Re: [PATCH net 1/3] net: mvneta: Fix spinlock usage

Jisheng Zhang
2016-03-09T06:47:37Z

Dear Gregory,

On Tue, 8 Mar 2016 13:57:04 +0100 Gregory CLEMENT wrote:

> In the previous patch, the spinlock was not initialized. While it didn't
> cause any trouble yet it could be a problem to use it uninitialized.
>
> The most annoying part was the critical section protected by the spinlock
> in mvneta_stop(). Some of the functions could sleep as pointed when
> activated CONFIG_DEBUG_ATOMIC_SLEEP. Actually, in mvneta_stop() we only
> need to protect the is_stopped flagged, indeed the code of the notifier
> for CPU online is protected by the same spinlock, so when we get the
> lock, the notifer work is done.
>
> Reported-by: Patrick Uiterwijk <patrick@puiterwijk.org>
> Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
> ---
> drivers/net/ethernet/marvell/mvneta.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index b0ae69f84493..8dc7df2edff6 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -3070,17 +3070,17 @@ static int mvneta_stop(struct net_device *dev)
> struct mvneta_port *pp = netdev_priv(dev);
>
> /* Inform that we are stopping so we don't want to setup the
> - * driver for new CPUs in the notifiers
> + * driver for new CPUs in the notifiers. The code of the
> + * notifier for CPU online is protected by the same spinlock,
> + * so when we get the lock, the notifer work is done.
> */
> spin_lock(&pp->lock);
> pp->is_stopped = true;
> + spin_unlock(&pp->lock);

This fix sleep in atomic issue. But
I see race here. Let's assume is_stopped is false.

cpu0: cpu1:
mvneta_percpu_notifier(): mvneta_stop():

if (pp->is_stopped) {
spin_unlock(&pp->lock);
break;
}

pp->is_stopped = true;
spin_unlock(&pp->lock);

netif_tx_stop_all_queues(pp->dev);
for_each_online_cpu(other_cpu) {
....

Thanks,
Jisheng

> +
> mvneta_stop_dev(pp);
> mvneta_mdio_remove(pp);
> unregister_cpu_notifier(&pp->cpu_notifier);
> - /* Now that the notifier are unregistered, we can release le
> - * lock
> - */
> - spin_unlock(&pp->lock);
> on_each_cpu(mvneta_percpu_disable, pp, true);
> free_percpu_irq(dev->irq, pp->ports);
> mvneta_cleanup_rxqs(pp);
> @@ -3612,6 +3612,7 @@ static int mvneta_probe(struct platform_device *pdev)
> dev->ethtool_ops = &mvneta_eth_tool_ops;
>
> pp = netdev_priv(dev);
> + spin_lock_init(&pp->lock);
> pp->phy_node = phy_node;
> pp->phy_interface = phy_mode;
>