ioprio: fix RCU locking around task dereference

With 2.6.37-rc1, I observe sys_ioprio_set not taking the RCU lock [1]
across access to the task credentials.

Inspecting the code in fs/ioprio.c, the tasklist_lock is held for read
across the __task_cred call, which is presumably sufficient to prevent
the task credentials becoming stale.

===================================================

[ INFO: suspicious rcu_dereference_check() usage. ]

---------------------------------------------------

kernel/pid.c:419 invoked rcu_dereference_check() without protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 1

1 lock held by start-stop-daem/2246:

 #0:  (tasklist_lock){.?.?..}, at: [<ffffffff811a2dfa>]
sys_ioprio_set+0x8a/0x400

stack backtrace:

Pid: 2246, comm: start-stop-daem Not tainted 2.6.37-rc1-330cd+ #2

Call Trace:

 [<ffffffff8109f5f4>] lockdep_rcu_dereference+0xa4/0xc0

 [<ffffffff81085651>] find_task_by_pid_ns+0x81/0x90

 [<ffffffff8108567d>] find_task_by_vpid+0x1d/0x20

 [<ffffffff811a3160>] sys_ioprio_set+0x3f0/0x400

 [<ffffffff816efa79>] ? trace_hardirqs_on_thunk+0x3a/0x3f

 [<ffffffff81003482>] system_call_fastpath+0x16/0x1b

Take the RCU lock for read across acquiring the pointer to the task
credentials and dereferencing it.
Signed-off-by: Daniel J Blueman <daniel.blueman@gmail.com>

Fixed up by Jens to fix missing rcu_read_unlock() on mismatches.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/ioprio.c

@@ -139,7 +139,12 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 				break;
 
 			do_each_thread(g, p) {
-				if (__task_cred(p)->uid != who)
+				int match;
+
+				rcu_read_lock();
+				match = __task_cred(p)->uid == who;
+				rcu_read_unlock();
+				if (!match)
 					continue;
 				ret = set_task_ioprio(p, ioprio);
 				if (ret)
@@ -232,7 +237,12 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 				break;
 
 			do_each_thread(g, p) {
-				if (__task_cred(p)->uid != user->uid)
+				int match;
+
+				rcu_read_lock();
+				match = __task_cred(p)->uid == user->uid;
+				rcu_read_unlock();
+				if (!match)
 					continue;
 				tmpio = get_task_ioprio(p);
 				if (tmpio < 0)