Subject: Re: netdev ioctl & dev_base_lock : bad idea ?

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 10 Dec 2004 09:14:35 +1100

On Wed, 2004-12-08 at 23:13 -0800, David S. Miller wrote:
> On Thu, 09 Dec 2004 17:22:13 +1100
> Benjamin Herrenschmidt <[email-masked]> wrote:
> 
> > Right, and I missed the fact that we did indeed take the semaphore and
> > not the lock in the _set_ functions which is just fine, we can actually
> > schedule.... except in set_multicast...
> > 
> > Is there any reason we actually _need_ to get the xmit lock in this one
> > specifically ?
> 
> Since we implement NETIF_F_LLTX, the core packet transmit routines do
> no locking, the driver does it all.
> 
> So if we don't hold the tx lock in the set multicast routine, any other
> cpu can come into our hard_start_xmit function and poke at the hardware.
> 
> Upon further consideration, it seems that it may be OK to drop that tx
> lock right after we do the netif_stop_queue().  But we should regrab
> the tx lock when we do the subsequent netif_wake_queue().

Yes. In fact, I think it should be driver local locking policy, and not
enforced by net/core/*.

For example, for things like USB based networking (or other "remote"
busses like that), it's both very useful to be able to schedule in
set_multicast, and there is no need for any synchronisation with the
xmit code.

For things like sungem, I already have a driver local lock that can be
used if necessary.

Also, the lack of ability  to schedule means we can't suspend and resume
NAPI polling, which basically forces us to take a lock in the NAPI poll
side of the driver... I'm aiming at limiting the amount of locks we take
in sungem along with moving as much as I can to task level so I can do a
bit better power management without having big u/mdelay's all over.
 
Also, why would we need the xmit lock when calling netif_wake_queue() ?
I'm not sure I get that one (but I'm not too familiar with the net core
neither).

Ben.