Re: [PATCH iwl-net 0/3] ice: fix synchronization between .ndo_bpf() and reset

[Larysa Zaremba <larysa.zaremba@intel.com>]

On Thu, Jun 13, 2024 at 07:13:43AM -0700, Jakub Kicinski wrote:
> On Thu, 13 Jun 2024 10:54:12 +0200 Larysa Zaremba wrote:
> > > > The locking mechanisms I use here do not look pretty, but if I am not missing 
> > > > anything, the synchronization they provide must be robust.  
> > > 
> > > Robust as in they may be correct here, but you lose lockdep and all
> > > other infra normal mutex would give you.
> > 
> > I know, but __netif_queue_set_napi() requires rtnl_lock() inside the potential 
> > critical section and creates a deadlock this way. However, after reading 
> > patches that introduce this function, I think it is called too early in the
> > configuration. Seems like it should be called somewhere right after 
> > netif_set_real_num_rx/_tx_queues(), much later in the configuration where we 
> > already hold the rtnl_lock(). In such way, ice_vsi_rebuild() could be protected 
> > with an internal mutex. WDYT?
> 
> On a quick look I think that may work. For setting the NAPI it makes
> sense - netif_set_real_num_rx/_tx_queues() and netif_queue_set_napi()
> both inform netdev about the queue config, so its logical to keep them
> together. I was worried there may be an inconveniently placed
> netif_queue_set_napi() call which is clearing the NAPI pointer.
> But I don't see one.
>

Ok, will do this in v2. Thanks for the discussion.
 
> > > > A prettier way of protecting the same critical sections would be replacing 
> > > > ICE_CFG_BUSY around ice_vsi_rebuild() with rtnl_lock(), this would eliminate 
> > > > locking code from .ndo_bpf() altogether, ice_rebuild_pending() logic will have 
> > > > to stay.
> > > > 
> > > > At some point I have decided to avoid using rtnl_lock(), if I do not have to. I 
> > > > think this is a goal worth pursuing?  
> > > 
> > > Is the reset for failure recovery, rather than reconfiguration? 
> > > If so netif_device_detach() is generally the best way of avoiding
> > > getting called (I think I mentioned it to someone @intal recently).  
> > 
> > AFAIK, netif_device_detach() does not affect .ndo_bpf() calls. We were trying 
> > such approach with idpf and it does work for ethtool, but not for XDP.
> 
> I reckon that's an unintentional omission. In theory XDP is "pure
> software" but if the device is running driver will likely have to
> touch HW to reconfigure. So, if you're willing, do send a ndo_bpf 
> patch to add a detached check.

This does not seem that simple. In cases of program/pool detachment, 
.ndo_bpf() does not accept 'no' as an answer, so there is no easy existing way 
of handling !netif_device_present() either. And we have to notify the driver 
that pool/program is no longer needed no matter what. So what is left is somehow 
postpone pool/prog removal to after the netdev gets attached again.