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

Gregory CLEMENT
2016-03-09T07:49:47Z

Hi Jisheng,

On mer., mars 09 2016, Jisheng Zhang <jszhang@marvell.com> wrote:

> 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.

You forgot that the lock was hold in the mvneta_percpu_notifier so your
scenario can't happen.

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

spin_lock(&pp->lock);

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

the lock is hold in
mvneta_percpu_notifier(), so as
said in the comment this cpu is
waiting for on the following
line:
spin_lock(&pp->lock);

This code will be executed only
when the lock will be released
> pp->is_stopped = true;
> spin_unlock(&pp->lock);
>
>
> netif_tx_stop_all_queues(pp->dev);
> for_each_online_cpu(other_cpu) {
> ....
>
So what will happen is:
cpu0: cpu1:
mvneta_percpu_notifier(): mvneta_stop():

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

netif_tx_stop_all_queues(pp->dev);
for_each_online_cpu(other_cpu) {
....
spin_unlock(&pp->lock);
pp->is_stopped = true;
spin_unlock(&pp->lock);

Gregory

--
Gregory Clement, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.