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net: Introduce net_rwsem to protect net_namespace_list

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rtnl_lock() is used everywhere, and contention is very high.
When someone wants to iterate over alive net namespaces,
he/she has no a possibility to do that without exclusive lock.
But the exclusive rtnl_lock() in such places is overkill,
and it just increases the contention. Yes, there is already
for_each_net_rcu() in kernel, but it requires rcu_read_lock(),
and this can't be sleepable. Also, sometimes it may be need
really prevent net_namespace_list growth, so for_each_net_rcu()
is not fit there.

This patch introduces new rw_semaphore, which will be used
instead of rtnl_mutex to protect net_namespace_list. It is
sleepable and allows not-exclusive iterations over net
namespaces list. It allows to stop using rtnl_lock()
in several places (what is made in next patches) and makes
less the time, we keep rtnl_mutex. Here we just add new lock,
while the explanation of we can remove rtnl_lock() there are
in next patches.

Fine grained locks generally are better, then one big lock,
so let's do that with net_namespace_list, while the situation
allows that.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Kirill Tkhai 2018-03-29 19:20:32 +03:00 • committed by David S. Miller
parent 906edee91e
commit f0b07bb151
11 changed files with 37 additions and 5 deletions
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2
net/core/fib_notifier.c
View file

@ -33,6 +33,7 @@ static unsigned int fib_seq_sum(void)
struct net *net;
rtnl_lock();
down_read(&net_rwsem);
for_each_net(net) {
rcu_read_lock();
list_for_each_entry_rcu(ops, &net->fib_notifier_ops, list) {
@ -43,6 +44,7 @@ static unsigned int fib_seq_sum(void)
}
rcu_read_unlock();
}
up_read(&net_rwsem);
rtnl_unlock();
return fib_seq;