| Message ID | 20210609103326.278782-6-toke@redhat.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | Clean up and document RCU-based object protection for XDP_REDIRECT | expand |
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 > Cc: Guy Tzalik <gtzalik@amazon.com> > Cc: Saeed Bishara <saeedb@amazon.com> > Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> > --- > 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 >
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; }
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. Cc: Guy Tzalik <gtzalik@amazon.com> Cc: Saeed Bishara <saeedb@amazon.com> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> --- drivers/net/ethernet/amazon/ena/ena_netdev.c | 3 --- 1 file changed, 3 deletions(-)