linux-kernel - Re: [PATCH iwl-net 0/3] ice: fix synchronization between .ndo

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Message-ID: <ZmqztPo6UDIC6gKx@lzaremba-mobl.ger.corp.intel.com>
Date: Thu, 13 Jun 2024 10:54:12 +0200
From: Larysa Zaremba <larysa.zaremba@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: <intel-wired-lan@...ts.osuosl.org>, Jesse Brandeburg
	<jesse.brandeburg@...el.com>, Tony Nguyen <anthony.l.nguyen@...el.com>,
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, "Daniel
 Borkmann" <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>, Maciej Fijalkowski
	<maciej.fijalkowski@...el.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>,
	<magnus.karlsson@...el.com>, Michal Kubiak <michal.kubiak@...el.com>
Subject: Re: [PATCH iwl-net 0/3] ice: fix synchronization between .ndo_bpf()
 and reset

On Wed, Jun 12, 2024 at 02:09:35PM -0700, Jakub Kicinski wrote:
> On Wed, 12 Jun 2024 08:56:38 +0200 Larysa Zaremba wrote:
> > On Tue, Jun 11, 2024 at 07:38:37PM -0700, Jakub Kicinski wrote:
> > > On Mon, 10 Jun 2024 17:37:12 +0200 Larysa Zaremba wrote:  
> > > > Fix the problems that are triggered by tx_timeout and ice_xdp() calls,
> > > > including both pool and program operations.  
> > > 
> > > Is there really no way for ice to fix the locking? :(
> > > The busy loops and trylocks() are not great, and seem like duct tape.
> > 
> > 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?

> > 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.