Re: [PATCH net-next v2 2/3] net: dsa: add Arrow SpeedChips XRS700x driver

On Fri, Nov 27, 2020 at 8:15 PM Jakub Kicinski [off-list ref] wrote:

On Sat, 28 Nov 2020 03:41:06 +0200 Vladimir Oltean wrote:

quoted

Jakub, I would like to hear more from you. I would still like to try
this patch out. You clearly have a lot more background with the code.

Well, I've seen people run into the problem of this NDO not being able
to sleep, but I don't have much background or knowledge of what impact
the locking will have on real systems.

We will need to bring this up with Eric (probably best after the turkey
weekend is over).

In the meantime if you feel like it you may want to add some tracing /
printing to check which processes are accessing /proc/net/dev on your
platforms of interest, see if there is anything surprising.

quoted

You said in an earlier reply that you should have also documented that
ndo_get_stats64 is one of the few NDOs that does not take the RTNL. Is
there a particular reason for that being so, and a reason why it can't
change?

I just meant that as a way of documenting the status quo. I'm not aware
of any other place reading stats under RCU (which doesn't mean it
doesn't exist :)).

That said it is a little tempting to add a new per-netdev mutex here,
instead of congesting RTNL lock further, since today no correct driver
should depend on the RTNL lock.

Another possible option could be replacing for_each_netdev_rcu with
for_each_netdev_srcu and using list_for_each_entry_srcu (though it's
currently used nowhere else in the kernel). Has anyone considered
using sleepable RCUs or thought of a reason they wouldn't work or
wouldn't be desirable? For more info search for SRCU in
Documentation/RCU/RTFP.txt