[PATCH net 3/3] igb: add XDP transition guards in igb_xdp_setup()

[Alex Dvoretsky <advoretsky@gmail.com>]

igb_xdp_setup() calls igb_close() + igb_open() when transitioning
between XDP and non-XDP mode on a running device. This has two issues:

1. ndo_xsk_wakeup() runs under rcu_read_lock() and may still access
   the rings while igb_xdp_setup() removes the XDP program. Without
   waiting for an RCU grace period, igb_close() can tear down the
   rings while ndo_xsk_wakeup() is still executing. Add
   synchronize_rcu() before igb_close() when removing an XDP program
   to ensure all in-flight RCU readers complete first.

2. The igb_close()/igb_open() window leaves trans_start stale from
   before the close: the TX watchdog can fire a spurious timeout and
   queue a reset_task that races with igb_open(). Add
   netif_trans_update() after igb_open() to refresh the timestamp, and
   cancel_work() to cancel any reset_task that may have been queued
   while the device was down.

Note: cancel_work_sync() cannot be used here because igb_reset_task()
takes rtnl_lock, which is already held by the ndo_bpf caller. Plain
cancel_work() is sufficient: if reset_task is already running, it blocks
on rtnl_lock and will check __IGB_DOWN when it acquires it.

Fixes: 9cbc948b5a20 ("igb: add XDP support")
Cc: stable@vger.kernel.org
Signed-off-by: Alex Dvoretsky <advoretsky@gmail.com>
---
 drivers/net/ethernet/intel/igb/igb_main.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index ddb7ce9e97bf..9ba944bf67b4 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -2913,6 +2913,9 @@ static int igb_xdp_setup(struct net_device *dev, struct netdev_bpf *bpf)
 
 	/* device is up and bpf is added/removed, must setup the RX queues */
 	if (need_reset && running) {
+		if (!prog)
+			/* Wait for RCU readers (e.g. ndo_xsk_wakeup). */
+			synchronize_rcu();
 		igb_close(dev);
 	} else {
 		for (i = 0; i < adapter->num_rx_queues; i++)
@@ -2936,6 +2939,14 @@ static int igb_xdp_setup(struct net_device *dev, struct netdev_bpf *bpf)
 	if (running)
 		igb_open(dev);
 
+	/* Refresh watchdog timestamp after reopen and cancel any
+	 * reset task queued while the device was down.
+	 */
+	if (need_reset && running) {
+		netif_trans_update(dev);
+		cancel_work(&adapter->reset_task);
+	}
+
 	return 0;
 }
 
-- 
2.51.0