From: Saeed Mahameed <sae...@mellanox.com> 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 <sae...@mellanox.com> 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.
