using rcu_read_lock() after calling dst_neigh_lookup

using rcu_read_lock() after calling dst_neigh_lookup

[Hadar Hen Zion]

Hi Dave,

Drivers which are calling dst_neigh_lookup() are also using 
rcu_read_lock() before accessing the neigh pointer (and asking it's ll 
address data and its validity state).

You can find the same behavior in:

drivers/infiniband/core/addr.c, drivers/infiniband/hw/i40iw/i40iw_cm.c, 
drivers/infiniband/hw/nes/nes_cm.c, etc.

(the above locations are just an example).

While the documentation in neighbour.c says:

  "Neighbour entries are protected:
    - with reference count.
    - with rwlock neigh->lock
    Reference count prevents destruction.
    neigh->lock mainly serializes ll address data and its validity state."

So what is the right way to protect the neigh entry parameters? I 
couldn't find why rcu_read_lock() is helping here (dst_neigh_lookup 
already takes a reference on the neigh).

Thank you,

Hadar

Re: using rcu_read_lock() after calling dst_neigh_lookup

[David Miller]

From: Hadar Hen Zion <hadarh@mellanox.com>
Date: Tue, 10 Jan 2017 16:22:51 +0200

> While the documentation in neighbour.c says:
> 
>  "Neighbour entries are protected:
>    - with reference count.
>    - with rwlock neigh->lock
>    Reference count prevents destruction.
>    neigh->lock mainly serializes ll address data and its validity state."
> 
> So what is the right way to protect the neigh entry parameters? I
> couldn't find why rcu_read_lock() is helping here (dst_neigh_lookup
> already takes a reference on the neigh).

Documentation is, unfortunately, out of date.

When there is a mis-match, usually the behavior of the code in the
tree trumps whatever the documentation says.