[tip:core/locking] futex: Avoid wake_futex for a PI futex_q

From: tip-bot for Darren Hart
Date: Wed Nov 21 2012 - 15:17:03 EST


Commit-ID: 0e8f7a5954be13d0c8dcbca3204a9e962498c46e
Gitweb: http://git.kernel.org/tip/0e8f7a5954be13d0c8dcbca3204a9e962498c46e
Author: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
AuthorDate: Tue, 20 Nov 2012 23:36:45 -0800
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Wed, 21 Nov 2012 21:05:34 +0100

futex: Avoid wake_futex for a PI futex_q

Dave Jones reported a bug with futex_lock_pi() that his trinity test
exposed. Sometime between queue_me() and taking the q.lock_ptr, the
lock_ptr became NULL, resulting in a crash.

While futex_wake() is careful to not call wake_futex() on futex_q's with
a pi_state or an rt_waiter (which are either waiting for a
futex_unlock_pi() or a PI futex_requeue()), futex_wake_op() and
futex_requeue() do not perform the same test.

Update futex_wake_op() and futex_requeue() to test for q.pi_state and
q.rt_waiter and abort with -EINVAL if detected. To ensure any future
breakage is caught, add a WARN() to wake_futex() if the same condition
is true.

This fix has seen 3 hours of testing with "trinity -c futex" on an
x86_64 VM with 4 CPUS.

Reported-by: Dave Jones <davej@xxxxxxxxx>
Signed-off-by: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: John Kacur <jkacur@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/3b25c8ba053760892871713ff6e81660433f6734.1353483196.git.dvhart@xxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
kernel/futex.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 3717e7b..5699b21 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -840,6 +840,11 @@ static void wake_futex(struct futex_q *q)
{
struct task_struct *p = q->task;

+ if (q->pi_state || q->rt_waiter) {
+ WARN(1, "%s: refusing to wake PI futex\n", __FUNCTION__);
+ return;
+ }
+
/*
* We set q->lock_ptr = NULL _before_ we wake up the task. If
* a non-futex wake up happens on another CPU then the task
@@ -1075,6 +1080,10 @@ retry_private:

plist_for_each_entry_safe(this, next, head, list) {
if (match_futex (&this->key, &key1)) {
+ if (this->pi_state || this->rt_waiter) {
+ ret = -EINVAL;
+ goto out_unlock;
+ }
wake_futex(this);
if (++ret >= nr_wake)
break;
@@ -1087,6 +1096,10 @@ retry_private:
op_ret = 0;
plist_for_each_entry_safe(this, next, head, list) {
if (match_futex (&this->key, &key2)) {
+ if (this->pi_state || this->rt_waiter) {
+ ret = -EINVAL;
+ goto out_unlock;
+ }
wake_futex(this);
if (++op_ret >= nr_wake2)
break;
@@ -1095,6 +1108,7 @@ retry_private:
ret += op_ret;
}

+out_unlock:
double_unlock_hb(hb1, hb2);
out_put_keys:
put_futex_key(&key2);
@@ -1384,9 +1398,13 @@ retry_private:
/*
* FUTEX_WAIT_REQEUE_PI and FUTEX_CMP_REQUEUE_PI should always
* be paired with each other and no other futex ops.
+ *
+ * We should never be requeueing a futex_q with a pi_state,
+ * which is awaiting a futex_unlock_pi().
*/
if ((requeue_pi && !this->rt_waiter) ||
- (!requeue_pi && this->rt_waiter)) {
+ (!requeue_pi && this->rt_waiter) ||
+ this->pi_state) {
ret = -EINVAL;
break;
}
--
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