[PATCH net v2] igb: remove napi_synchronize() in igb_down()

[Alex Dvoretsky <advoretsky@gmail.com>]

When an AF_XDP zero-copy application terminates abruptly (e.g., kill -9),
the XSK buffer pool is destroyed but NAPI polling continues.
igb_clean_rx_irq_zc() repeatedly returns the full budget, preventing
napi_complete_done() from clearing NAPI_STATE_SCHED.

igb_down() calls napi_synchronize() before napi_disable() for each queue
vector. napi_synchronize() spins waiting for NAPI_STATE_SCHED to clear,
which never happens. igb_down() blocks indefinitely, the TX watchdog
fires, and the TX queue remains permanently stalled.

napi_disable() already handles this correctly: it sets NAPI_STATE_DISABLE.
After a full-budget poll, __napi_poll() checks napi_disable_pending(). If
set, it forces completion and clears NAPI_STATE_SCHED, breaking the loop
that napi_synchronize() cannot.

napi_synchronize() was added in commit 41f149a285da ("igb: Fix possible
panic caused by Rx traffic arrival while interface is down").
napi_disable() provides stronger guarantees: it prevents further
scheduling and waits for any active poll to exit.
Other Intel drivers (ixgbe, ice, i40e) use napi_disable() without a
preceding napi_synchronize() in their down paths.

Remove redundant napi_synchronize() call.

Fixes: 2c6196013f84 ("igb: Add AF_XDP zero-copy Rx support")
Cc: stable@vger.kernel.org
Signed-off-by: Alex Dvoretsky <advoretsky@gmail.com>
---
Thanks for the suggestion, Maciej. I tested removing napi_synchronize()
and it fixes the issue cleanly — napi_disable() handles the stuck poll
via NAPI_STATE_DISABLE without needing any hot-path changes.

v2:
  - Replaced 3-patch series with single napi_synchronize() removal,
    per Maciej Fijalkowski's suggestion. napi_disable() handles the
    stuck NAPI poll via NAPI_STATE_DISABLE, making the __IGB_DOWN
    checks in igb_clean_rx_irq_zc() and igb_tx_timeout(), and the
    transition guards in igb_xdp_setup(), all unnecessary.
  - Tested on Intel I210 (igb) with AF_XDP zero-copy: full E2E
    traffic suite, graceful shutdown, and 5x kill-9 stress cycles.
    Zero tx_timeout events.

 drivers/net/ethernet/intel/igb/igb_main.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index 12e8e30d8a2d..a1b3c5e4f7d2 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -2203,7 +2203,6 @@ void igb_down(struct igb_adapter *adapter)

 	for (i = 0; i < adapter->num_q_vectors; i++) {
 		if (adapter->q_vector[i]) {
-			napi_synchronize(&adapter->q_vector[i]->napi);
 			igb_set_queue_napi(adapter, i, NULL);
 			napi_disable(&adapter->q_vector[i]->napi);
 		}
--
2.51.0