2008-02-15 15:43:39

by Marin Mitov

Subject: Is netif_tx_lock() SMP PREEMPT safe?

Hi all,

As in: include/linux/netdevice.h (kernel-2.6.24.2) one finds:

static inline void __netif_tx_lock(struct net_device *dev, int cpu)
{
spin_lock(&dev->_xmit_lock);
dev->xmit_lock_owner = cpu;
}

static inline void netif_tx_lock(struct net_device *dev)
{
__netif_tx_lock(dev, smp_processor_id());
}

Does netif_tx_lock(struct net_device *dev) expands into:

cpu = smp_processor_id();
<preempt & shift to another cpu (bogus)>
spin_lock(&dev->_xmit_lock);
dev->xmit_lock_owner = cpu; /* cpu is not the lock owner */

Or to:

spin_lock(&dev->_xmit_lock);
dev->xmit_lock_owner = smp_processor_id();

which is correct?

Thanks in advance for your answer.

Regards

Marin Mitov

2008-02-15 16:01:24

by Eric Dumazet

Subject: Re: Is netif_tx_lock() SMP PREEMPT safe?

Marin Mitov a ?crit :
> Hi all,
>
> As in: include/linux/netdevice.h (kernel-2.6.24.2) one finds:
>
> static inline void __netif_tx_lock(struct net_device *dev, int cpu)
> {
> spin_lock(&dev->_xmit_lock);
> dev->xmit_lock_owner = cpu;
> }
>
> static inline void netif_tx_lock(struct net_device *dev)
> {
> __netif_tx_lock(dev, smp_processor_id());
> }
>
> Does netif_tx_lock(struct net_device *dev) expands into:
>
> cpu = smp_processor_id();
> <preempt & shift to another cpu (bogus)>
> spin_lock(&dev->_xmit_lock);
> dev->xmit_lock_owner = cpu; /* cpu is not the lock owner */
>
> Or to:
>
> spin_lock(&dev->_xmit_lock);
> dev->xmit_lock_owner = smp_processor_id();
>
> which is correct?
>
>
Hi Marin

This expands to the first version, but netif_tx_lock() is allways called
with preemption disabled.

(Or checks in smp_processor_id() would just trigger)


Eric
(Cced netdev for network related stuff)