When we transmit a fragmented skb, we may run into a race like the
following scenario (assume txq->cur_tx is next to txq->dirty_tx):
cpu 0 cpu 1
fec_enet_txq_submit_skb
reserve a bdp for the first fragment
fec_enet_txq_submit_frag_skb
update the bdp for the other fragment
update txq->cur_tx
fec_enet_tx_queue
bdp =
fec_enet_get_nextdesc(txq->dirty_tx, fep, queue_id);
This bdp is the bdp
reserved for the first segment. Given
that this bdp
BD_ENET_TX_READY bit is not set and txq->cur_tx
is already pointed to a
bdp beyond this one. We think this is a
completed bdp and try to
reclaim it.
update the bdp for the first segment
update txq->cur_tx
So we shouldn't update the txq->cur_tx until all the update to the
bdps used for fragments are performed. Also add the corresponding
memory barrier to guarantee that the update to the bdps, dirty_tx and
cur_tx performed in the proper order.
Signed-off-by: Kevin Hao <haoke...@gmail.com>
---
I have run into a kernel hang due to this race on a custom rt kernel. Even
I can't reproduce it on the vanilla kernel, it seems that the same race
window does exist. No difference in performance before or after this patch.
drivers/net/ethernet/freescale/fec_main.c | 35 ++++++++++++++++++-------------
1 file changed, 20 insertions(+), 15 deletions(-)
diff --git a/drivers/net/ethernet/freescale/fec_main.c
b/drivers/net/ethernet/freescale/fec_main.c
index 32e3807c650e..ca792368f5df 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -364,7 +364,7 @@ fec_enet_clear_csum(struct sk_buff *skb, struct net_device
*ndev)
return 0;
}
-static int
+static struct bufdesc *
fec_enet_txq_submit_frag_skb(struct fec_enet_priv_tx_q *txq,
struct sk_buff *skb,
struct net_device *ndev)
@@ -439,10 +439,7 @@ fec_enet_txq_submit_frag_skb(struct fec_enet_priv_tx_q
*txq,
bdp->cbd_sc = status;
}
- txq->cur_tx = bdp;
-
- return 0;
-
+ return bdp;
dma_mapping_error:
bdp = txq->cur_tx;
for (i = 0; i < frag; i++) {
@@ -450,7 +447,7 @@ dma_mapping_error:
dma_unmap_single(&fep->pdev->dev, bdp->cbd_bufaddr,
bdp->cbd_datlen, DMA_TO_DEVICE);
}
- return NETDEV_TX_OK;
+ return ERR_PTR(-ENOMEM);
}
static int fec_enet_txq_submit_skb(struct fec_enet_priv_tx_q *txq,
@@ -467,7 +464,6 @@ static int fec_enet_txq_submit_skb(struct
fec_enet_priv_tx_q *txq,
unsigned int estatus = 0;
unsigned int index;
int entries_free;
- int ret;
entries_free = fec_enet_get_free_txdesc_num(fep, txq);
if (entries_free < MAX_SKB_FRAGS + 1) {
@@ -485,6 +481,7 @@ static int fec_enet_txq_submit_skb(struct
fec_enet_priv_tx_q *txq,
/* Fill in a Tx ring entry */
bdp = txq->cur_tx;
+ last_bdp = bdp;
status = bdp->cbd_sc;
status &= ~BD_ENET_TX_STATS;
@@ -513,9 +510,9 @@ static int fec_enet_txq_submit_skb(struct
fec_enet_priv_tx_q *txq,
}
if (nr_frags) {
- ret = fec_enet_txq_submit_frag_skb(txq, skb, ndev);
- if (ret)
- return ret;
+ last_bdp = fec_enet_txq_submit_frag_skb(txq, skb, ndev);
+ if (IS_ERR(last_bdp))
+ return NETDEV_TX_OK;
} else {
status |= (BD_ENET_TX_INTR | BD_ENET_TX_LAST);
if (fep->bufdesc_ex) {
@@ -544,7 +541,6 @@ static int fec_enet_txq_submit_skb(struct
fec_enet_priv_tx_q *txq,
ebdp->cbd_esc = estatus;
}
- last_bdp = txq->cur_tx;
index = fec_enet_get_bd_index(txq->tx_bd_base, last_bdp, fep);
/* Save skb pointer */
txq->tx_skbuff[index] = skb;
@@ -563,6 +559,10 @@ static int fec_enet_txq_submit_skb(struct
fec_enet_priv_tx_q *txq,
skb_tx_timestamp(skb);
+ /* Make sure the update to bdp and tx_skbuff are performed before
+ * cur_tx.
+ */
+ wmb();
txq->cur_tx = bdp;
/* Trigger transmission start */
@@ -1218,10 +1218,11 @@ fec_enet_tx_queue(struct net_device *ndev, u16 queue_id)
/* get next bdp of dirty_tx */
bdp = fec_enet_get_nextdesc(bdp, fep, queue_id);
- while (((status = bdp->cbd_sc) & BD_ENET_TX_READY) == 0) {
-
- /* current queue is empty */
- if (bdp == txq->cur_tx)
+ while (bdp != READ_ONCE(txq->cur_tx)) {
+ /* Order the load of cur_tx and cbd_sc */
+ rmb();
+ status = READ_ONCE(bdp->cbd_sc);
+ if (status & BD_ENET_TX_READY)
break;
index = fec_enet_get_bd_index(txq->tx_bd_base, bdp, fep);
@@ -1275,6 +1276,10 @@ fec_enet_tx_queue(struct net_device *ndev, u16 queue_id)
/* Free the sk buffer associated with this last transmit */
dev_kfree_skb_any(skb);
+ /* Make sure the update to bdp and tx_skbuff are performed
+ * before dirty_tx
+ */
+ wmb();
txq->dirty_tx = bdp;
/* Update pointer to next buffer descriptor to be transmitted */
--
2.1.0
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