[PATCH] ixgbe: Panic during XDP_TX with > 64 CPUs

[John Hickey <jjh@daedalian.us>]

In commit 'ixgbe: let the xdpdrv work with more than 64 cpus'
(4fe815850bdc8d4cc94e06fe1de069424a895826), support was added to allow
XDP programs to run on systems with more than 64 CPUs by locking the
XDP TX rings and indexing them using cpu % 64 (IXGBE_MAX_XDP_QS).

Upon trying this out patch via the Intel 5.18.6 out of tree driver
on a system with more than 64 cores, the kernel paniced with an
array-index-out-of-bounds at the return in ixgbe_determine_xdp_ring in
ixgbe.h, which means ixgbe_determine_xdp_q_idx was just returning the
cpu instead of cpu % IXGBE_MAX_XDP_QS.

I think this is how it happens:

Upon loading the first XDP program on a system with more than 64 CPUs,
ixgbe_xdp_locking_key is incremented in ixgbe_xdp_setup.  However,
immediately after this, the rings are reconfigured by ixgbe_setup_tc.
ixgbe_setup_tc calls ixgbe_clear_interrupt_scheme which calls
ixgbe_free_q_vectors which calls ixgbe_free_q_vector in a loop.
ixgbe_free_q_vector decrements ixgbe_xdp_locking_key once per call if
it is non-zero.  Commenting out the decrement in ixgbe_free_q_vector
stopped my system from panicing.

I suspect to make the original patch work, I would need to load an XDP
program and then replace it in order to get ixgbe_xdp_locking_key back
above 0 since ixgbe_setup_tc is only called when transitioning between
XDP and non-XDP ring configurations, while ixgbe_xdp_locking_key is
incremented every time ixgbe_xdp_setup is called.

Also, ixgbe_setup_tc can be called via ethtool --set-channels, so this
becomes another path to decrement ixgbe_xdp_locking_key to 0 on systems
with greater than 64 CPUs.

For this patch, I have changed static_branch_inc to static_branch_enable
in ixgbe_setup_xdp.  We aren't counting references and I don't see any
reason to turn it off, since all the locking appears to be in the XDP_TX
path, which isn't run if a XDP program isn't loaded.

Signed-off-by: John Hickey <jjh@daedalian.us>
---
 drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c  | 3 ---
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 2 +-
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
index f8156fe4b1dc..0ee943db3dc9 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
@@ -1035,9 +1035,6 @@ static void ixgbe_free_q_vector(struct ixgbe_adapter *adapter, int v_idx)
 	adapter->q_vector[v_idx] = NULL;
 	__netif_napi_del(&q_vector->napi);
 
-	if (static_key_enabled(&ixgbe_xdp_locking_key))
-		static_branch_dec(&ixgbe_xdp_locking_key);
-
 	/*
 	 * after a call to __netif_napi_del() napi may still be used and
 	 * ixgbe_get_stats64() might access the rings on this vector,
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index ab8370c413f3..cd2fb72c67be 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -10283,7 +10283,7 @@ static int ixgbe_xdp_setup(struct net_device *dev, struct bpf_prog *prog)
 	if (nr_cpu_ids > IXGBE_MAX_XDP_QS * 2)
 		return -ENOMEM;
 	else if (nr_cpu_ids > IXGBE_MAX_XDP_QS)
-		static_branch_inc(&ixgbe_xdp_locking_key);
+		static_branch_enable(&ixgbe_xdp_locking_key);
 
 	old_prog = xchg(&adapter->xdp_prog, prog);
 	need_reset = (!!prog != !!old_prog);
-- 
2.37.2