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

Jisheng Zhang
2016-03-09T08:18:10Z

Dear Gregory,

On Wed, 9 Mar 2016 08:49:40 +0100 Gregory CLEMENT wrote:

> 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;
> > }

OOPS, I misread the code here as "the lock will be unlocked" ;)
Sorry for noise.

If you want, feel free to add

Reviewed-by: Jisheng Zhang <jszhang@marvell.com>

> >
>
> 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
>