[PATCH] ioprio: rcu_read_lock/unlock protect find_task_by_vpid call (V2)

[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.

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

 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
                     
V2: rcu critical section expanded according to comment by Paul E. McKenne:
| Looking over the patch again, the scope of the RCU read-side critical
| section needs to expand to cover the use of the pointer as well as the
| call to find_task_by_vpid().
                                                                

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

---

diff --git a/fs/ioprio.c b/fs/ioprio.c
index 748cfb9..64cd50b 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -111,12 +111,14 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 	read_lock(&tasklist_lock);
 	switch (which) {
 		case IOPRIO_WHO_PROCESS:
+			rcu_read_lock();
 			if (!who)
 				p = current;
 			else
 				p = find_task_by_vpid(who);
 			if (p)
 				ret = set_task_ioprio(p, ioprio);
+			rcu_read_unlock();
 			break;
 		case IOPRIO_WHO_PGRP:
 			if (!who)
@@ -200,12 +202,14 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 	read_lock(&tasklist_lock);
 	switch (which) {
 		case IOPRIO_WHO_PROCESS:
+			rcu_read_lock();
 			if (!who)
 				p = current;
 			else
 				p = find_task_by_vpid(who);
 			if (p)
 				ret = get_task_ioprio(p);
+			rcu_read_unlock();
 			break;
 		case IOPRIO_WHO_PGRP:
 			if (!who)