On Fri, Jan 6, 2017 at 10:01 AM, David Miller <da...@davemloft.net> wrote: > From: Eric Dumazet <eric.duma...@gmail.com> > Date: Fri, 06 Jan 2017 09:32:56 -0800 > >> 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.
