Re: [PATCH net-next 3/3] vrf: Remove unnecessary synchronize_rcu() invocations

[Ido Schimmel <idosch@nvidia.com>]

On Wed, Mar 25, 2026 at 08:17:30AM -0600, David Ahern wrote:
> On 3/24/26 2:39 PM, Eric Dumazet wrote:
> > On Tue, Mar 24, 2026 at 8:56 AM Ido Schimmel <idosch@nvidia.com> wrote:
> >>
> >> vrf_rtable_release() and vrf_rt6_release() are called as part of the VRF
> >> device's ndo_uninit() callback. As such, an RCU grace period passes
> >> between the removal of the VRF slaves and the closing of the VRF device
> >> until these functions are invoked.
> >>
> >> Therefore, it should not be possible for any concurrent RCU readers to
> >> try and acquire a reference on the dst entry while we are potentially
> >> releasing the last reference via dst_release().
> >>
> >> Given the above, the synchronize_rcu() invocations in these functions
> >> seem unnecessary and only add unnecessary delay when deleting VRF
> >> devices. Remove them.
> >>
> >> Reviewed-by: Petr Machata <petrm@nvidia.com>
> >> Signed-off-by: Ido Schimmel <idosch@nvidia.com>
> >> ---
> >>  drivers/net/vrf.c | 2 --
> >>  1 file changed, 2 deletions(-)
> >>
> >> diff --git a/drivers/net/vrf.c b/drivers/net/vrf.c
> >> index bfc9ea91ac20..75edb8eba87f 100644
> >> --- a/drivers/net/vrf.c
> >> +++ b/drivers/net/vrf.c
> >> @@ -753,7 +753,6 @@ static void vrf_rt6_release(struct net_device *dev, struct net_vrf *vrf)
> >>         struct rt6_info *rt6 = rtnl_dereference(vrf->rt6);
> >>
> >>         RCU_INIT_POINTER(vrf->rt6, NULL);
> > 
> > We could argue that the vrf->rt6 clearing is not needed,
> > or should be done before the RCU grace period mentioned in the
> > changelog started ?
> > 
> 
> AIUI Ido's point is that the synchronize_net() in
> unregister_netdevice_many_notify() occurs before the ndo_uninit call
> making the synchronize_rcu here unnecessary. There is also a second
> synchronize_net after it.
> 
> I get your point about skipping the clearing the poiniters.

We can probably remove more code than just the synchronize_rcu() calls.
Looking into that right now.