[PATCH RFC 03/25] net: Introduce net_sem for protection of pernet_list

Kirill Tkhai
2017-11-17T18:27:41Z

Curently mutex is used to protect pernet operations list. It makes
cleanup_net() to execute ->exit methods of the same operations set,
which was used on the time of ->init, even after net namespace is
unlinked from net_namespace_list.

But the problem is it's need to synchronize_rcu() after net is removed
from net_namespace_list():

Destroy net_ns:
cleanup_net()
mutex_lock(&net_mutex)
list_del_rcu(&net->list)
synchronize_rcu() <--- Sleep there for ages
list_for_each_entry_reverse(ops, &pernet_list, list)
ops_exit_list(ops, &net_exit_list)
list_for_each_entry_reverse(ops, &pernet_list, list)
ops_free_list(ops, &net_exit_list)
mutex_unlock(&net_mutex)

This primitive is not fast, especially on the systems with many processors
and/or when preemptible RCU is enabled in config. So, all the time, while
cleanup_net() is waiting for RCU grace period, creation of new net namespaces
is not possible, the tasks, who makes it, are sleeping on the same mutex:

Create net_ns:
copy_net_ns()
mutex_lock_killable(&net_mutex) <--- Sleep there for ages

I observed 20-30 seconds hangs of "unshare -n" on ordinary 8-cpu laptop
with preemptible RCU enabled.

The solution is to convert net_mutex to the rw_semaphore and add small locks
to really small number of pernet_operations, what really need them. Then,
pernet_operations::init/::exit methods, modifying the net-related data,
will require down_read() locking only, while down_write() will be used
for changing pernet_list.

This gives signify performance increase, like you may see here:
https://www.spinics.net/lists/netdev/msg467095.html

It's 4.6 times performance increase on one-thread test.
Multi-thread tests increase may be close to 4.6 multiplied
to number of threads.

This patch starts replacing net_mutex to net_sem. It adds rw_semaphore,
describes the variables it protects, and makes to use where appropriate.
net_mutex is still present, and next patches will kick it out step-by-step.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
include/linux/rtnetlink.h | 1 +
net/core/net_namespace.c | 37 +++++++++++++++++++++++++------------
net/core/rtnetlink.c | 4 ++--
3 files changed, 28 insertions(+), 14 deletions(-)

diff --git a/include/linux/rtnetlink.h b/include/linux/rtnetlink.h
index 2032ce2eb20b..f640fc87fe1d 100644
--- a/include/linux/rtnetlink.h
+++ b/include/linux/rtnetlink.h
@@ -35,6 +35,7 @@ extern int rtnl_is_locked(void);

extern wait_queue_head_t netdev_unregistering_wq;
extern struct mutex net_mutex;
+extern struct rw_semaphore net_sem;

#ifdef CONFIG_PROVE_LOCKING
extern bool lockdep_rtnl_is_held(void);
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 2e512965bf42..2254b1639209 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -41,6 +41,11 @@ struct net init_net = {
EXPORT_SYMBOL(init_net);

static bool init_net_initialized;
+/*
+ * net_sem: protects: pernet_list, net_generic_ids,
+ * init_net_initialized and first_* pointers.
+ */
+DECLARE_RWSEM(net_sem);

#define MIN_PERNET_OPS_ID \
((sizeof(struct net_generic) + sizeof(void *) - 1) / sizeof(void *))
@@ -411,12 +416,16 @@ struct net *copy_net_ns(unsigned long flags,
net->ucounts = ucounts;
get_user_ns(user_ns);

- rv = mutex_lock_killable(&net_mutex);
+ rv = down_read_killable(&net_sem);
if (rv < 0)
goto put_userns;
-
+ rv = mutex_lock_killable(&net_mutex);
+ if (rv < 0)
+ goto up_read;
rv = setup_net(net, user_ns);
mutex_unlock(&net_mutex);
+up_read:
+ up_read(&net_sem);
if (rv < 0) {
put_userns:
put_user_ns(user_ns);
@@ -443,6 +452,7 @@ static void cleanup_net(struct work_struct *work)
list_replace_init(&cleanup_list, &net_kill_list);
spin_unlock_irq(&cleanup_list_lock);

+ down_read(&net_sem);
mutex_lock(&net_mutex);

/* Don't let anyone else find us. */
@@ -484,6 +494,7 @@ static void cleanup_net(struct work_struct *work)
ops_free_list(ops, &net_exit_list);

mutex_unlock(&net_mutex);
+ up_read(&net_sem);

/* Ensure there are no outstanding rcu callbacks using this
* network namespace.
@@ -510,8 +521,10 @@ static void cleanup_net(struct work_struct *work)
*/
void net_ns_barrier(void)
{
+ down_write(&net_sem);
mutex_lock(&net_mutex);
mutex_unlock(&net_mutex);
+ up_write(&net_sem);
}
EXPORT_SYMBOL(net_ns_barrier);

@@ -838,12 +851,12 @@ static int __init net_ns_init(void)

rcu_assign_pointer(init_net.gen, ng);

- mutex_lock(&net_mutex);
+ down_write(&net_sem);
if (setup_net(&init_net, &init_user_ns))
panic("Could not setup the initial network namespace");

init_net_initialized = true;
- mutex_unlock(&net_mutex);
+ up_write(&net_sem);

register_pernet_subsys(&net_ns_ops);

@@ -983,9 +996,9 @@ static void unregister_pernet_operations(struct pernet_operations *ops)
int register_pernet_subsys(struct pernet_operations *ops)
{
int error;
- mutex_lock(&net_mutex);
+ down_write(&net_sem);
error = register_pernet_operations(first_device, ops);
- mutex_unlock(&net_mutex);
+ up_write(&net_sem);
return error;
}
EXPORT_SYMBOL_GPL(register_pernet_subsys);
@@ -1001,9 +1014,9 @@ EXPORT_SYMBOL_GPL(register_pernet_subsys);
*/
void unregister_pernet_subsys(struct pernet_operations *ops)
{
- mutex_lock(&net_mutex);
+ down_write(&net_sem);
unregister_pernet_operations(ops);
- mutex_unlock(&net_mutex);
+ up_write(&net_sem);
}
EXPORT_SYMBOL_GPL(unregister_pernet_subsys);

@@ -1029,11 +1042,11 @@ EXPORT_SYMBOL_GPL(unregister_pernet_subsys);
int register_pernet_device(struct pernet_operations *ops)
{
int error;
- mutex_lock(&net_mutex);
+ down_write(&net_sem);
error = register_pernet_operations(&pernet_list, ops);
if (!error && (first_device == &pernet_list))
first_device = &ops->list;
- mutex_unlock(&net_mutex);
+ up_write(&net_sem);
return error;
}
EXPORT_SYMBOL_GPL(register_pernet_device);
@@ -1049,11 +1062,11 @@ EXPORT_SYMBOL_GPL(register_pernet_device);
*/
void unregister_pernet_device(struct pernet_operations *ops)
{
- mutex_lock(&net_mutex);
+ down_write(&net_sem);
if (&ops->list == first_device)
first_device = first_device->next;
unregister_pernet_operations(ops);
- mutex_unlock(&net_mutex);
+ up_write(&net_sem);
}
EXPORT_SYMBOL_GPL(unregister_pernet_device);

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index dabba2a91fc8..cb06d43c4230 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -390,11 +390,11 @@ static void rtnl_lock_unregistering_all(void)
void rtnl_link_unregister(struct rtnl_link_ops *ops)
{
/* Close the race with cleanup_net() */
- mutex_lock(&net_mutex);
+ down_write(&net_sem);
rtnl_lock_unregistering_all();
__rtnl_link_unregister(ops);
rtnl_unlock();
- mutex_unlock(&net_mutex);
+ up_write(&net_sem);
}
EXPORT_SYMBOL_GPL(rtnl_link_unregister);