This is a note to let you know that I've just added the patch titled
serial: 8250: Fix serial8250_tx_empty() race with DMA Tx
to the 6.3-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
serial-8250-fix-serial8250_tx_empty-race-with-dma-tx.patch
and it can be found in the queue-6.3 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 146a37e05d620cef4ad430e5d1c9c077fe6fa76f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= <ilpo.jarvinen@xxxxxxxxxxxxxxx>
Date: Fri, 17 Mar 2023 13:33:18 +0200
Subject: serial: 8250: Fix serial8250_tx_empty() race with DMA Tx
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From: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
commit 146a37e05d620cef4ad430e5d1c9c077fe6fa76f upstream.
There's a potential race before THRE/TEMT deasserts when DMA Tx is
starting up (or the next batch of continuous Tx is being submitted).
This can lead to misdetecting Tx empty condition.
It is entirely normal for THRE/TEMT to be set for some time after the
DMA Tx had been setup in serial8250_tx_dma(). As Tx side is definitely
not empty at that point, it seems incorrect for serial8250_tx_empty()
claim Tx is empty.
Fix the race by also checking in serial8250_tx_empty() whether there's
DMA Tx active.
Note: This fix only addresses in-kernel race mainly to make using
TCSADRAIN/FLUSH robust. Userspace can still cause other races but they
seem userspace concurrency control problems.
Fixes: 9ee4b83e51f74 ("serial: 8250: Add support for dmaengine")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20230317113318.31327-3-ilpo.jarvinen@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/tty/serial/8250/8250.h | 12 ++++++++++++
drivers/tty/serial/8250/8250_port.c | 7 ++++---
2 files changed, 16 insertions(+), 3 deletions(-)
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -365,6 +365,13 @@ static inline void serial8250_do_prepare
if (dma->prepare_rx_dma)
dma->prepare_rx_dma(p);
}
+
+static inline bool serial8250_tx_dma_running(struct uart_8250_port *p)
+{
+ struct uart_8250_dma *dma = p->dma;
+
+ return dma && dma->tx_running;
+}
#else
static inline int serial8250_tx_dma(struct uart_8250_port *p)
{
@@ -380,6 +387,11 @@ static inline int serial8250_request_dma
return -1;
}
static inline void serial8250_release_dma(struct uart_8250_port *p) { }
+
+static inline bool serial8250_tx_dma_running(struct uart_8250_port *p)
+{
+ return false;
+}
#endif
static inline int ns16550a_goto_highspeed(struct uart_8250_port *up)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2016,18 +2016,19 @@ static int serial8250_tx_threshold_handl
static unsigned int serial8250_tx_empty(struct uart_port *port)
{
struct uart_8250_port *up = up_to_u8250p(port);
+ unsigned int result = 0;
unsigned long flags;
- u16 lsr;
serial8250_rpm_get(up);
spin_lock_irqsave(&port->lock, flags);
- lsr = serial_lsr_in(up);
+ if (!serial8250_tx_dma_running(up) && uart_lsr_tx_empty(serial_lsr_in(up)))
+ result = TIOCSER_TEMT;
spin_unlock_irqrestore(&port->lock, flags);
serial8250_rpm_put(up);
- return uart_lsr_tx_empty(lsr) ? TIOCSER_TEMT : 0;
+ return result;
}
unsigned int serial8250_do_get_mctrl(struct uart_port *port)
Patches currently in stable-queue which might be from ilpo.jarvinen@xxxxxxxxxxxxxxx are
queue-6.3/tty-prevent-writing-chars-during-tcsetattr-tcsadrain-flush.patch
queue-6.3/serial-fix-tiocsrs485-locking.patch
queue-6.3/serial-8250-fix-serial8250_tx_empty-race-with-dma-tx.patch
