Re: 3.0.4: Soft lockup in netfront in SMP build

[Jeremy Fitzhardinge <jeremy@goop.org>]

Graham, Simon wrote:
> Just run into a (real) soft lockup running 3.0.4 - stack is at the end of this, but basically:
>
> . network_open acquires the rx spin lock with spin_lock() and then checks for
>   work on the queue and calls (I think) netif_rx_schedule with the lock still held which
>   can call into the hypervisor.
> . An interrupt is delivered to the bottom half of netfront which ends up calling netif_poll
>   which blocks attempting to acquire the rx spin lock.
>
> Oops! 
>
> I see from the unstable tree that this code was recently modified to use spin_lock_bh() instead of spin_lock() as part of a mega-merge of IA64 code - clearly we cant merge this changeset into 3.0.4.
>
> I haven't looked too closely at all of the code yet, but I'm wondering if a judicious change of spin_lock to spin_lock_bh in netfront would be the best approach?
>   

I found a few locking problems when I ran netfront with lockdep
enabled.  Fixes were committed to xen-unstable in 14844:abea8d171503 and
14851:22460cfaca71.  I was wondering if there had been any real cases of
these deadlocking.

    J