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

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Message-ID: <87czzu7xkq.fsf@waldekranz.com>
Date:   Tue, 01 Dec 2020 09:13:57 +0100
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, andrew@...n.ch,
        vivien.didelot@...il.com, f.fainelli@...il.com,
        j.vosburgh@...il.com, vfalico@...il.com, andy@...yhouse.net,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 2/4] net: dsa: Link aggregation support

On Tue, Dec 01, 2020 at 03:37, Vladimir Oltean <olteanv@...il.com> wrote:
> On Mon, Nov 30, 2020 at 03:06:08PM +0100, Tobias Waldekranz wrote:
>> +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.