netdev - [PATCH net v2 1/4] bnxt: don't lock the tx queue from napi poll

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Message-Id: <20210811213749.3276687-2-kuba@kernel.org>
Date:   Wed, 11 Aug 2021 14:37:46 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     davem@...emloft.net
Cc:     michael.chan@...adcom.com, huangjw@...adcom.com,
        eddie.wai@...adcom.com, prashant@...adcom.com, gospo@...adcom.com,
        netdev@...r.kernel.org, edwin.peer@...adcom.com,
        Jakub Kicinski <kuba@...nel.org>
Subject: [PATCH net v2 1/4] bnxt: don't lock the tx queue from napi poll

We can't take the tx lock from the napi poll routine, because
netpoll can poll napi at any moment, including with the tx lock
already held.

It seems that the tx lock is only protecting against the disable
path, appropriate barriers are already in place to make sure
cleanup can safely run concurrently with start_xmit. I don't see
any other reason why 'stopped && avail > thresh' needs to be
re-checked under the lock.

Remove the tx lock and use synchronize_net() to make sure
closing the device does not race we restarting the queues.
Annotate accesses to dev_state against data races.

Fixes: c0c050c58d84 ("bnxt_en: New Broadcom ethernet driver.")
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
--
v2: keep the unlikely in bnxt_tx_int() [Edwin]
---
 drivers/net/ethernet/broadcom/bnxt/bnxt.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt.c b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
index 865fcb8cf29f..365f8ae91acb 100644
--- a/drivers/net/ethernet/broadcom/bnxt/bnxt.c
+++ b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
@@ -731,14 +731,9 @@ static void bnxt_tx_int(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
 	smp_mb();
 
 	if (unlikely(netif_tx_queue_stopped(txq)) &&
-	    (bnxt_tx_avail(bp, txr) > bp->tx_wake_thresh)) {
-		__netif_tx_lock(txq, smp_processor_id());
-		if (netif_tx_queue_stopped(txq) &&
-		    bnxt_tx_avail(bp, txr) > bp->tx_wake_thresh &&
-		    txr->dev_state != BNXT_DEV_STATE_CLOSING)
-			netif_tx_wake_queue(txq);
-		__netif_tx_unlock(txq);
-	}
+	    bnxt_tx_avail(bp, txr) > bp->tx_wake_thresh &&
+	    READ_ONCE(txr->dev_state) != BNXT_DEV_STATE_CLOSING)
+		netif_tx_wake_queue(txq);
 }
 
 static struct page *__bnxt_alloc_rx_page(struct bnxt *bp, dma_addr_t *mapping,
@@ -9264,9 +9259,11 @@ void bnxt_tx_disable(struct bnxt *bp)
 	if (bp->tx_ring) {
 		for (i = 0; i < bp->tx_nr_rings; i++) {
 			txr = &bp->tx_ring[i];
-			txr->dev_state = BNXT_DEV_STATE_CLOSING;
+			WRITE_ONCE(txr->dev_state, BNXT_DEV_STATE_CLOSING);
 		}
 	}
+	/* Make sure napi polls see @dev_state change */
+	synchronize_net();
 	/* Drop carrier first to prevent TX timeout */
 	netif_carrier_off(bp->dev);
 	/* Stop all TX queues */
@@ -9280,8 +9277,10 @@ void bnxt_tx_enable(struct bnxt *bp)
 
 	for (i = 0; i < bp->tx_nr_rings; i++) {
 		txr = &bp->tx_ring[i];
-		txr->dev_state = 0;
+		WRITE_ONCE(txr->dev_state, 0);
 	}
+	/* Make sure napi polls see @dev_state change */
+	synchronize_net();
 	netif_tx_wake_all_queues(bp->dev);
 	if (bp->link_info.link_up)
 		netif_carrier_on(bp->dev);
-- 
2.31.1