ioprio: grab rcu_read_lock in sys_ioprio_{set,get}()
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>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Modified by Jens to remove the now redundant inner rcu lock and
unlock since they are now protected by the outer lock.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/fs/ioprio.c b/fs/ioprio.c
index 2f7d05c..7da2a06 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -103,22 +103,15 @@
}
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:
- 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)
@@ -141,12 +134,7 @@
break;
do_each_thread(g, p) {
- int match;
-
- rcu_read_lock();
- match = __task_cred(p)->uid == who;
- rcu_read_unlock();
- if (!match)
+ if (__task_cred(p)->uid != who)
continue;
ret = set_task_ioprio(p, ioprio);
if (ret)
@@ -160,7 +148,7 @@
ret = -EINVAL;
}
- read_unlock(&tasklist_lock);
+ rcu_read_unlock();
return ret;
}
@@ -204,17 +192,15 @@
int ret = -ESRCH;
int tmpio;
- read_lock(&tasklist_lock);
+ rcu_read_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)
@@ -241,12 +227,7 @@
break;
do_each_thread(g, p) {
- int match;
-
- rcu_read_lock();
- match = __task_cred(p)->uid == user->uid;
- rcu_read_unlock();
- if (!match)
+ if (__task_cred(p)->uid != user->uid)
continue;
tmpio = get_task_ioprio(p);
if (tmpio < 0)
@@ -264,6 +245,6 @@
ret = -EINVAL;
}
- read_unlock(&tasklist_lock);
+ rcu_read_unlock();
return ret;
}