linux/net/core
Daniel Wagner 8a8e04df47 cgroup: Assign subsystem IDs during compile time
WARNING: With this change it is impossible to load external built
controllers anymore.

In case where CONFIG_NETPRIO_CGROUP=m and CONFIG_NET_CLS_CGROUP=m is
set, corresponding subsys_id should also be a constant. Up to now,
net_prio_subsys_id and net_cls_subsys_id would be of the type int and
the value would be assigned during runtime.

By switching the macro definition IS_SUBSYS_ENABLED from IS_BUILTIN
to IS_ENABLED, all *_subsys_id will have constant value. That means we
need to remove all the code which assumes a value can be assigned to
net_prio_subsys_id and net_cls_subsys_id.

A close look is necessary on the RCU part which was introduces by
following patch:

  commit f845172531
  Author:	Herbert Xu <herbert@gondor.apana.org.au>  Mon May 24 09:12:34 2010
  Committer:	David S. Miller <davem@davemloft.net>  Mon May 24 09:12:34 2010

  cls_cgroup: Store classid in struct sock

  Tis code was added to init_cgroup_cls()

	  /* We can't use rcu_assign_pointer because this is an int. */
	  smp_wmb();
	  net_cls_subsys_id = net_cls_subsys.subsys_id;

  respectively to exit_cgroup_cls()

	  net_cls_subsys_id = -1;
	  synchronize_rcu();

  and in module version of task_cls_classid()

	  rcu_read_lock();
	  id = rcu_dereference(net_cls_subsys_id);
	  if (id >= 0)
		  classid = container_of(task_subsys_state(p, id),
					 struct cgroup_cls_state, css)->classid;
	  rcu_read_unlock();

Without an explicit explaination why the RCU part is needed. (The
rcu_deference was fixed by exchanging it to rcu_derefence_index_check()
in a later commit, but that is a minor detail.)

So here is my pondering why it was introduced and why it safe to
remove it now. Note that this code was copied over to net_prio the
reasoning holds for that subsystem too.

The idea behind the RCU use for net_cls_subsys_id is to make sure we
get a valid pointer back from task_subsys_state(). task_subsys_state()
is just blindly accessing the subsys array and returning the
pointer. Obviously, passing in -1 as id into task_subsys_state()
returns an invalid value (out of lower bound).

So this code makes sure that only after module is loaded and the
subsystem registered, the id is assigned.

Before unregistering the module all old readers must have left the
critical section. This is done by assigning -1 to the id and issuing a
synchronized_rcu(). Any new readers wont call task_subsys_state()
anymore and therefore it is safe to unregister the subsystem.

The new code relies on the same trick, but it looks at the subsys
pointer return by task_subsys_state() (remember the id is constant
and therefore we allways have a valid index into the subsys
array).

No precautions need to be taken during module loading
module. Eventually, all CPUs will get a valid pointer back from
task_subsys_state() because rebind_subsystem() which is called after
the module init() function will assigned subsys[net_cls_subsys_id] the
newly loaded module subsystem pointer.

When the subsystem is about to be removed, rebind_subsystem() will
called before the module exit() function. In this case,
rebind_subsys() will assign subsys[net_cls_subsys_id] a NULL pointer
and then it calls synchronize_rcu(). All old readers have left by then
the critical section. Any new reader wont access the subsystem
anymore.  At this point we are safe to unregister the subsystem. No
synchronize_rcu() call is needed.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: netdev@vger.kernel.org
Cc: cgroups@vger.kernel.org
2012-09-14 09:57:43 -07:00
..
datagram.c net: skb_free_datagram_locked() doesnt drop all packets 2012-06-27 15:40:57 -07:00
dev_addr_lists.c net: addr_list: add exclusive dev_uc_add and dev_mc_add 2012-04-15 13:06:04 -04:00
dev.c net/core/dev.c: fix kernel-doc warning 2012-08-20 03:00:55 -07:00
drop_monitor.c drop_monitor: dont sleep in atomic context 2012-06-04 11:42:01 -04:00
dst.c net: force dst_default_metrics to const section 2012-08-08 16:00:28 -07:00
ethtool.c ethtool: Make more commands available to unprivileged processes 2012-06-12 18:51:09 -07:00
fib_rules.c ipv4: Elide fib_validate_source() completely when possible. 2012-06-29 01:36:36 -07:00
filter.c netvm: allow skb allocation to use PFMEMALLOC reserves 2012-07-31 18:42:46 -07:00
flow_dissector.c ipv6: add ipv6_addr_hash() helper 2012-07-18 11:28:46 -07:00
flow.c net: Add a flow_cache_flush_deferred function 2011-12-21 16:48:08 -05:00
gen_estimator.c Remove all #inclusions of asm/system.h 2012-03-28 18:30:03 +01:00
gen_stats.c gen_stats: Stop using NLA_PUT*(). 2012-04-02 04:33:44 -04:00
iovec.c net: get rid of some pointless casts to sockaddr 2012-03-11 19:11:22 -07:00
link_watch.c net: linkwatch: allow vlans to get carrier changes faster 2011-09-15 15:36:34 -04:00
Makefile sock_diag: Move the sock_ code to net/core/ 2011-12-06 13:58:02 -05:00
neighbour.c neigh: Convert over to dst_neigh_lookup_skb(). 2012-07-05 01:12:00 -07:00
net_namespace.c net: Statically initialize init_net.dev_base_head 2012-07-18 13:32:27 -07:00
net-sysfs.c wireless: remove wext sysfs 2012-06-05 15:32:15 -04:00
net-sysfs.h xps: Add CONFIG_XPS 2010-11-28 18:24:14 -08:00
net-traces.c net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
netevent.c net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
netpoll.c netpoll: re-enable irq in poll_napi() 2012-08-14 14:33:33 -07:00
netprio_cgroup.c cgroup: Assign subsystem IDs during compile time 2012-09-14 09:57:43 -07:00
pktgen.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-05-20 21:53:04 -04:00
request_sock.c ipv4:correct description for tcp_max_syn_backlog 2011-12-06 13:02:28 -05:00
rtnetlink.c This patch series contains a major revamp of how we collect entropy 2012-07-31 19:07:42 -07:00
scm.c net: netprio: fd passed in SCM_RIGHTS datagram not set correctly 2012-08-16 14:56:11 -07:00
secure_seq.c net: fix some sparse errors 2012-01-17 10:31:12 -05:00
skbuff.c netvm: allow skb allocation to use PFMEMALLOC reserves 2012-07-31 18:42:46 -07:00
sock_diag.c net: make sock diag per-namespace 2012-07-16 22:31:34 -07:00
sock.c cgroup: Assign subsystem IDs during compile time 2012-09-14 09:57:43 -07:00
stream.c net: Fix the condition passed to sk_wait_event() 2010-10-03 20:41:32 -07:00
sysctl_net_core.c net: Delete all remaining instances of ctl_path 2012-04-20 21:22:30 -04:00
timestamping.c net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
user_dma.c net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
utils.c net: Fixed coding style issues relating to braces. 2012-04-12 16:35:48 -04:00