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    Subject[PATCH 4.15 109/128] net: sched: drop qdisc_reset from dev_graft_qdisc
    Date
    4.15-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: John Fastabend <john.fastabend@gmail.com>


    [ Upstream commit 7bbde83b1860c28a1cc35516352c4e7e5172c29a ]

    In qdisc_graft_qdisc a "new" qdisc is attached and the 'qdisc_destroy'
    operation is called on the old qdisc. The destroy operation will wait
    a rcu grace period and call qdisc_rcu_free(). At which point
    gso_cpu_skb is free'd along with all stats so no need to zero stats
    and gso_cpu_skb from the graft operation itself.

    Further after dropping the qdisc locks we can not continue to call
    qdisc_reset before waiting an rcu grace period so that the qdisc is
    detached from all cpus. By removing the qdisc_reset() here we get
    the correct property of waiting an rcu grace period and letting the
    qdisc_destroy operation clean up the qdisc correctly.

    Note, a refcnt greater than 1 would cause the destroy operation to
    be aborted however if this ever happened the reference to the qdisc
    would be lost and we would have a memory leak.

    Signed-off-by: John Fastabend <john.fastabend@gmail.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/sched/sch_generic.c | 28 +++++++++++++++++++---------
    1 file changed, 19 insertions(+), 9 deletions(-)

    --- a/net/sched/sch_generic.c
    +++ b/net/sched/sch_generic.c
    @@ -762,10 +762,6 @@ struct Qdisc *dev_graft_qdisc(struct net
    root_lock = qdisc_lock(oqdisc);
    spin_lock_bh(root_lock);

    - /* Prune old scheduler */
    - if (oqdisc && refcount_read(&oqdisc->refcnt) <= 1)
    - qdisc_reset(oqdisc);
    -
    /* ... and graft new one */
    if (qdisc == NULL)
    qdisc = &noop_qdisc;
    @@ -916,6 +912,16 @@ static bool some_qdisc_is_busy(struct ne
    return false;
    }

    +static void dev_qdisc_reset(struct net_device *dev,
    + struct netdev_queue *dev_queue,
    + void *none)
    +{
    + struct Qdisc *qdisc = dev_queue->qdisc_sleeping;
    +
    + if (qdisc)
    + qdisc_reset(qdisc);
    +}
    +
    /**
    * dev_deactivate_many - deactivate transmissions on several devices
    * @head: list of devices to deactivate
    @@ -926,7 +932,6 @@ static bool some_qdisc_is_busy(struct ne
    void dev_deactivate_many(struct list_head *head)
    {
    struct net_device *dev;
    - bool sync_needed = false;

    list_for_each_entry(dev, head, close_list) {
    netdev_for_each_tx_queue(dev, dev_deactivate_queue,
    @@ -936,20 +941,25 @@ void dev_deactivate_many(struct list_hea
    &noop_qdisc);

    dev_watchdog_down(dev);
    - sync_needed |= !dev->dismantle;
    }

    /* Wait for outstanding qdisc-less dev_queue_xmit calls.
    * This is avoided if all devices are in dismantle phase :
    * Caller will call synchronize_net() for us
    */
    - if (sync_needed)
    - synchronize_net();
    + synchronize_net();

    /* Wait for outstanding qdisc_run calls. */
    - list_for_each_entry(dev, head, close_list)
    + list_for_each_entry(dev, head, close_list) {
    while (some_qdisc_is_busy(dev))
    yield();
    + /* The new qdisc is assigned at this point so we can safely
    + * unwind stale skb lists and qdisc statistics
    + */
    + netdev_for_each_tx_queue(dev, dev_qdisc_reset, NULL);
    + if (dev_ingress_queue(dev))
    + dev_qdisc_reset(dev, dev_ingress_queue(dev), NULL);
    + }
    }

    void dev_deactivate(struct net_device *dev)

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