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Miller" , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, Ido Schimmel , Jiri Pirko , eric.dumazet@gmail.com Subject: Re: [PATCH v2 net-next 01/14] rtnetlink: prepare nla_put_iflink() to run under RCU In-Reply-To: (Eric Dumazet's message of "Sat, 24 Feb 2024 12:08:56 +0100") Date: Mon, 26 Feb 2024 08:59:47 +0000 Message-ID: References: <20240222105021.1943116-1-edumazet@google.com> <20240222105021.1943116-2-edumazet@google.com> User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Eric Dumazet writes: >> >> And use of them here: >> >> > diff --git a/drivers/net/can/vxcan.c b/drivers/net/can/vxcan.c >> > index 98c669ad5141479b509ee924ddba3da6bca554cd..f7fabba707ea640cab8863e63bb19294e333ba2c 100644 >> > --- a/drivers/net/can/vxcan.c >> > +++ b/drivers/net/can/vxcan.c >> > @@ -119,7 +119,7 @@ static int vxcan_get_iflink(const struct net_device *dev) >> > >> > rcu_read_lock(); >> > peer = rcu_dereference(priv->peer); >> > - iflink = peer ? peer->ifindex : 0; >> > + iflink = peer ? READ_ONCE(peer->ifindex) : 0; >> > rcu_read_unlock(); >> > >> > return iflink; >> >> >> > We do not need an rcu_read_lock() only to fetch dev->ifindex, if this >> > is what concerns you. >> >> In which case, it seems that no .ndo_get_iflink implementations should >> need the rcu_read_* calls? > > rcu_read_lock() is needed in all cases a dereference is performed, > expecting RCU protection of the pointer. > > In vxcan_get_iflink(), we access priv->peer, then peer->ifindex. > > rcu_read_lock() is needed because of the second dereference, peer->ifindex. > > Without rcu_read_lock(), peer could be freed before we get a chance to > read peer->ifindex. Thanks for the detailed explanation.