linux-kernel - Re: [PATCH v2 1/4] wifi: ath12k: fix dest ring-buffer corruption

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Message-ID: <ed06d62a-4b98-443a-a2ed-c92058bb521a@quicinc.com>
Date: Thu, 5 Jun 2025 18:49:37 +0800
From: Baochen Qiang <quic_bqiang@...cinc.com>
To: Johan Hovold <johan@...nel.org>, P Praneesh <quic_ppranees@...cinc.com>
CC: Johan Hovold <johan+linaro@...nel.org>,
        Jeff Johnson
	<jjohnson@...nel.org>,
        Miaoqing Pan <quic_miaoqing@...cinc.com>,
        "Remi
 Pommarel" <repk@...plefau.lt>, <linux-wireless@...r.kernel.org>,
        <ath12k@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] wifi: ath12k: fix dest ring-buffer corruption



On 6/5/2025 6:00 PM, Johan Hovold wrote:
> On Thu, Jun 05, 2025 at 04:41:32PM +0800, Baochen Qiang wrote:
>> On 6/4/2025 10:45 PM, Johan Hovold wrote:
>>> Add the missing memory barrier to make sure that destination ring
>>> descriptors are read after the head pointers to avoid using stale data
>>> on weakly ordered architectures like aarch64.
>>>
>>> The barrier is added to the ath12k_hal_srng_access_begin() helper for
>>> symmetry with follow-on fixes for source ring buffer corruption which
>>> will add barriers to ath12k_hal_srng_access_end().
>>>
>>> Note that this may fix the empty descriptor issue recently worked around
>>> by commit 51ad34a47e9f ("wifi: ath12k: Add drop descriptor handling for
>>> monitor ring").
>>
>> why? I would expect drunk cookies are valid in case of HAL_MON_DEST_INFO0_EMPTY_DESC,
>> rather than anything caused by reordering.
> 
> Based on a quick look it seemed like this could possibly fall in the
> same category as some of the other workarounds I've spotted while
> looking into these ordering issues (e.g. f9fff67d2d7c ("wifi: ath11k:
> Fix SKB corruption in REO destination ring")).
> 
> If you say this one is clearly unrelated, I'll drop the comment.

Praneesh, could you comment here since you made that change?

> 
>>> @@ -343,9 +343,6 @@ static int ath12k_ce_completed_recv_next(struct ath12k_ce_pipe *pipe,
>>>  		goto err;
>>>  	}
>>>  
>>> -	/* Make sure descriptor is read after the head pointer. */
>>> -	dma_rmb();
>>> -
>>>  	*nbytes = ath12k_hal_ce_dst_status_get_length(desc);
>>>  
>>>  	*skb = pipe->dest_ring->skb[sw_index];
>>> diff --git a/drivers/net/wireless/ath/ath12k/hal.c b/drivers/net/wireless/ath/ath12k/hal.c
>>> index 91d5126ca149..9eea13ed5565 100644
>>> --- a/drivers/net/wireless/ath/ath12k/hal.c
>>> +++ b/drivers/net/wireless/ath/ath12k/hal.c
>>> @@ -2126,13 +2126,24 @@ void *ath12k_hal_srng_src_get_next_reaped(struct ath12k_base *ab,
>>>  
>>>  void ath12k_hal_srng_access_begin(struct ath12k_base *ab, struct hal_srng *srng)
>>>  {
>>> +	u32 hp;
>>> +
>>>  	lockdep_assert_held(&srng->lock);
>>>  
>>> -	if (srng->ring_dir == HAL_SRNG_DIR_SRC)
>>> +	if (srng->ring_dir == HAL_SRNG_DIR_SRC) {
>>>  		srng->u.src_ring.cached_tp =
>>>  			*(volatile u32 *)srng->u.src_ring.tp_addr;
>>> -	else
>>> -		srng->u.dst_ring.cached_hp = READ_ONCE(*srng->u.dst_ring.hp_addr);
>>> +	} else {
>>> +		hp = READ_ONCE(*srng->u.dst_ring.hp_addr);
>>> +
>>> +		if (hp != srng->u.dst_ring.cached_hp) {
>>
>> This consumes additional CPU cycles in hot path, which is a concern to me.
>>
>> Based on that, I prefer the v1 implementation.
> 
> The conditional avoids a memory barrier in case the ring is empty, so
> for all callers but ath12k_ce_completed_recv_next() it's an improvement
> over v1 in that sense.
> 
> I could make the barrier unconditional, which will only add one barrier
> to ath12k_ce_completed_recv_next() in case the ring is empty compared to
> v1. Perhaps that's a good compromise if you worry about the extra
> comparison?

I guess the unconditional barrier also has impact on performance? If so I am not sure
which one is better then ...

Let's just keep it as is and see what others think.

> 
> I very much want to avoid having both explicit barriers in the caller
> and barriers in the hal end() helper. I think it should be either or.
>  
>>> +			srng->u.dst_ring.cached_hp = hp;
>>> +			/* Make sure descriptor is read after the head
>>> +			 * pointer.
>>> +			 */
>>> +			dma_rmb();
>>> +		}
>>> +	}
> 
> Johan