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

[Praneesh P]

On 6/5/2025 4:19 PM, Baochen Qiang wrote:
> 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?
Empty/Drop descriptor is intentionally issued by the hardware during 
backpressure scenario
and is unrelated to the issue discussed in this patch series.
> > > > @@ -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