Re: [PATCH v2] wifi: ath12k: fix false positive RCU warnings on PREEMPT_RT

Jeff Johnson
2026-04-22T13:33:50Z

On 4/21/2026 2:27 PM, Jeff Johnson wrote:
> On 4/21/2026 10:25 AM, Yu-Hsiang Tseng wrote:
>> Two functions in ath12k assert that the caller holds an RCU read lock:
>> ath12k_mac_get_arvif() and ath12k_p2p_noa_update_vdev_iter(). Both use:
>>
>> WARN_ON(!rcu_read_lock_any_held());
>>
>> On PREEMPT_RT kernels built with CONFIG_PROVE_RCU=n, this produces a
>> false positive splat whenever these functions are invoked from paths
>> that do hold the RCU read lock (e.g. firmware stats processing or
>> mac80211 interface iteration).
>>
>> Root cause:
>>
>> - On !PROVE_RCU, rcu_read_lock_any_held() is a static inline that
>> returns !preemptible() as a proxy for "in an RCU read section".
>>
>> - On PREEMPT_RT, rcu_read_lock() does not disable preemption. A
>> task can therefore be preemptible while legitimately holding an
>> RCU read lock.
>>
>> - Callers such as ath12k_wmi_tlv_rssi_chain_parse() (via guard(rcu)())
>> and ieee80211_iterate_active_interfaces_atomic() do hold the RCU
>> read lock, so these warnings are incorrect.
>>
>> Typical splat seen on a WCN7850 station with periodic fw stats
>> processing:
>>
>> WARNING: drivers/net/wireless/ath/ath12k/mac.c:791 at
>> ath12k_mac_get_arvif+0x9e/0xd0 [ath12k]
>> Tainted: G W O 6.19.13-rt #1 PREEMPT_RT
>> Call Trace:
>> ath12k_wmi_tlv_rssi_chain_parse+0x69/0x170 [ath12k]
>> ath12k_wmi_tlv_iter+0x7f/0x120 [ath12k]
>> ath12k_wmi_tlv_fw_stats_parse+0x342/0x6b0 [ath12k]
>> ath12k_wmi_op_rx+0xe9e/0x3150 [ath12k]
>> ath12k_htc_rx_completion_handler+0x3df/0x5b0 [ath12k]
>> ath12k_ce_per_engine_service+0x325/0x3e0 [ath12k]
>> ath12k_pci_ce_workqueue+0x20/0x40 [ath12k]
>>
>> Replace the WARN_ON() with RCU_LOCKDEP_WARN(), which is gated on
>> debug_lockdep_rcu_enabled() and therefore compiles out entirely
>> when PROVE_RCU is disabled. PROVE_RCU kernels continue to get the
>> full lockdep-based check.
>>
>> Fixes: 3dd2c68f206e ("wifi: ath12k: prepare vif data structure for MLO handling")
>> Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.1.c5-00302-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.115823.3
>
> Note that Tested-on: is not a official upstream tag, it is an ath-specific
> tag. Since it is not an official tag, it should be specified separately from
> the official tags:
>
> <commit text>
> <blank line>
> Tested-on: ...
> <blank line>
> <official upstream tags>
> Signed-off-by: ...
>
> Unless there are are other review comments there is no need to send a v3 to
> address this -- I can make this change when I pick up the patch.
Amended patch in my pending branch: