FAILED: patch "[PATCH] tty: serial: samsung: Remove redundant port lock acquisition" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x a3bb136bff5e6a5e48cdd813246c9c4686feaaa9
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026060429-saddling-hacked-b97c@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a3bb136bff5e6a5e48cdd813246c9c4686feaaa9 Mon Sep 17 00:00:00 2001
From: Tudor Ambarus <tudor.ambarus@linaro.org>
Date: Fri, 15 May 2026 12:41:21 +0000
Subject: [PATCH] tty: serial: samsung: Remove redundant port lock acquisition
 in rx helpers

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 <stable@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>
Link: https://patch.msgid.link/20260515-samsung-tty-flow-control-deadlock-v1-1-93255edbc9bc@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
index e27806bf2cf3..17cd5bb100b1 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)