netdev - Re: [PATCH net-next] net: add two more call_rcu

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Date: Wed, 24 Apr 2024 10:01:11 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>, "David S . Miller"
 <davem@...emloft.net>,  Jakub Kicinski <kuba@...nel.org>
Cc: David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Joel Fernandes <joel@...lfernandes.org>, "Paul E .
	McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH net-next] net: add two more call_rcu_hurry()

On Tue, 2024-04-23 at 20:54 +0000, Eric Dumazet wrote:
> I had failures with pmtu.sh selftests lately,
> with netns dismantles firing ref_tracking alerts [1].
> 
> After much debugging, I found that some queued
> rcu callbacks were delayed by minutes, because
> of CONFIG_RCU_LAZY=y option.
> 
> Joel Fernandes had a similar issue in the past,
> fixed with commit 483c26ff63f4 ("net: Use call_rcu_hurry()
> for dst_release()")
> 
> In this commit, I make sure nexthop_free_rcu()
> and free_fib_info_rcu() are not delayed too much
> because they both can release device references.

Great debugging! 

I'm wondering how many other similar situations we have out there???

Have you considered instead adding a synchronize_rcu() alongside the
rcu_barrier() in netdev_wait_allrefs_any()? If I read correctly commit
483c26ff63f4, That should kick all the possibly pending lazy rcu
operation.

The patch LGTM, I'm just "thinking aloud".

Thanks,

Paolo