[PATCH AUTOSEL 6.1 1/3] ethernet: intel: fix building with large NR_CPUS

[Sasha Levin <sashal@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

[ Upstream commit 24171a5a4a952c26568ff0d2a0bc8c4708a95e1d ]

With large values of CONFIG_NR_CPUS, three Intel ethernet drivers fail to
compile like:

In function ‘i40e_free_q_vector’,
    inlined from ‘i40e_vsi_alloc_q_vectors’ at drivers/net/ethernet/intel/i40e/i40e_main.c:12112:3:
  571 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
include/linux/rcupdate.h:1084:17: note: in expansion of macro ‘BUILD_BUG_ON’
 1084 |                 BUILD_BUG_ON(offsetof(typeof(*(ptr)), rhf) >= 4096);    \
drivers/net/ethernet/intel/i40e/i40e_main.c:5113:9: note: in expansion of macro ‘kfree_rcu’
 5113 |         kfree_rcu(q_vector, rcu);
      |         ^~~~~~~~~

The problem is that the 'rcu' member in 'q_vector' is too far from the start
of the structure. Move this member before the CPU mask instead, in all three
drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Tested-by: Sunitha Mekala <sunithax.d.mekala@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

**YES**

This commit should be backported to stable kernel trees for the
following reasons:

1. **Compilation Failure Fix**: This fixes a build-time compilation
   failure that occurs with large values of CONFIG_NR_CPUS. The error
   prevents the kernel from building successfully, which is a
   significant issue for systems configured with high CPU counts.

2. **Root Cause**: The issue stems from the `kfree_rcu()` macro in
   `include/linux/rcupdate.h:1084` which includes a
   `BUILD_BUG_ON(offsetof(typeof(*(ptr)), rhf) >= 4096)` check. This
   compile-time assertion ensures that the RCU head field is within the
   first 4096 bytes of the structure.

3. **Simple and Safe Fix**: The fix is straightforward - it moves the
   `struct rcu_head rcu` member earlier in the structure, placing it
   before the `cpumask_t affinity_mask` member. This is a safe change
   because:
   - It only reorders structure members
   - The RCU head doesn't need to be at any specific location
     functionally
   - Moving it earlier reduces the offset from the structure start
   - No functional code changes are required

4. **Affects Multiple Drivers**: The issue affects three Intel ethernet
   drivers (i40e, ixgbe, fm10k), all of which are fixed identically by
   moving the rcu member earlier in their respective q_vector
   structures.

5. **Configuration-Dependent Bug**: This bug only manifests with large
   CONFIG_NR_CPUS values (likely >= 512 or higher), where the
   `cpumask_t` type becomes large enough to push the rcu member beyond
   the 4096-byte offset limit. Systems with high core counts are
   becoming more common in production environments.

6. **No Risk of Regression**: The change is minimal and doesn't alter
   any functionality. It's purely a structural reordering that maintains
   all existing behavior while fixing the compilation issue.

7. **Meets Stable Criteria**: This fix clearly meets the stable kernel
   criteria as it:
   - Fixes a real bug (compilation failure)
   - Is minimal and contained
   - Has no risk of introducing new issues
   - Affects users with legitimate configurations

The commit is an excellent candidate for stable backporting as it fixes
a real compilation issue that prevents kernel builds on systems with
large CPU counts, and the fix is trivial with no risk of regression.

 drivers/net/ethernet/intel/fm10k/fm10k.h | 3 ++-
 drivers/net/ethernet/intel/i40e/i40e.h   | 2 +-
 drivers/net/ethernet/intel/ixgbe/ixgbe.h | 3 ++-
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/intel/fm10k/fm10k.h b/drivers/net/ethernet/intel/fm10k/fm10k.h
index 6119a41088381..65a2816142d96 100644
--- a/drivers/net/ethernet/intel/fm10k/fm10k.h
+++ b/drivers/net/ethernet/intel/fm10k/fm10k.h
@@ -189,13 +189,14 @@ struct fm10k_q_vector {
 	struct fm10k_ring_container rx, tx;
 
 	struct napi_struct napi;
+	struct rcu_head rcu;	/* to avoid race with update stats on free */
+
 	cpumask_t affinity_mask;
 	char name[IFNAMSIZ + 9];
 
 #ifdef CONFIG_DEBUG_FS
 	struct dentry *dbg_q_vector;
 #endif /* CONFIG_DEBUG_FS */
-	struct rcu_head rcu;	/* to avoid race with update stats on free */
 
 	/* for dynamic allocation of rings associated with this q_vector */
 	struct fm10k_ring ring[] ____cacheline_internodealigned_in_smp;
diff --git a/drivers/net/ethernet/intel/i40e/i40e.h b/drivers/net/ethernet/intel/i40e/i40e.h
index 22ac8c48ca340..61590e92f3abc 100644
--- a/drivers/net/ethernet/intel/i40e/i40e.h
+++ b/drivers/net/ethernet/intel/i40e/i40e.h
@@ -980,6 +980,7 @@ struct i40e_q_vector {
 	u16 reg_idx;		/* register index of the interrupt */
 
 	struct napi_struct napi;
+	struct rcu_head rcu;	/* to avoid race with update stats on free */
 
 	struct i40e_ring_container rx;
 	struct i40e_ring_container tx;
@@ -990,7 +991,6 @@ struct i40e_q_vector {
 	cpumask_t affinity_mask;
 	struct irq_affinity_notify affinity_notify;
 
-	struct rcu_head rcu;	/* to avoid race with update stats on free */
 	char name[I40E_INT_NAME_STR_LEN];
 	bool arm_wb_state;
 	bool in_busy_poll;
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe.h b/drivers/net/ethernet/intel/ixgbe/ixgbe.h
index 2bf387e52e202..f49b99b175ef4 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe.h
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe.h
@@ -498,9 +498,10 @@ struct ixgbe_q_vector {
 	struct ixgbe_ring_container rx, tx;
 
 	struct napi_struct napi;
+	struct rcu_head rcu;	/* to avoid race with update stats on free */
+
 	cpumask_t affinity_mask;
 	int numa_node;
-	struct rcu_head rcu;	/* to avoid race with update stats on free */
 	char name[IFNAMSIZ + 9];
 
 	/* for dynamic allocation of rings associated with this q_vector */
-- 
2.39.5