On Wed, Jun 04, 2025 at 10:16:23AM +0800, Baochen Qiang wrote: > On 6/3/2025 7:51 PM, Johan Hovold wrote: > > On Tue, Jun 03, 2025 at 06:52:37PM +0800, Baochen Qiang wrote: > >> On 6/2/2025 4:03 PM, Johan Hovold wrote: > > > >>> No, the barrier is needed between reading the head pointer and accessing > >>> descriptor fields, that's what matters. > >>> > >>> You can still end up with reading stale descriptor data even when > >>> ath11k_hal_srng_dst_get_next_entry() returns non-NULL due to speculation > >>> (that's what happens on the X13s). > >> > >> The fact is that a dma_rmb() does not even prevent speculation, no matter where it is > >> placed, right? > > > > It prevents the speculated load from being used. > > Sorry, still not get it. To my knowledge whether the speculated load (steps 3 and 4) would > get used depends on whether the condition check pass in step 2. How does a dma_rmb() make > any difference in this process? It orders the two loads from the device so that the descriptor is not (speculatively) loaded before the head pointer. When the CPU sees the updated head pointer it may otherwise proceed with using stale descriptor data. The barrier prevents this. Johan
