netdev - Re: [PATCH bpf-next 11/17] mlx4: remove rcu_read

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Date:   Thu, 10 Jun 2021 10:10:54 +0300
From:   Tariq Toukan <ttoukan.linux@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        bpf@...r.kernel.org, netdev@...r.kernel.org
Cc:     Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Tariq Toukan <tariqt@...dia.com>
Subject: Re: [PATCH bpf-next 11/17] mlx4: remove rcu_read_lock() around XDP
 program invocation



On 6/9/2021 1:33 PM, Toke Høiland-Jørgensen wrote:
> The mlx4 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. Also switch the RCU
> dereferences in the driver loop itself to the _bh variants.
> 
> Cc: Tariq Toukan <tariqt@...dia.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---

Thanks for your patch.
Reviewed-by: Tariq Toukan <tariqt@...dia.com>

Regards,
Tariq