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From:   Vladimir Oltean <vladimir.oltean@nxp.com>
To:     "netdev@vger.kernel.org" <netdev@vger.kernel.org>
CC:     Vinicius Costa Gomes <vinicius.gomes@intel.com>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Jiri Pirko <jiri@resnulli.us>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Voon Weifeng <weifeng.voon@intel.com>,
        Vladimir Oltean <olteanv@gmail.com>,
        Kurt Kanzenbach <kurt@linutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net 3/3] net/sched: taprio: dereference oper and admin
 sched under RCU in taprio_destroy
Thread-Topic: [PATCH net 3/3] net/sched: taprio: dereference oper and admin
 sched under RCU in taprio_destroy
Thread-Index: AQHYyEcskjCPZ7fjpUGbRQIOqb0/Za3feGyA
Date:   Wed, 14 Sep 2022 21:53:58 +0000
Message-ID: <20220914215357.i7kvrnuczrltdsoj@skbuf>
References: <20220914143439.1544363-1-vladimir.oltean@nxp.com>
 <20220914143439.1544363-4-vladimir.oltean@nxp.com>
In-Reply-To: <20220914143439.1544363-4-vladimir.oltean@nxp.com>
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On Wed, Sep 14, 2022 at 05:34:39PM +0300, Vladimir Oltean wrote:
> -	if (q->oper_sched)
> -		call_rcu(&q->oper_sched->rcu, taprio_free_sched_cb);
> +	rcu_read_lock();
> +
> +	oper =3D rcu_dereference(q->oper_sched);
> +	admin =3D rcu_dereference(q->admin_sched);
> +
> +	if (oper)
> +		call_rcu(&oper->rcu, taprio_free_sched_cb);
> =20
> -	if (q->admin_sched)
> -		call_rcu(&q->admin_sched->rcu, taprio_free_sched_cb);
> +	if (admin)
> +		call_rcu(&admin->rcu, taprio_free_sched_cb);
> +
> +	rcu_read_unlock();
>  }

I decided to code up this patch at the last minute, since the sparse
warning was bugging me. But after more testing (including with lockdep,
which says absolutely nothing), I notice some RCU stalls after this
patch. It looks like call_rcu() really doesn't like to be called under
rcu_read_lock(). Please discard this patch set from patchwork, I'll
resend when things will work properly.=