Re: [PATCH v2 net-next 2/4] net: dsa: Link aggregation support

On Tue, Dec 01, 2020 at 03:37, Vladimir Oltean [off-list ref] wrote:

On Mon, Nov 30, 2020 at 03:06:08PM +0100, Tobias Waldekranz wrote:

quoted

+static void dsa_lag_release(struct kref *refcount)
+{
+	struct dsa_lag *lag = container_of(refcount, struct dsa_lag, refcount);
+
+	rcu_assign_pointer(lag->dev, NULL);
+	synchronize_rcu();
+	memset(lag, 0, sizeof(*lag));
+}

What difference does it make if lag->dev is set to NULL right away or
after a grace period? Squeezing one last packet from that bonding interface?
Pointer updates are atomic operations on all architectures that the
kernel supports, and, as long as you use WRITE_ONCE and READ_ONCE memory
barriers, there should be no reason for RCU protection that I can see.
And unlike typical uses of RCU, you do not free lag->dev, because you do
not own lag->dev. Instead, the bonding interface pointed to by lag->dev
is going to be freed (in case of a deletion using ip link) after an RCU
grace period anyway. And the receive data path is under an RCU read-side
critical section anyway. So even if you set lag->dev to NULL using
WRITE_ONCE, the existing in-flight readers from the RX data path that
had called dsa_lag_dev_by_id() will still hold a reference to a valid
bonding interface.

I completely agree with your analysis. I will remove all the RCU
primitives in v3. Thank you.