On Thu, 21 Jan 2021 00:17:08 -0600 Lijun Pan wrote:
> Move the dma_rmb() between pending_scrq() and ibmvnic_next_scrq()
> into the end of pending_scrq(), and explain why.
> Explain in detail why the dma_rmb() is placed at the end of
> ibmvnic_next_scrq().
>
> Fixes: b71ec9522346 ("ibmvnic: Ensure that SCRQ entry reads are correctly
> ordered")
> Signed-off-by: Lijun Pan <l...@linux.ibm.com>
I fail to see why this is a fix. You move the barrier from caller to
callee but there are no new barriers here. Did I miss some?
> diff --git a/drivers/net/ethernet/ibm/ibmvnic.c
> b/drivers/net/ethernet/ibm/ibmvnic.c
> index 9778c83150f1..8e043683610f 100644
> --- a/drivers/net/ethernet/ibm/ibmvnic.c
> +++ b/drivers/net/ethernet/ibm/ibmvnic.c
> @@ -2511,12 +2511,6 @@ static int ibmvnic_poll(struct napi_struct *napi, int
> budget)
>
> if (!pending_scrq(adapter, rx_scrq))
> break;
> - /* The queue entry at the current index is peeked at above
> - * to determine that there is a valid descriptor awaiting
> - * processing. We want to be sure that the current slot
> - * holds a valid descriptor before reading its contents.
> - */
> - dma_rmb();
> next = ibmvnic_next_scrq(adapter, rx_scrq);
> rx_buff =
> (struct ibmvnic_rx_buff *)be64_to_cpu(next->
> @@ -3256,13 +3250,6 @@ static int ibmvnic_complete_tx(struct ibmvnic_adapter
> *adapter,
> int total_bytes = 0;
> int num_packets = 0;
>
> - /* The queue entry at the current index is peeked at above
> - * to determine that there is a valid descriptor awaiting
> - * processing. We want to be sure that the current slot
> - * holds a valid descriptor before reading its contents.
> - */
> - dma_rmb();
> -
> next = ibmvnic_next_scrq(adapter, scrq);
> for (i = 0; i < next->tx_comp.num_comps; i++) {
> if (next->tx_comp.rcs[i])
> @@ -3636,11 +3623,25 @@ static int pending_scrq(struct ibmvnic_adapter
> *adapter,
> struct ibmvnic_sub_crq_queue *scrq)
> {
> union sub_crq *entry = &scrq->msgs[scrq->cur];
> + int rc;
>
> if (entry->generic.first & IBMVNIC_CRQ_CMD_RSP)
> - return 1;
> + rc = 1;
> else
> - return 0;
> + rc = 0;
> +
> + /* Ensure that the entire SCRQ descriptor scrq->msgs
> + * has been loaded before reading its contents.
This comment is confusing. IMHO the comments you're removing are much
clearer.
> + * This barrier makes sure this function's entry, esp.
> + * entry->generic.first & IBMVNIC_CRQ_CMD_RSP
> + * 1. is loaded before ibmvnic_next_scrq()'s
> + * entry->generic.first & IBMVNIC_CRQ_CMD_RSP;
> + * 2. OR is loaded before ibmvnic_poll()'s
> + * disable_scrq_irq()'s scrq->hw_irq.
> + */
> + dma_rmb();
> +
> + return rc;
> }
>
> static union sub_crq *ibmvnic_next_scrq(struct ibmvnic_adapter *adapter,
