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

On Fri, Jan 6, 2017 at 10:01 AM, David Miller [off-list ref] 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.

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

The patch doesn't work anyway in the net-procfs code path upon closer
examination.  Because we are using seq_ops and taking the RCU lock at
the beginning of the sequence, we cannot take RTNL.  That means
drivers cannot use mutex as well.

For tg3, I think I will just use tp->lock spinlock to protect
get_stats64 and the freeing of the stats memory.