netdev - Re: [PATCH bpf-next v3 15/16] stmmac: remove rcu_read

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Message-ID: <20210618094735.GA20711@linux.intel.com>
Date:   Fri, 18 Jun 2021 17:47:35 +0800
From:   Wong Vee Khee <vee.khee.wong@...ux.intel.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        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>,
        Jakub Kicinski <kuba@...nel.org>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Song Yoong Siang <Yoong.Siang.Song@...el.com>
Subject: Re: [PATCH bpf-next v3 15/16] stmmac: remove rcu_read_lock() around
 XDP program invocation

On Thu, Jun 17, 2021 at 11:27:47PM +0200, Toke Høiland-Jørgensen wrote:
> 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.
> 
> Cc: Giuseppe Cavallaro <peppe.cavallaro@...com>
> Cc: Alexandre Torgue <alexandre.torgue@...s.st.com>
> Cc: Jose Abreu <joabreu@...opsys.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>

Acked-by: Wong Vee Khee <vee.khee.wong@...ux.intel.com>
Tested-by: Song, Yoong Siang <yoong.siang.song@...el.com>

> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)