Re: [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@xxxxxxxxxxx>