* [PATCH] tty: serial: samsung: Remove redundant port lock acquisition in rx helpers
@ 2026-05-15 12:41 Tudor Ambarus
2026-05-24 19:42 ` Krzysztof Kozlowski
0 siblings, 1 reply; 2+ messages in thread
From: Tudor Ambarus @ 2026-05-15 12:41 UTC (permalink / raw)
To: Krzysztof Kozlowski, Alim Akhtar, Greg Kroah-Hartman, Jiri Slaby,
Ben Dooks
Cc: linux-arm-kernel, linux-samsung-soc, linux-kernel, linux-serial,
john.ogness, peter.griffin, andre.draszik, jyescas, kernel-team,
stable, John Ogness, Tudor Ambarus
Sashiko identified a deadlock when the console flow is engaged [1].
When console flow control is enabled (UPF_CONS_FLOW),
s3c24xx_serial_stop_tx() calls s3c24xx_serial_rx_enable() and
s3c24xx_serial_start_tx() calls s3c24xx_serial_rx_disable().
The serial core framework invokes the .stop_tx() and .start_tx()
callbacks with the port->lock spinlock already held. Furthermore, all
internal driver paths that invoke stop_tx (such as the DMA TX
completion handler s3c24xx_serial_tx_dma_complete() or the PIO TX IRQ
handler s3c24xx_serial_tx_irq()) also acquire port->lock prior to
calling it. (Note that s3c24xx_serial_start_tx() is only invoked by the
serial core).
However, s3c24xx_serial_rx_enable() and s3c24xx_serial_rx_disable()
unconditionally attempt to acquire port->lock again using
uart_port_lock_irqsave(). Since spinlocks are not recursive, this
causes a deadlock on the same CPU when console flow control is engaged.
Remove the redundant lock acquisition from both rx helper functions.
Cc: stable@vger•kernel.org
Fixes: b497549a035c ("[ARM] S3C24XX: Split serial driver into core and per-cpu drivers")
Reported-by: John Ogness <john.ogness@linutronix•de>
Closes: https://sashiko.dev/#/patchset/20260506121606.5805-1-john.ogness%40linutronix.de [1]
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro•org>
---
drivers/tty/serial/samsung_tty.c | 8 --------
1 file changed, 8 deletions(-)
diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
index 2f94fc798cff..63d0232dffc2 100644
--- a/drivers/tty/serial/samsung_tty.c
+++ b/drivers/tty/serial/samsung_tty.c
@@ -245,12 +245,9 @@ static bool s3c24xx_serial_txempty_nofifo(const struct uart_port *port)
static void s3c24xx_serial_rx_enable(struct uart_port *port)
{
struct s3c24xx_uart_port *ourport = to_ourport(port);
- unsigned long flags;
int count = 10000;
u32 ucon, ufcon;
- uart_port_lock_irqsave(port, &flags);
-
while (--count && !s3c24xx_serial_txempty_nofifo(port))
udelay(100);
@@ -263,23 +260,18 @@ static void s3c24xx_serial_rx_enable(struct uart_port *port)
wr_regl(port, S3C2410_UCON, ucon);
ourport->rx_enabled = 1;
- uart_port_unlock_irqrestore(port, flags);
}
static void s3c24xx_serial_rx_disable(struct uart_port *port)
{
struct s3c24xx_uart_port *ourport = to_ourport(port);
- unsigned long flags;
u32 ucon;
- uart_port_lock_irqsave(port, &flags);
-
ucon = rd_regl(port, S3C2410_UCON);
ucon &= ~S3C2410_UCON_RXIRQMODE;
wr_regl(port, S3C2410_UCON, ucon);
ourport->rx_enabled = 0;
- uart_port_unlock_irqrestore(port, flags);
}
static void s3c24xx_serial_stop_tx(struct uart_port *port)
---
base-commit: 16e95bfb79b5d9d01dc7651d98caf3c2ace331cd
change-id: 20260515-samsung-tty-flow-control-deadlock-1d426171bf41
Best regards,
--
Tudor Ambarus <tudor.ambarus@linaro•org>
^ permalink raw reply related [flat|nested] 2+ messages in thread* Re: [PATCH] tty: serial: samsung: Remove redundant port lock acquisition in rx helpers
2026-05-15 12:41 [PATCH] tty: serial: samsung: Remove redundant port lock acquisition in rx helpers Tudor Ambarus
@ 2026-05-24 19:42 ` Krzysztof Kozlowski
0 siblings, 0 replies; 2+ messages in thread
From: Krzysztof Kozlowski @ 2026-05-24 19:42 UTC (permalink / raw)
To: Tudor Ambarus, Alim Akhtar, Greg Kroah-Hartman, Jiri Slaby,
Ben Dooks
Cc: linux-arm-kernel, linux-samsung-soc, linux-kernel, linux-serial,
john.ogness, peter.griffin, andre.draszik, jyescas, kernel-team,
stable, John Ogness
On 15/05/2026 14:41, Tudor Ambarus wrote:
> Sashiko identified a deadlock when the console flow is engaged [1].
>
> When console flow control is enabled (UPF_CONS_FLOW),
> s3c24xx_serial_stop_tx() calls s3c24xx_serial_rx_enable() and
> s3c24xx_serial_start_tx() calls s3c24xx_serial_rx_disable().
>
> The serial core framework invokes the .stop_tx() and .start_tx()
> callbacks with the port->lock spinlock already held. Furthermore, all
> internal driver paths that invoke stop_tx (such as the DMA TX
> completion handler s3c24xx_serial_tx_dma_complete() or the PIO TX IRQ
> handler s3c24xx_serial_tx_irq()) also acquire port->lock prior to
> calling it. (Note that s3c24xx_serial_start_tx() is only invoked by the
> serial core).
>
> However, s3c24xx_serial_rx_enable() and s3c24xx_serial_rx_disable()
> unconditionally attempt to acquire port->lock again using
> uart_port_lock_irqsave(). Since spinlocks are not recursive, this
> causes a deadlock on the same CPU when console flow control is engaged.
>
> Remove the redundant lock acquisition from both rx helper functions.
>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss•qualcomm.com>
Best regards,
Krzysztof
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