Re: My system stops during startup with curretn git tree of 2.6.25-rc2

Laszlo Attila Toth
2008-02-18T14:13:42Z

Jiri Kosina wrote:
> On Mon, 18 Feb 2008, Laszlo Attila Toth wrote:
>
>> Okay, but I can't figure out what's the problem with it. I don't have
>> wireless card on my linux box also I can't test it but everything else
>> works. Swap is mounted. The concurrency cannot be a problem because the
>> write operation is protected by a lock.
>
> - write_lock_bh(&dev_base_lock);
> - dev->link_mode = nla_get_u8(tb[IFLA_LINKMODE]);
> - write_unlock_bh(&dev_base_lock);
> + if (dev->link_mode != nla_get_u8(tb[IFLA_LINKMODE])) {
> + write_lock_bh(&dev_base_lock);
> + dev->link_mode = nla_get_u8(tb[IFLA_LINKMODE]);
> + write_lock_bh(&dev_base_lock);
> + modified = 1;
> + }
> }
>
> 1) you are accessing dev->link_mode and tb[] outside the dev_base_lock

yes, because tb[IFLA_LINKMODE] is not used by someone else in this case
only dev->link_mode. Although its value is unpredictable in case of a
concurrent access in the condition, it does not affect the final value
of dev->link_mode but the length of the critical section remains
minimal. The if statement may be inside the lock.

> 2) there is obvious and immediate deadlock -- you acquire the
> dev_base_lock twice, without any unlock, just look at the chunk above

Indeed:
"Feb 16 16:51:49 sandman kernel: BUG: rwlock recursion on CPU#0,"

I missed it. I copied the code from another patch which didn't contain
the two locking statements and when I copied them back it became a
copy-paste bug.

> 3) even with this deadlock fixed, Rafael states that either NM or
> wpa_supplicant (I don't recall from top of my head) still don't work

That's bad. Does my suggestion solve the problem? Again:

- if (modified)
- netdev_state_change(dev);
+ if (modified && dev->flags & IFF_UP)
+ call_netdevice_notifiers(NETDEV_CHANGE, dev)

Regards,
Attila