[Devel] [PATCH rh7] vznetstat: fix RCU-protected "info_V{4, 6}" pointers initialization

[Stanislav Kinsburskiy]

This pointers have to be referenced and assigned under rcu_read_lock().
Moreover, there is no need to take tc_lock: it protects stats hash, but not
info pointers.

Fixes the following warning:

[   31.658913] ===============================
[   31.659035] [ INFO: suspicious RCU usage. ]
[   31.659155] 3.10.0+ #121 Not tainted
[   31.659258] -------------------------------
[   31.659377] kernel/ve/vznetstat/vznetstat.c:95 suspicious
rcu_dereference_check() usage!
[   31.659597]
[   31.659597] other info that might help us debug this:
[   31.659597]
[   31.659831]
[   31.659831] rcu_scheduler_active = 1, debug_locks = 0
[   31.660026] 1 lock held by vztactl/831:
[   31.660136]  #0:  (tc_lock){.+.+..}, at: [<ffffffffa0052e64>]
venet_acct_set_classes+0xf4/0x240 [vznetstat]
[   31.660454]
[   31.660454] stack backtrace:
[   31.660584] CPU: 0 PID: 831 Comm: vztactl ve: 0 Not tainted 3.10.0+ #121
ovz.9.8-21-g6c64ed7
[   31.660823] Hardware name: Parallels Software International Inc. Parallels
Virtual Platform/Parallels Virtual Platform, BIOS 6.9.23856.1122235 03/18/2015
[   31.661201]  0000000000000000 000000003e77c710 ffff8800a960bd88
ffffffff816298fe
[   31.661436]  ffff8800a960bdb8 ffffffff810f9667 0000000000000000
ffff8801446e2f08
[   31.661661]  0000000000000000 000000000000000c ffff8800a960bdf0
ffffffffa0052f61
[   31.661910] Call Trace:
[   31.661989]  [<ffffffff816298fe>] dump_stack+0x19/0x1b
[   31.662134]  [<ffffffff810f9667>] lockdep_rcu_suspicious+0xe7/0x120
[   31.662308]  [<ffffffffa0052f61>] venet_acct_set_classes+0x1f1/0x240
[vznetstat]
[   31.662511]  [<ffffffffa005390b>] venet_acct_ioctl+0x2bb/0xaf0 [vznetstat]
[   31.662706]  [<ffffffffa003f0d5>] vzctl_ioctl+0x45/0x60 [vzdev]
[   31.662875]  [<ffffffff812296d5>] do_vfs_ioctl+0x265/0x540
[   31.663041]  [<ffffffff81236177>] ? fget_light+0x377/0x4e0
[   31.663193]  [<ffffffff81229a04>] SyS_ioctl+0x54/0xa0
[   31.663334]  [<ffffffff8163bf09>] system_call_fastpath+0x16/0x1b


Signed-off-by: Stanislav Kinsburskiy <skinsbursky at virtuozzo.com>
---
 kernel/ve/vznetstat/vznetstat.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/ve/vznetstat/vznetstat.c b/kernel/ve/vznetstat/vznetstat.c
index 99feafb..c47865a 100644
--- a/kernel/ve/vznetstat/vznetstat.c
+++ b/kernel/ve/vznetstat/vznetstat.c
@@ -87,7 +87,7 @@ static int venet_acct_set_classes(const void __user *user_info, int length, int
 			goto out_free;
 	}
 
-	write_lock_irq(&tc_lock);
+	rcu_read_lock();
 	if (v6) {
 		old = rcu_dereference(info_v6);
 		rcu_assign_pointer(info_v6, info);
@@ -95,7 +95,7 @@ static int venet_acct_set_classes(const void __user *user_info, int length, int
 		old = rcu_dereference(info_v4);
 		rcu_assign_pointer(info_v4, info);
 	}
-	write_unlock_irq(&tc_lock);
+	rcu_read_unlock();
 
 	synchronize_net();
 	/* IMPORTANT. I think reset of statistics collected should not be