[bpf-next,v2,16/16] net: ti: remove rcu_read_lock() around XDP program invocation

Commit Message

Toke Høiland-Jørgensen June 15, 2021, 2:54 p.m. UTC

The cpsw driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Grygorii Strashko <[e-mail]>
Cc: [e-mail]
Tested-by: Grygorii Strashko <[e-mail]>
Reviewed-by: Grygorii Strashko <[e-mail]>
Signed-off-by: Toke Høiland-Jørgensen <[e-mail]>
---
 drivers/net/ethernet/ti/cpsw_priv.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

Patch

diff --git a/drivers/net/ethernet/ti/cpsw_priv.c b/drivers/net/ethernet/ti/cpsw_priv.c
index 5862f0a4a975..25fa7304a542 100644
--- a/drivers/net/ethernet/ti/cpsw_priv.c
+++ b/drivers/net/ethernet/ti/cpsw_priv.c
@@ -1328,14 +1328,13 @@  int cpsw_run_xdp(struct cpsw_priv *priv, int ch, struct xdp_buff *xdp,
 	struct bpf_prog *prog;
 	u32 act;
 
-	rcu_read_lock();
-
 	prog = READ_ONCE(priv->xdp_prog);
-	if (!prog) {
-		ret = CPSW_XDP_PASS;
-		goto out;
-	}
+	if (!prog)
+		return CPSW_XDP_PASS;
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(prog, xdp);
 	/* XDP prog might have changed packet data and boundaries */
 	*len = xdp->data_end - xdp->data;
@@ -1378,10 +1377,8 @@  int cpsw_run_xdp(struct cpsw_priv *priv, int ch, struct xdp_buff *xdp,
 	ndev->stats.rx_bytes += *len;
 	ndev->stats.rx_packets++;
 out:
-	rcu_read_unlock();
 	return ret;
 drop:
-	rcu_read_unlock();
 	page_pool_recycle_direct(cpsw->page_pool[ch], page);
 	return ret;
 }