Re: [PATCH bpf-next 05/17] ena: remove rcu_read_lock() around XDP program invocation
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: 2021-06-09 13:57:27
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bpf
On Wed, Jun 09, 2021 at 12:33:14PM +0200, Toke Høiland-Jørgensen wrote:
The ena 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.
It might be worth adding a comment, perhaps where the rcu_read_lock() used to be, stating what the protection is. Maybe something like this? /* * This code is invoked within a single NAPI poll cycle * and thus under local_bh_disable(), which provides the * needed RCU protection. */ Thanx, Paul
quoted hunk ↗ jump to hunk
Cc: Guy Tzalik <redacted> Cc: Saeed Bishara <redacted> Signed-off-by: Toke Høiland-Jørgensen <redacted> --- drivers/net/ethernet/amazon/ena/ena_netdev.c | 3 --- 1 file changed, 3 deletions(-)diff --git a/drivers/net/ethernet/amazon/ena/ena_netdev.c b/drivers/net/ethernet/amazon/ena/ena_netdev.c index 881f88754bf6..a4378b14af4c 100644 --- a/drivers/net/ethernet/amazon/ena/ena_netdev.c +++ b/drivers/net/ethernet/amazon/ena/ena_netdev.c@@ -385,7 +385,6 @@ static int ena_xdp_execute(struct ena_ring *rx_ring, struct xdp_buff *xdp) u64 *xdp_stat; int qid; - rcu_read_lock(); xdp_prog = READ_ONCE(rx_ring->xdp_bpf_prog); if (!xdp_prog)@@ -443,8 +442,6 @@ static int ena_xdp_execute(struct ena_ring *rx_ring, struct xdp_buff *xdp) ena_increase_stat(xdp_stat, 1, &rx_ring->syncp); out: - rcu_read_unlock(); - return verdict; }-- 2.31.1