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

[Jiri Pirko]

Sat, Jan 28, 2023 at 02:12:12AM CET, jjh@xxxxxxxxxxxx wrote:
>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@xxxxxxxxxxxx>

This is missing "Fixes" tag and "net" keyword in "[patch]" subject
section.


>---
> 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
>