if 2 threads with differents priorities use
poll_out. In fact, the lock data->tx_lock could be lock by a thread with
lower priority and then a thread with higher priority can't take the
lock. There was a race condition here:

/* Wait for TXE flag to be raised */
while (1) {
	if (atomic_cas(&data->tx_lock, 0, 1)) {
		/* !!!!!!!! RACE CONDITION !!!!!!!!!!!!!!
		if (LL_USART_IsActiveFlag_TXE(UartInstance)) {
			break;
		}
		atomic_set(&data->tx_lock, 0);
	}
}

To fix race condition, the interrupts are locked in poll_out.

Signed-off-by: Julien D'ascenzio <julien.dascenzio@paratronic.fr>
This commit is contained in:
Julien D'ascenzio 2021-12-01 10:19:44 +01:00 • committed by Christopher Friedt
commit e4234aeb89
2 changed files with 50 additions and 11 deletions
Download patch file Download diff file Expand all files Collapse all files

59
drivers/serial/uart_stm32.c
View file

@ -525,21 +525,36 @@ static void uart_stm32_poll_out(const struct device *dev,
unsigned char c)
{
USART_TypeDef *UartInstance = UART_STRUCT(dev);
#ifdef CONFIG_PM
struct uart_stm32_data *data = DEV_DATA(dev);
#endif
int key;
/* Wait for TXE flag to be raised */
/* Wait for TXE flag to be raised
* When TXE flag is raised, we lock interrupts to prevent interrupts (notably that of usart)
* or thread switch. Then, we can safely send our character. The character sent will be
* interlaced with the characters potentially send with interrupt transmission API
*/
while (1) {
if (atomic_cas(&data->tx_lock, 0, 1)) {
if (LL_USART_IsActiveFlag_TXE(UartInstance)) {
key = irq_lock();
if (LL_USART_IsActiveFlag_TXE(UartInstance)) {
break;
}
atomic_set(&data->tx_lock, 0);
irq_unlock(key);
}
if (!k_is_in_isr()) {
/* yield execution to another thread of the same or higher priority. */
k_yield();
}
}
#ifdef CONFIG_PM
if (!data->tx_poll_stream_on) {
/* If an interrupt transmission is in progress, the pm constraint is already managed by the
* call of uart_stm32_irq_tx_[en|dis]able
*/
if (!data->tx_poll_stream_on && !data->tx_int_stream_on) {
data->tx_poll_stream_on = true;
/* Don't allow system to suspend until stream
@ -555,7 +570,7 @@ static void uart_stm32_poll_out(const struct device *dev,
#endif /* CONFIG_PM */
LL_USART_TransmitData8(UartInstance, (uint8_t)c);
atomic_set(&data->tx_lock, 0);
irq_unlock(key);
}
static int uart_stm32_err_check(const struct device *dev)
@ -615,6 +630,14 @@ static int uart_stm32_fifo_fill(const struct device *dev,
{
USART_TypeDef *UartInstance = UART_STRUCT(dev);
uint8_t num_tx = 0U;
int key;
if (!LL_USART_IsActiveFlag_TXE(UartInstance)) {
return num_tx;
}
/* Lock interrupts to prevent nested interrupts or thread switch */
key = irq_lock();
while ((size - num_tx > 0) &&
LL_USART_IsActiveFlag_TXE(UartInstance)) {
@ -624,6 +647,8 @@ static int uart_stm32_fifo_fill(const struct device *dev,
LL_USART_TransmitData8(UartInstance, tx_data[num_tx++]);
}
irq_unlock(key);
return num_tx;
}
@ -652,29 +677,43 @@ static int uart_stm32_fifo_read(const struct device *dev, uint8_t *rx_data,
static void uart_stm32_irq_tx_enable(const struct device *dev)
{
USART_TypeDef *UartInstance = UART_STRUCT(dev);
#ifdef CONFIG_PM
struct uart_stm32_data *data = DEV_DATA(dev);
while (atomic_cas(&data->tx_lock, 0, 1) == false) {
}
int key;
#endif
#ifdef CONFIG_PM
key = irq_lock();
data->tx_poll_stream_on = false;
data->tx_int_stream_on = true;
uart_stm32_pm_constraint_set(dev);
#endif
LL_USART_EnableIT_TC(UartInstance);
#ifdef CONFIG_PM
irq_unlock(key);
#endif
}
static void uart_stm32_irq_tx_disable(const struct device *dev)
{
USART_TypeDef *UartInstance = UART_STRUCT(dev);
#ifdef CONFIG_PM
struct uart_stm32_data *data = DEV_DATA(dev);
int key;
key = irq_lock();
#endif
LL_USART_DisableIT_TC(UartInstance);
atomic_set(&data->tx_lock, 0);
#ifdef CONFIG_PM
data->tx_int_stream_on = false;
uart_stm32_pm_constraint_release(dev);
#endif
#ifdef CONFIG_PM
uart_stm32_pm_constraint_release(dev);
irq_unlock(key);
#endif
}

2
drivers/serial/uart_stm32.h
View file

@ -57,7 +57,6 @@ struct uart_stm32_data {
uint32_t baud_rate;
/* clock device */
const struct device *clock;
atomic_t tx_lock;
#ifdef CONFIG_UART_INTERRUPT_DRIVEN
uart_irq_callback_user_data_t user_cb;
void *user_data;
@ -74,6 +73,7 @@ struct uart_stm32_data {
#endif
#ifdef CONFIG_PM
bool tx_poll_stream_on;
bool tx_int_stream_on;
bool pm_constraint_on;
#endif
};