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

[Johan Hovold <johan@kernel.org>]

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.

> > @@ -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 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