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

[Johan Hovold]

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