FAILED: patch "[PATCH] serial: 8250: always disable IRQ during THRE test" 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 24b98e8664e157aff0814a0f49895ee8223f382f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026032133-finalize-barometer-aa69@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

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

From 24b98e8664e157aff0814a0f49895ee8223f382f Mon Sep 17 00:00:00 2001
From: Peng Zhang <zhangpeng.00@bytedance.com>
Date: Tue, 24 Feb 2026 13:16:39 +0100
Subject: [PATCH] serial: 8250: always disable IRQ during THRE test

commit 039d4926379b ("serial: 8250: Toggle IER bits on only after irq
has been set up") moved IRQ setup before the THRE test, in combination
with commit 205d300aea75 ("serial: 8250: change lock order in
serial8250_do_startup()") the interrupt handler can run during the
test and race with its IIR reads. This can produce wrong THRE test
results and cause spurious registration of the
serial8250_backup_timeout timer. Unconditionally disable the IRQ for
the short duration of the test and re-enable it afterwards to avoid
the race.

Fixes: 039d4926379b ("serial: 8250: Toggle IER bits on only after irq has been set up")
Depends-on: 205d300aea75 ("serial: 8250: change lock order in serial8250_do_startup()")
Cc: stable <stable@kernel.org>
Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Alban Bedel <alban.bedel@lht.dlh.de>
Tested-by: Maximilian Lueer <maximilian.lueer@lht.dlh.de>
Link: https://patch.msgid.link/20260224121639.579404-1-alban.bedel@lht.dlh.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index cc94af2d578a..a743964c9d22 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2147,8 +2147,7 @@ static void serial8250_THRE_test(struct uart_port *port)
 	if (up->port.flags & UPF_NO_THRE_TEST)
 		return;
 
-	if (port->irqflags & IRQF_SHARED)
-		disable_irq_nosync(port->irq);
+	disable_irq(port->irq);
 
 	/*
 	 * Test for UARTs that do not reassert THRE when the transmitter is idle and the interrupt
@@ -2170,8 +2169,7 @@ static void serial8250_THRE_test(struct uart_port *port)
 		serial_port_out(port, UART_IER, 0);
 	}
 
-	if (port->irqflags & IRQF_SHARED)
-		enable_irq(port->irq);
+	enable_irq(port->irq);
 
 	/*
 	 * If the interrupt is not reasserted, or we otherwise don't trust the iir, setup a timer to