[PATCH] rcu_read_lock/unlock protect find_task_by_vpid call

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde "Add RCU check for 
find_task_by_vpid()" introduced rcu_lockdep_assert to find_task_by_pid_ns.
Assertion failed in sys_ioprio_get. The patch is fixing assertion
failure in ioprio_set as well. 

 ===================================================
 [ INFO: suspicious rcu_dereference_check() usage. ]
 ---------------------------------------------------
 kernel/pid.c:419 invoked rcu_dereference_check() without protection!
 
 rcu_scheduler_active = 1, debug_locks = 0
 1 lock held by iotop/4254:
 #0:  (tasklist_lock){.?.?..}, at: [<ffffffff811104b4>] sys_ioprio_get+0x22/0x2da
 
 stack backtrace:
 Pid: 4254, comm: iotop Not tainted
 Call Trace:
 [<ffffffff810656f2>] lockdep_rcu_dereference+0xaa/0xb2
 [<ffffffff81053c67>] find_task_by_pid_ns+0x4f/0x68
 [<ffffffff81053c9d>] find_task_by_vpid+0x1d/0x1f
 [<ffffffff811104e2>] sys_ioprio_get+0x50/0x2da
 [<ffffffff81002182>] system_call_fastpath+0x16/0x1b
 

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

---

diff --git a/fs/ioprio.c b/fs/ioprio.c
index 748cfb9..666343d 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -113,8 +113,11 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 		case IOPRIO_WHO_PROCESS:
 			if (!who)
 				p = current;
-			else
+			else {
+				rcu_read_lock();
 				p = find_task_by_vpid(who);
+				rcu_read_unlock();
+			}
 			if (p)
 				ret = set_task_ioprio(p, ioprio);
 			break;
@@ -202,8 +205,11 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 		case IOPRIO_WHO_PROCESS:
 			if (!who)
 				p = current;
-			else
+			else {
+				rcu_read_lock();
 				p = find_task_by_vpid(who);
+				rcu_read_unlock();
+			}
 			if (p)
 				ret = get_task_ioprio(p);
 			break;