Re: [PATCH 1/1] net: Add rtnl_lock for netif_device_attach/detach

[Li, ZhenHua]

The problem I am trying to fix is: when netif_device_attach/detached is 
called, it get a return value from netif_running, but at this moment, in 
another thread, the stat of this dev changes. But in 
netif_device_attach, it does not know stat changed, and this may cause bugs.


I think you are right, this patch cannot fix race with another thread 
that takes the lock. But that's what is happening now(with out this 
patch).  I do not yet find a way to fix it completely.

And another problem is: we only need a lock for this dev , not full all 
dev. So how about adding a single lock for each net device?

Regards
Zhenhua

On 04/16/2014 03:38 PM, Veaceslav Falico wrote:
> On Wed, Apr 16, 2014 at 03:08:02PM +0800, Li, Zhen-Hua wrote:
> > From: "Li, Zhen-Hua" <zhen-hual@xxxxxx>
> >
> > As netif_running is called in netif_device_attach/detach. There should be
> > rtnl_lock/unlock called, to avoid dev stat change during
> > netif_device_attach
> > and detach being called.
> > I checked NIC some drivers,  some of them have netif_device_attach/detach
> > called between rtnl_lock/unlock, while some drivers do not.
>
> It can race with any other thread that takes the lock - i.e. suppose you
> have a driver that doesn't take the lock and calls netif_device_attach(),
> while another thread (completely unrelated to the issue) holds rtnl_lock -
> this way the trylock will return false, the thread that took rtnl releases
> it - and you'll see the exact same behaviour as without your patch.
>
> I'm not sure about the issue you're trying to fix here - there might be a
> better approach which I'm not aware of, however with your approach you
> should really either remove the rtnl locking from all drivers that use this
> function (and insert a normal rtnl_lock here) or, vice-versa, add it to all
> drivers and add an ASSERT_RTNL to netif_device_detach/attach.
>
> > This patch is tring to find a generic way to fix this for all NIC
> > drivers.
> >
> > Signed-off-by: Li, Zhen-Hua <zhen-hual@xxxxxx>
> > ---
> > net/core/dev.c |   18 ++++++++++++++++++
> > 1 file changed, 18 insertions(+)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 5b3042e..795bbc5 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -2190,10 +2190,19 @@ EXPORT_SYMBOL(__dev_kfree_skb_any);
> >  */
> > void netif_device_detach(struct net_device *dev)
> > {
> > +    /**
> > +     * As netif_running is called , rtnl_lock and unlock are needed to
> > +     * avoid __LINK_STATE_START bit changes during this function call.
> > +     */
> > +    int need_unlock;
> > +
> > +    need_unlock = rtnl_trylock();
> >     if (test_and_clear_bit(__LINK_STATE_PRESENT, &dev->state) &&
> >         netif_running(dev)) {
> >         netif_tx_stop_all_queues(dev);
> >     }
> > +    if (need_unlock)
> > +        rtnl_unlock();
> > }
> > EXPORT_SYMBOL(netif_device_detach);
> >
> > @@ -2205,11 +2214,20 @@ EXPORT_SYMBOL(netif_device_detach);
> >  */
> > void netif_device_attach(struct net_device *dev)
> > {
> > +    /**
> > +     * As netif_running is called , rtnl_lock and unlock are needed to
> > +     * avoid __LINK_STATE_START bit changes during this function call.
> > +     */
> > +    int need_unlock;
> > +
> > +    need_unlock = rtnl_trylock();
> >     if (!test_and_set_bit(__LINK_STATE_PRESENT, &dev->state) &&
> >         netif_running(dev)) {
> >         netif_tx_wake_all_queues(dev);
> >         __netdev_watchdog_up(dev);
> >     }
> > +    if (need_unlock)
> > +        rtnl_unlock();
> > }
> > EXPORT_SYMBOL(netif_device_attach);
> >
> > --
> > 1.7.10.4

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