Message-ID: <0e17cadc-5cab-44f2-8998-67ecde033a06@intel.com>
Date: Wed, 4 Feb 2026 07:12:19 +0100
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Jacob Keller <jacob.e.keller@...el.com>, Petr Oros <poros@...hat.com>,
	Jakub Kicinski <kuba@...nel.org>
CC: <ivecera@...hat.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Andrew Lunn <andrew+netdev@...n.ch>, "Eric
 Dumazet" <edumazet@...gle.com>, Stanislav Fomichev <sdf@...ichev.me>, "Tony
 Nguyen" <anthony.l.nguyen@...el.com>, <intel-wired-lan@...ts.osuosl.org>,
	Paolo Abeni <pabeni@...hat.com>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [Intel-wired-lan] [PATCH net] iavf: fix deadlock in reset
 handling


[...]

>> If dropping the lock is a no-go, the only viable path forward is to 
>> split the
>> reset_task so that the waiting portion is decoupled from the netdev_lock
>> critical section.
>>
> We used to do this back before the netdev shaper ops. We didn't acquire 
> either netdev lock or RTNL during reset.
> 
> I thought we had some code in the past that would handle netdev stuff 
> outside of reset.. but I don't really know and git blame is not making 
> it easy to find this information.

IIRC it was iavf_config_task that is used to complete stuff requested
under RTNL but later

> 
> Perhaps we don't actually need to hold the netdev lock over the reset 
> task.. except Przemek's refactor to remove the critical lock now makes 
> us fully dependent on the netdev lock in this case for reset :(
> 
>> The fact remains that MTU configuration and ring parameter changes are
>> currently broken in iavf. Changing the MTU on a Virtual Function is a
>> fundamental configuration not an obscure edge case that can remain 
>> non- functional.
>>
> 
> Agreed. This needs a resolution. It is just very tricky to figure out 
> what the solution should be.
> 
> We need to hold the netdev lock during reset, and we need to have our 
> handlers wait for reset to complete in order to be certain their task is 
> done... but reset is a separate thread so we can't really communicate to 
> it that we're holding the lock, and attempts to do that would be a huge 
> problem.
> 
> We don't want to go back to the critical lock and all of its horrible 
> problems either. The commit that removed it is here: 120f28a6f314 
> ("iavf: get rid of the crit lock")
> 
>> I would appreciate any further guidance on how you would prefer...
>>
> 
> I wish I had some better ideas..
> 
> Bad ideas I've thought about so far:
> 
> 1) this patch with its drop lock and wait, which we discussed as 
> problematic before. It creates a lot of issues since it means the 
> operations are no longer atomic and we could potentially get stuck with 
> some other operation in the event of another thread starting some core 
> netdev task. No good.
> 
> 2) not holding netdev_lock in reset, which is now a nogo since we 
> removed the crit_lock, and apparently we held netdev_lock prior to that 
> too anyways...
> 
> 3) we could maybe do some sort of ref counting dance where we take some 
> reference in threads that queue a reset, and the reset task would know 
> if that reference was non-zero then another driver thread is holding 
> netdev_lock so its safe to go into reset without locking... but this 
> feels extremely ugly and error prone to me...
> 
> 4) convert reset handling to a separate function that depends on the 
> netdev_lock, and call that directly from within the threads that 
> currently "wait for reset" while holding the netdev lock. Thus, we 
> basically move this entire call chain into the thread already holding 
> the lock, and call it from the context of the function like the MTU 
> change, etc. 

reset_thread() {
	while (!stopped) {
		netdev_lock();
		reset_step();
		netdev_unlock();
	}
}

looks cool, IIRC I did something similar with the "after crit lock
removal refactor continuation", but I've put it on hold
https://github.com/pkitszel/linux/commits/undeadlock/
The linked code went even further and merged all of our admin-worker 
threads into one and the whole was protected by the netdev_lock :)

This feels like its also a huge issue, and could
> potentially lead to some sort of issue where we need to still block the 
> reset thread from going if we reset at the end of the netdev_lock thread..

that should queue on the "do later stuff queue", which we don't have
right now (and keep some of such stuff in the state machine), but would
be useful for many other actions too (like virtchnl messages, for which
we have queue-of-1 right now

> 
> I don't really like any of these solutions, even if (3) and (4) aren't 
> fully ruled out as completely broken... they probably have all kinds of 
> issues...

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