Re: [PATCH] neighbour: Move the netdev_put() out of RCU callback

[Kuniyuki Iwashima <kuniyu@google.com>]

On Sat, 07 Nov 2026 19:34:07 +0000, Kuniyuki Iwashima wrote:
> On Sat, 07 Nov 2026 11:47:45 +0000, Zqiang wrote:
> > > 
> > > On Fri, Oct 9, 2026 at 2:19 AM Zqiang <qiang.zhang@linux.dev> wrote:
> > > 
> > > > 
> > > > For builts with CONFIG_NET_DEV_REFCNT_TRACKER=y kernels, netdev_put()
> > > >  records the caller's stack trace via ref_tracker_free() for debugging
> > > >  reference leaks. however, the netdev_put() is within pneigh_destroy()
> > > >  RCU callback, this recorded stack is uninformative, it does not reveal
> > > >  which code path initiated the deletion.
> > > > 
> > > Have you seen pneigh double-free ? What motivates this change ?
> > 
> > Firstly, Apologies for delayed reply.
> > 
> > 
> > 
> > Actually, I didn't see any double-free happening,  just wanted it to
> > record a meaningful callstack which on release path.  After all, if double-free
> > really occurs, the callstack of the RCU callback execution path will not be helpful.
> > 
> > 
> > > 
> > > > 
> > > > This commit therefore move netdev_put() from the RCU callback into the
> > > >  caller's process context, and replace call_rcu() with kfree_rcu().
> > > > 
> > > >  Fixes: d539d8fbd8fc ("neighbour: Free pneigh_entry after RCU grace period.")
> > > > 
> > > Please elaborate why your change is safe in the commit message
> > > and remove the tag.
> > 
> > Does it refer to using kfree_rcu instead of call_rcu ensures safely ?
> 
> I mean why dropping netdev refcnt waiting RCU gp is safe.

*without waiting


> 
> 
> > 
> > Thanks
> > Zqiang
> > 
> > > 
> > > > 
> > > > Signed-off-by: Zqiang <qiang.zhang@linux.dev>
> > > > 
> > > nit: full name
> > > 
> > > > 
> > > > ---
> > > >  net/core/neighbour.c | 14 ++++----------
> > > >  1 file changed, 4 insertions(+), 10 deletions(-)
> > > > 
> > > >  diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> > > >  index 6ed8eb075146..4263689fc415 100644
> > > >  --- a/net/core/neighbour.c
> > > >  +++ b/net/core/neighbour.c
> > > >  @@ -818,14 +818,6 @@ int pneigh_create(struct neigh_table *tbl,
> > > >  return err;
> > > >  }
> > > > 
> > > >  -static void pneigh_destroy(struct rcu_head *rcu)
> > > >  -{
> > > >  - struct pneigh_entry *n = container_of(rcu, struct pneigh_entry, rcu);
> > > >  -
> > > >  - netdev_put(n->dev, &n->dev_tracker);
> > > >  - kfree(n);
> > > >  -}
> > > >  -
> > > >  int pneigh_delete(struct neigh_table *tbl, const void *pkey,
> > > >  struct net_device *dev)
> > > >  {
> > > >  @@ -849,7 +841,8 @@ int pneigh_delete(struct neigh_table *tbl, const void *pkey,
> > > >  if (tbl->pdestructor)
> > > >  tbl->pdestructor(n);
> > > > 
> > > >  - call_rcu(&n->rcu, pneigh_destroy);
> > > >  + netdev_put(n->dev, &n->dev_tracker);
> > > >  + kfree_rcu(n, rcu);
> > > >  return 0;
> > > >  }
> > > >  }
> > > >  @@ -891,7 +884,8 @@ static void pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev,
> > > >  if (tbl->pdestructor)
> > > >  tbl->pdestructor(n);
> > > > 
> > > >  - call_rcu(&n->rcu, pneigh_destroy);
> > > >  + netdev_put(n->dev, &n->dev_tracker);
> > > >  + kfree_rcu(n, rcu);
> > > >  }
> > > >  }
> > > > 
> > > >  --
> > > >  2.17.1
> > > >
> > >