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

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Date: Wed, 24 Apr 2024 15:04:41 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	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 Wed, Apr 24, 2024 at 3:01 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
>  Hi Paolo
>
> On Wed, Apr 24, 2024 at 10:01 AM Paolo Abeni <pabeni@...hat.com> wrote:
> >
> > 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???
>
> I think there is another candidate for inet_free_ifa()
>
> diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
> index 7592f242336b7fdf67e79dbd75407cf03e841cfc..cd2f0af7240899795abff0087730db2bb755c36e
> 100644
> --- a/net/ipv4/devinet.c
> +++ b/net/ipv4/devinet.c
> @@ -231,7 +231,7 @@ static void inet_rcu_free_ifa(struct rcu_head *head)
>
>  static void inet_free_ifa(struct in_ifaddr *ifa)
>  {
> -       call_rcu(&ifa->rcu_head, inet_rcu_free_ifa);
> +       call_rcu_hurry(&ifa->rcu_head, inet_rcu_free_ifa);
>  }
>
>
> >
> > 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.
>
> synchronize_rcu() could return very fast, even if queued rcu items are
> still lingering.
>
> I tried the following patch, this does not help.
>
> Were you thinking of something else ?
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 8bdc59074b29c287e432c73fffbe93c63d539ad2..a727290011693081b13ac6065e2ff2810bb5739d
> 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -10559,6 +10559,7 @@ static struct net_device
> *netdev_wait_allrefs_any(struct list_head *list)
>                         rebroadcast_time = jiffies;
>                 }
>
> +               synchronize_rcu_expedited();
>                 if (!wait) {
>                         rcu_barrier();
>                         wait = WAIT_REFS_MIN_MSECS;
>
>

Note that if we are in this part of the code, we are already in some
trouble, so the call_rcu_hurry() would avoid this.

I will try ::

diff --git a/net/core/dev.c b/net/core/dev.c
index 8bdc59074b29c287e432c73fffbe93c63d539ad2..b4aa5b7070897edbe8a26fbb51d23e85a7a09dc4
100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10559,8 +10559,8 @@ static struct net_device
*netdev_wait_allrefs_any(struct list_head *list)
                        rebroadcast_time = jiffies;
                }

+               rcu_barrier();
                if (!wait) {
-                       rcu_barrier();
                        wait = WAIT_REFS_MIN_MSECS;
                } else {
                        msleep(wait);