Re: [PATCH net-next] net: don't relock netdev when on qdisc_create replay

[Stanislav Fomichev <stfomichev@gmail.com>]

On 03/18, Simon Horman wrote:
> On Thu, Mar 13, 2025 at 03:04:07AM -0700, Stanislav Fomichev wrote:
> > Eric reports that by the time we call netdev_lock_ops after
> > rtnl_unlock/rtnl_lock, the dev might point to an invalid device.
> > Don't relock the device after request_module and don't try
> > to unlock it in the caller (tc_modify_qdisc) in case of replay.
> > 
> > Fixes: a0527ee2df3f ("net: hold netdev instance lock during qdisc ndo_setup_tc")
> > Reported-by: Eric Dumazet <edumazet@google.com>
> > Link: https://lore.kernel.org/netdev/20250305163732.2766420-1-sdf@fomichev.me/T/#me8dfd778ea4c4463acab55644e3f9836bc608771
> > Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
> > ---
> >  net/sched/sch_api.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
> > index abace7665cfe..f1ec6ec0cf05 100644
> > --- a/net/sched/sch_api.c
> > +++ b/net/sched/sch_api.c
> > @@ -1278,13 +1278,14 @@ static struct Qdisc *qdisc_create(struct net_device *dev,
> >  			 * tell the caller to replay the request.  We
> >  			 * indicate this using -EAGAIN.
> >  			 * We replay the request because the device may
> > -			 * go away in the mean time.
> > +			 * go away in the mean time. Note that we also
> > +			 * don't relock the device because it might
> > +			 * be gone at this point.
> >  			 */
> >  			netdev_unlock_ops(dev);
> >  			rtnl_unlock();
> >  			request_module(NET_SCH_ALIAS_PREFIX "%s", name);
> >  			rtnl_lock();
> > -			netdev_lock_ops(dev);
> >  			ops = qdisc_lookup_ops(kind);
> >  			if (ops != NULL) {
> 
> Hi Stan,
> 
> I see that if this condition is met then the replay logic
> in the next hunk works as intended by this patch.
> 
> But what if this condition is not met?
> It seems to me that qdisc_create(), and thus __tc_modify_qdisc()
> will return with an unlocked device, but the replay logic
> won't take effect in tc_modify_qdisc().
> 
> Am I missing something?

Oh, yes, thanks for catching this. Let me think on how to handle the
-ENOENT as well..

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