[PATCH 0/3] can: enable NAPI before starting the controller in three drivers

[PATCH 0/3] can: enable NAPI before starting the controller in three drivers

[Patrick Zhang]

Hi,

Three CAN drivers open the device in the order "start the controller
and unmask its RX interrupt, then napi_enable()". Each of them has an
ISR that masks the RX interrupt before calling napi_schedule(), and a
poll that is the only place the RX interrupt is unmasked again. A frame
that arrives between the controller start and napi_enable() is handled
by the ISR, which masks RX and calls napi_schedule() on a NAPI instance
that is still disabled. napi_schedule_prep() sees NAPI_STATE_SCHED set
and drops the schedule. napi_enable() clears SCHED without scheduling.
Nothing ever polls, so RX stays masked and the interface receives
nothing until the next close. Transmit keeps working and no error
counter moves, so the failure is silent.

The affected drivers are ctucanfd, xilinx_can and ifi_canfd. The fix is
the same in each: call napi_enable() before the controller start, so
that an early RX interrupt can schedule the poll, and call
napi_disable() on the error path if the start can fail.

Patch 1 (ctucanfd) was tested on QEMU with the ctucan_pci device.
Patch 2 (xilinx_can) was tested on a Versal board. Patch 3 (ifi_canfd)
is from code inspection only. I do not have the hardware, so it is
compile tested only. The sequence is the same as in the other two
drivers, and I would appreciate a test from someone who has an IFI CAN
FD core.

The bug has been present since each driver was added, so each patch
has a Fixes: tag pointing at the commit that added the driver.
Patches 1 and 2 are marked for stable backport. Patch 3 is not,
because it is untested. The patches are independent and can be
applied separately.

I looked at the other NAPI users under drivers/net/can. at91_can had
the same persistent-mask pattern in 6.6 but no longer masks RX in its
ISR since the conversion to rx-offload, so I left it alone; ti_hecc and
virtio_can start the device before napi_enable() but do not leave RX
masked, so a dropped first schedule only delays the first frame. All
the others enable NAPI before the device can interrupt.

Patrick Zhang (3):
  can: ctucanfd: enable NAPI before starting the controller
  can: xilinx_can: enable NAPI before starting the controller
  can: ifi_canfd: enable NAPI before starting the controller

 drivers/net/can/ctucanfd/ctucanfd_base.c | 6 +++++-
 drivers/net/can/ifi_canfd/ifi_canfd.c    | 5 ++++-
 drivers/net/can/xilinx_can.c             | 6 +++++-
 3 files changed, 14 insertions(+), 3 deletions(-)


base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
-- 
2.50.1

[PATCH 1/3] can: ctucanfd: enable NAPI before starting the controller

[Patrick Zhang]

ctucan_open() starts the controller and unmasks RBNEI in
ctucan_chip_start(), prints "ctu_can_fd device registered", and only
then calls napi_enable(). A frame that arrives in that window raises
RBNEI. ctucan_interrupt() masks RBNEI and calls napi_schedule(). NAPI is
still disabled, so napi_schedule_prep() finds NAPI_STATE_SCHED set and
the schedule is dropped. napi_enable() then clears SCHED without
scheduling anything. Only ctucan_rx_poll() unmasks RBNEI again, and it
never runs. The interface receives nothing until the next close.
Transmit still works. The RX FIFO fills up and the core sets DOR, but
the overrun is only accounted in ctucan_rx_poll(), so rx_errors and
rx_over_errors stay at zero and nothing is logged. Every re-open (ip
link down/up, a driver rebind) runs the same race again.

The netdev_info() in the window makes it much wider on a slow console.
This can be reproduced by sending continuous traffic with cangen from
another node and repeating "ip link set down/up" on the test device.

Enable NAPI before the controller starts, so that an early RX interrupt
can schedule the poll. NAPI can be enabled while the controller is in
reset, because nothing schedules it until an interrupt arrives. If
ctucan_chip_start() fails, disable NAPI again. Otherwise the next open
would call napi_enable() on an already enabled instance and trip the
BUG_ON() in napi_enable_locked().

Tested on QEMU with the ctucan_pci device.

Fixes: 2dcb8e8782d8 ("can: ctucanfd: add support for CTU CAN FD open-source IP core - bus independent part.")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Patrick Zhang <patrzhan@amazon.com>
---
 drivers/net/can/ctucanfd/ctucanfd_base.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/ctucanfd/ctucanfd_base.c b/drivers/net/can/ctucanfd/ctucanfd_base.c
index 07d4aa43c..82bf6c645 100644
--- a/drivers/net/can/ctucanfd/ctucanfd_base.c
+++ b/drivers/net/can/ctucanfd/ctucanfd_base.c
@@ -1231,6 +1231,10 @@ static int ctucan_open(struct net_device *ndev)
 		goto err_irq;
 	}
 
+	/* Keep before ctucan_chip_start(): the ISR masks RBNEI and only the
+	 * poll unmasks it.
+	 */
+	napi_enable(&priv->napi);
 	ret = ctucan_chip_start(ndev);
 	if (ret < 0) {
 		netdev_err(ndev, "ctucan_chip_start failed!\n");
@@ -1238,12 +1242,12 @@ static int ctucan_open(struct net_device *ndev)
 	}
 
 	netdev_info(ndev, "ctu_can_fd device registered\n");
-	napi_enable(&priv->napi);
 	netif_start_queue(ndev);
 
 	return 0;
 
 err_chip_start:
+	napi_disable(&priv->napi);
 	free_irq(ndev->irq, ndev);
 err_irq:
 	close_candev(ndev);

base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
-- 
2.50.1

[PATCH 2/3] can: xilinx_can: enable NAPI before starting the controller

[Patrick Zhang]

xcan_open() calls xcan_chip_start(), which writes XCAN_IER_OFFSET to
unmask the RX interrupts and then sets XCAN_SRR_CEN_MASK to enable the
controller, and only then calls napi_enable(). A frame that arrives in
that window raises the RX interrupt. xcan_interrupt() clears the RX
bits in XCAN_IER_OFFSET and calls napi_schedule(). NAPI is still
disabled, so napi_schedule_prep() finds NAPI_STATE_SCHED set and the
schedule is dropped. Only xcan_rx_poll() unmasks RX again, and it never
runs. The interface receives nothing until the next close. Transmit
still works and no rx_errors subcounter moves, so the stall is silent.
Every re-open (ip link down/up, a driver rebind) runs the same race
again.

The window is only the tail of xcan_chip_start(), so the failure is
rare. Adding an mdelay(50) at the end of xcan_chip_start() makes it
reproducible with continuous traffic from another node and a single
"ip link set down/up" on the test device.

Enable NAPI before the controller starts, so that an early RX interrupt
can schedule the poll. NAPI can be enabled while the controller is in
reset, because nothing schedules it until an interrupt arrives. If
xcan_chip_start() fails, disable NAPI again. Otherwise the next open
would call napi_enable() on an already enabled instance and trip the
BUG_ON() in napi_enable_locked().

This is the same race as in ctucanfd, fixed by the previous patch.

Tested on a Versal board.

Fixes: b1201e44f50b ("can: xilinx CAN controller support")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Patrick Zhang <patrzhan@amazon.com>
---
 drivers/net/can/xilinx_can.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/xilinx_can.c b/drivers/net/can/xilinx_can.c
index 43d7f2282..5c2412bf9 100644
--- a/drivers/net/can/xilinx_can.c
+++ b/drivers/net/can/xilinx_can.c
@@ -1570,18 +1570,22 @@ static int xcan_open(struct net_device *ndev)
 	if (ret)
 		goto err_irq;
 
+	/* Keep before xcan_chip_start(): the ISR masks RX interrupts and only
+	 * the poll unmasks them.
+	 */
+	napi_enable(&priv->napi);
 	ret = xcan_chip_start(ndev);
 	if (ret < 0) {
 		netdev_err(ndev, "xcan_chip_start failed!\n");
 		goto err_candev;
 	}
 
-	napi_enable(&priv->napi);
 	netif_start_queue(ndev);
 
 	return 0;
 
 err_candev:
+	napi_disable(&priv->napi);
 	close_candev(ndev);
 err_irq:
 	free_irq(ndev->irq, ndev);
-- 
2.50.1

[PATCH 3/3] can: ifi_canfd: enable NAPI before starting the controller

[Patrick Zhang]

ifi_canfd_open() calls ifi_canfd_start(), which unmasks the interrupts
in IFI_CANFD_IRQMASK and then enables the controller through
IFI_CANFD_STCMD, and only then calls napi_enable(). A frame that
arrives in that window raises RXFIFO_NEMPTY. ifi_canfd_isr() masks all
interrupts with ifi_canfd_irq_enable(ndev, 0) and calls
napi_schedule(). NAPI is still disabled, so napi_schedule_prep() finds
NAPI_STATE_SCHED set and the schedule is dropped. Only ifi_canfd_poll()
unmasks the interrupts again, and it never runs. From then on the
interface receives nothing. ifi_canfd_start_xmit() stops the queue
after every frame and relies on the TX FIFO interrupt to wake it, and
that interrupt is masked too, so transmit stops after the next frame.
The stall lasts until the next close. Every re-open runs the same race
again.

Enable NAPI before the controller starts, so that an early RX interrupt
can schedule the poll. NAPI can be enabled while the controller is in
reset, because nothing schedules it until an interrupt arrives.
ifi_canfd_start() cannot fail, so no error path changes.

This is the same race as in ctucanfd and xilinx_can, fixed by the
previous patches. I do not have IFI CAN FD hardware, so this change is
compile tested only and the failure sequence is taken from the code.

Fixes: 0c4d9c94a130 ("can: ifi: Add IFI CANFD IP support")
Assisted-by: LLM
Signed-off-by: Patrick Zhang <patrzhan@amazon.com>
---
 drivers/net/can/ifi_canfd/ifi_canfd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/ifi_canfd/ifi_canfd.c b/drivers/net/can/ifi_canfd/ifi_canfd.c
index 0f83335e4..feb738eb0 100644
--- a/drivers/net/can/ifi_canfd/ifi_canfd.c
+++ b/drivers/net/can/ifi_canfd/ifi_canfd.c
@@ -846,9 +846,12 @@ static int ifi_canfd_open(struct net_device *ndev)
 		goto err_irq;
 	}
 
+	/* Keep before ifi_canfd_start(): the ISR masks the interrupts and only
+	 * the poll unmasks them.
+	 */
+	napi_enable(&priv->napi);
 	ifi_canfd_start(ndev);
 
-	napi_enable(&priv->napi);
 	netif_start_queue(ndev);
 
 	return 0;
-- 
2.50.1