[PATCH 1/3] wifi: ath11k: fix dest ring-buffer corruption

[Miaoqing Pan]

On 6/4/2025 4:07 PM, Johan Hovold wrote:
> On Wed, Jun 04, 2025 at 03:57:57PM +0800, Miaoqing Pan wrote:
>> On 6/4/2025 3:06 PM, Johan Hovold wrote:
>>> On Wed, Jun 04, 2025 at 01:32:08PM +0800, Miaoqing Pan wrote:
>>>> On 6/4/2025 10:34 AM, Miaoqing Pan wrote:
>>>>> On 6/3/2025 7:51 PM, Johan Hovold wrote:
>>>>>> On Tue, Jun 03, 2025 at 06:52:37PM +0800, Baochen Qiang wrote:
> 
>>>>>>> The sequence is
>>>>>>>
>>>>>>>       1# reading HP
>>>>>>>           srng->u.dst_ring.cached_hp = READ_ONCE(*srng-
>>>>>>>> u.dst_ring.hp_addr);
>>>>>>>
>>>>>>>       2# validate HP
>>>>>>>           if (srng->u.dst_ring.tp == srng->u.dst_ring.cached_hp)
>>>>>>>               return NULL;
>>>>>>>
>>>>>>>       3# get desc
>>>>>>>           desc = srng->ring_base_vaddr + srng->u.dst_ring.tp;
>>>>>>>
>>>>>>>       4# accessing desc
>>>>>>>           ath11k_hal_desc_reo_parse_err(... desc, ...)
>>>>>>>
>>>>>>> Clearly each step depends on the results of previous steps. In this
>>>>>>> case the compiler/CPU
>>>>>>> is expected to be smart enough to not do any reordering, isn't it?
>>>>>>
>>>>>> Steps 3 and 4 can be done speculatively before the load in step 1 is
>>>>>> complete as long as the result is discarded if it turns out not to be
>>>>>> needed.
>>>
>>>>> If the condition in step 2 is true and step 3 speculatively loads
>>>>> descriptor from TP before step 1, could this cause issues?
>>>>
>>>> Sorry for typo, if the condition in step 2 is false and step 3
>>>> speculatively loads descriptor from TP before step 1, could this cause
>>>> issues?
>>>
>>> Almost correct; the descriptor can be loaded (from TP) before the head
>>> pointer is loaded and thus before the condition in step 2 has been
>>> evaluated. And if the condition in step 2 later turns out to be false,
>>> step 4 may use stale data from before the head pointer was updated.
>>
>> Actually, there's a missing step between step 3 and step 4: TP+1.
>>
>> TP+1:
>> 	srng->u.dst_ring.tp += srng->entry_size
> 
> Sure, but that is not relevant for the issue at hand.
> 
>> TP is managed by the CPU and points to the current first unprocessed
>> descriptor, while HP and the descriptor are asynchronously updated by
>> DMA. So are you saying that the descriptor obtained through speculative
>> loading has not yet been updated, or is in the process of being updated?
> 
> Exactly.
> 
> Johan

Thanks, make sense.

Reviewed-by: Miaoqing Pan <quic_miaoqing at quicinc.com>