Re: [PATCH net] net: Fix inconsistent rtnl_lock usage on dev_get_stats().

On Fri, 2017-01-06 at 13:01 -0500, David Miller wrote:

From: Eric Dumazet <redacted>
Date: Fri, 06 Jan 2017 09:32:56 -0800

quoted

This makes no sense to me.

RTNL is absolutely not needed to get device stats.

We try to not add RTNL, especially when not required.

Sure, RTNETLINK dumps currently hold RTNL, but we had various attempts
in the past to get rid of this behavior.

If a device driver expects RTNL being locked, it is clearly a bug that
needs a fix anyway.

This is extremely problematic when the driver has to synchronize some
piece of state between the get stats method and open/close.  It is
exactly the case we are trying to solve in tg3, and lots of drivers
end up hitting the same exact issue.

If open/close can happen asynchronously to get stats, it is very hard
to make dynamically allocated data structures or DMA buffers usable
from the stats call.

Yes, I had some issues lately with mlx4. netdevices are protected by
RCU, adding proper RCU logic for the stats is doable.

Drivers in this situation will just add a mutex specifically for this
situation if we don't consistently apply RTNL locking here.

Well, there are cases where RTNL is quite contended, but supervisions
like to get /proc/net/devices or various sysfs attributes
(netstat_show() can be called very very often
for /sys/class/net/*/statistics/*) in a reasonable amount of time.


I fear that such a change will add drifts, when devices are constantly
added/removed.