Re: [RFC net-next 1/2] net: net-porcfs: Reduce rcu lock critical section

From: Saeed Mahameed <redacted>
Date: Tue, 10 Apr 2018 10:08:11 -0700

The current net proc fs sequence file implementation to show current
namespace netdevs list statistics and mc lists holds the rcu lock
throughout the whole process, from dev seq start up to dev seq stop.

This is really greedy and demanding from device drivers since
ndo_get_stats64 called from dev_seq_show while the rcu lock is held.

The rcu lock is needed to guarantee that device chain is not modified
while the dev sequence file is walking through it and handling the
netdev in the same time.

To minimize this critical section and drastically reduce the time rcu lock
is being held, all we need is to grab the rcu lock only for the brief
moment where we are looking for the next netdev to handle, if found,
dev_hold it to guarantee it kept alive while accessed later in seq show
callback and release the rcu lock immediately.

The current netdev being handled will be released "dev_put" when the seq next
callback is called or dev seq stop is called.

Signed-off-by: Saeed Mahameed <redacted>

The tradeoff here is that now you are doing two unnecessary atomic
operations per stats dump.

That is what the RCU lock allows us to avoid.