netdev - Re: [bpf-next PATCH v2 2/2] bpf: xdp, remove no longer required rcu_read

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Date:   Sun, 12 Jan 2020 12:22:05 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>, bjorn.topel@...il.com,
        bpf@...r.kernel.org, toshiaki.makita1@...il.com
Cc:     netdev@...r.kernel.org, john.fastabend@...il.com, ast@...nel.org,
        daniel@...earbox.net
Subject: Re: [bpf-next PATCH v2 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()

John Fastabend <john.fastabend@...il.com> writes:

> Now that we depend on rcu_call() and synchronize_rcu() to also wait
> for preempt_disabled region to complete the rcu read critical section
> in __dev_map_flush() is no longer required. Except in a few special
> cases in drivers that need it for other reasons.
>
> These originally ensured the map reference was safe while a map was
> also being free'd. And additionally that bpf program updates via
> ndo_bpf did not happen while flush updates were in flight. But flush
> by new rules can only be called from preempt-disabled NAPI context.
> The synchronize_rcu from the map free path and the rcu_call from the
> delete path will ensure the reference there is safe. So lets remove
> the rcu_read_lock and rcu_read_unlock pair to avoid any confusion
> around how this is being protected.
>
> If the rcu_read_lock was required it would mean errors in the above
> logic and the original patch would also be wrong.
>
> Now that we have done above we put the rcu_read_lock in the driver
> code where it is needed in a driver dependent way. I think this
> helps readability of the code so we know where and why we are
> taking read locks. Most drivers will not need rcu_read_locks here
> and further XDP drivers already have rcu_read_locks in their code
> paths for reading xdp programs on RX side so this makes it symmetric
> where we don't have half of rcu critical sections define in driver
> and the other half in devmap.
>
> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> Signed-off-by: John Fastabend <john.fastabend@...il.com>

Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>