linux-kernel - [PATCH] serial: drop debugging WARN_ON_ONCE() from uart_put

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Date: Tue, 16 Apr 2024 09:19:04 +0200
From: "Jiri Slaby (SUSE)" <jirislaby@...nel.org>
To: gregkh@...uxfoundation.org
Cc: linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Jiri Slaby (SUSE)" <jirislaby@...nel.org>,
	Pengfei Xu <pengfei.xu@...el.com>
Subject: [PATCH] serial: drop debugging WARN_ON_ONCE() from uart_put_char()

Pengfei Xu reports, that the -next commit 1788cf6a91d9 (tty: serial:
switch from circ_buf to kfifo) tries to emit a WARNING and that leads to
lockdep errors. Obviously we cannot print anything from uart_put_char()!

This WARN_ON_ONCE() was/is a debug aid to check if the condition in
uart_put_char() can happen at all. Pengfei Xu confirmed it can. Unlike
me and kbuild bot in my queue.

Second, I completely forgot about it, so I did not remove it in the
final version, nor mentioned it in the commit log.

Drop it now as we are all good. And we even have stack traces (and a
reproducer)!

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@...nel.org>
Reported-by: Pengfei Xu <pengfei.xu@...el.com>
---
 drivers/tty/serial/serial_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 558f159ada15..bd4a72f8e821 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -573,7 +573,7 @@ static int uart_put_char(struct tty_struct *tty, u8 c)
 	int ret = 0;

 	port = uart_port_lock(state, flags);
-	if (WARN_ON_ONCE(!state->port.xmit_buf)) {
+	if (!state->port.xmit_buf) {
 		uart_port_unlock(port, flags);
 		return 0;
 	}
-- 
2.44.0