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Message-ID: <ZDaj2J/2CR03H/Od@Laptop-X1> Date: Wed, 12 Apr 2023 20:28:08 +0800 From: Hangbin Liu <liuhangbin@...il.com> To: Jay Vosburgh <jay.vosburgh@...onical.com> Cc: Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>, Jonathan Toppins <jtoppins@...hat.com>, Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>, Liang Li <liali@...hat.com>, Simon Horman <simon.horman@...igine.com>, Miroslav Lichvar <mlichvar@...hat.com> Subject: Re: [PATCHv3 net-next] bonding: add software tx timestamping support On Tue, Apr 11, 2023 at 11:33:23PM -0700, Jay Vosburgh wrote: > Jakub Kicinski <kuba@...nel.org> wrote: > > >On Mon, 10 Apr 2023 16:23:51 +0800 Hangbin Liu wrote: > >> @@ -5707,10 +5711,38 @@ static int bond_ethtool_get_ts_info(struct net_device *bond_dev, > >> ret = ops->get_ts_info(real_dev, info); > >> goto out; > >> } > >> + } else { > >> + /* Check if all slaves support software rx/tx timestamping */ > >> + rcu_read_lock(); > >> + bond_for_each_slave_rcu(bond, slave, iter) { > >> + ret = -1; > >> + ops = slave->dev->ethtool_ops; > >> + phydev = slave->dev->phydev; > >> + > >> + if (phy_has_tsinfo(phydev)) > >> + ret = phy_ts_info(phydev, &ts_info); > >> + else if (ops->get_ts_info) > >> + ret = ops->get_ts_info(slave->dev, &ts_info); > > > >Do we _really_ need to hold RCU lock over this? > >Imposing atomic context restrictions on driver callbacks should not be > >taken lightly. I'm 75% sure .ethtool_get_ts_info can only be called > >under rtnl lock off the top of my head, is that not the case? > > Ok, maybe I didn't look at that carefully enough, and now that I > do, it's really complicated. > > Going through it, I think the call path that's relevant is > taprio_change -> taprio_parse_clockid -> ethtool_ops->get_ts_info. > taprio_change is Qdisc_ops.change function, and tc_modify_qdisc should > come in with RTNL held. > > If I'm reading cscope right, the other possible caller of > Qdisc_ops.change is fifo_set_limit, and it looks like that function is > only called by functions that are themselves Qdisc_ops.change functions > (red_change -> __red_change, sfb_change, tbf_change) or Qdisc_ops.init > functions (red_init -> __red_change, sfb_init, tbf_init). > > There's also a qdisc_create_dflt -> Qdisc_ops.init call path, > but I don't know if literally all calls to qdisc_create_dflt hold RTNL. > > There's a lot of them, and I'm not sure how many of those could > ever end up calling into taprio_change (if, say, a taprio qdisc is > attached within another qdisc). > > qdisc_create also calls Qdisc_ops.init, but that one seems to > clearly expect to enter with RTNL. > > Any tc expert able to state for sure whether it's possible to > get into any of the above without RTNL? I suspect it isn't, but I'm not > 100% sure either. You dug more than me. Maybe we can add an ASSERT_RTNL() checking here first? But since we can't 100% sure we are holding the rtnl lock, I think we can keep the rcu lock for safe. I saw rlb_next_rx_slave() also did the same... > > > >> + if (!ret && (ts_info.so_timestamping & SOF_TIMESTAMPING_SOFTRXTX) == > >> + SOF_TIMESTAMPING_SOFTRXTX) { > > > >You could check in this loop if TX is supported... > > I see your point below about not wanting to create > SOFT_TIMESTAMPING_SOFTRXTX, but doesn't the logic need to test all three > of the flags _TX_SOFTWARE, _RX_SOFTWARE, and _SOFTWARE? I think Jakub means we have already add _RX_SOFTWARE and _SOFTWARE for bonding whatever slave's flag, then we just need to check slave's _TX_SOFTWARE flag. Thanks Hangbin
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