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
