stmmac: Remove rcu_read_lock() around XDP program invocation

The stmmac 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.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Jose Abreu <joabreu@synopsys.com>
Link: https://lore.kernel.org/bpf/20210624160609.292325-19-toke@redhat.com

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 16820873b01d54a53e69617d88c795156850eead..219535ab2c0cadc60f50b3659fcb238f4777ba68 100644 (file)
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4651,7 +4651,6 @@ static int stmmac_xdp_xmit_back(struct stmmac_priv *priv,
        return res;
 }
 
-/* This function assumes rcu_read_lock() is held by the caller. */
 static int __stmmac_xdp_run_prog(struct stmmac_priv *priv,
                                 struct bpf_prog *prog,
                                 struct xdp_buff *xdp)
@@ -4693,17 +4692,14 @@ static struct sk_buff *stmmac_xdp_run_prog(struct stmmac_priv *priv,
        struct bpf_prog *prog;
        int res;
 
-       rcu_read_lock();
-
        prog = READ_ONCE(priv->xdp_prog);
        if (!prog) {
                res = STMMAC_XDP_PASS;
-               goto unlock;
+               goto out;
        }
 
        res = __stmmac_xdp_run_prog(priv, prog, xdp);
-unlock:
-       rcu_read_unlock();
+out:
        return ERR_PTR(-res);
 }
 
@@ -4973,10 +4969,8 @@ read_again:
                buf->xdp->data_end = buf->xdp->data + buf1_len;
                xsk_buff_dma_sync_for_cpu(buf->xdp, rx_q->xsk_pool);
 
-               rcu_read_lock();
                prog = READ_ONCE(priv->xdp_prog);
                res = __stmmac_xdp_run_prog(priv, prog, buf->xdp);
-               rcu_read_unlock();
 
                switch (res) {
                case STMMAC_XDP_PASS: