[PATCH] ioprio: grab rcu_read_lock in sys_ioprio_{set,get}()

[Greg Thelen <gthelen@google.com>]

Using:
- CONFIG_LOCKUP_DETECTOR=y
- CONFIG_PREEMPT=y
- CONFIG_LOCKDEP=y
- CONFIG_PROVE_LOCKING=y
- CONFIG_PROVE_RCU=y
found a missing rcu lock during boot on a 512 MiB x86_64 ubuntu vm:
  ===================================================
  [ 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 = 0
  1 lock held by ureadahead/1355:
   #0:  (tasklist_lock){.+.+..}, at: [<ffffffff8115bc09>] sys_ioprio_set+0x7f/0x29e

  stack backtrace:
  Pid: 1355, comm: ureadahead Not tainted 2.6.37-dbg-DEV #1
  Call Trace:
   [<ffffffff8109c10c>] lockdep_rcu_dereference+0xaa/0xb3
   [<ffffffff81088cbf>] find_task_by_pid_ns+0x44/0x5d
   [<ffffffff81088cfa>] find_task_by_vpid+0x22/0x24
   [<ffffffff8115bc3e>] sys_ioprio_set+0xb4/0x29e
   [<ffffffff8147cf21>] ? trace_hardirqs_off_thunk+0x3a/0x3c
   [<ffffffff8105c409>] sysenter_dispatch+0x7/0x2c
   [<ffffffff8147cee2>] ? trace_hardirqs_on_thunk+0x3a/0x3f

The fix is to:
a) grab rcu lock in sys_ioprio_{set,get}() and
b) avoid grabbing tasklist_lock.
Discussion in: http://marc.info/?l=linux-kernel&m=128951324702889

Signed-off-by: Greg Thelen <gthelen@google.com>
---
 fs/ioprio.c |   13 ++++---------
 1 files changed, 4 insertions(+), 9 deletions(-)

diff --git a/fs/ioprio.c b/fs/ioprio.c
index 748cfb9..7da2a06 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -103,12 +103,7 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 	}
 
 	ret = -ESRCH;
-	/*
-	 * We want IOPRIO_WHO_PGRP/IOPRIO_WHO_USER to be "atomic",
-	 * so we can't use rcu_read_lock(). See re-copy of ->ioprio
-	 * in copy_process().
-	 */
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	switch (which) {
 		case IOPRIO_WHO_PROCESS:
 			if (!who)
@@ -153,7 +148,7 @@ free_uid:
 			ret = -EINVAL;
 	}
 
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 	return ret;
 }
 
@@ -197,7 +192,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 	int ret = -ESRCH;
 	int tmpio;
 
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	switch (which) {
 		case IOPRIO_WHO_PROCESS:
 			if (!who)
@@ -250,6 +245,6 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 			ret = -EINVAL;
 	}
 
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 	return ret;
 }
-- 
1.7.3.1