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Subject[PATCH RT v3] futex: update futex_q lock_ptr on requeue proxy lock
From: Darren Hart <dvhltc@us.ibm.com>

futex_requeue() can acquire the lock on behalf of a waiter early on or during
the requeue loop if it is uncontended or in the event of a lock steal or owner
died. On wakeup, the waiter (in futex_wait_requeue_pi()) cleans up the pi_state
owner using the lock_ptr to protect against concurrent access to the pi_state.
The pi_state is hung off futex_q's on the requeue target futex hash bucket so
the lock_ptr needs to be updated accordingly.

The problem manifested by triggering the WARN_ON in lookup_pi_state() about the
pid != pi_state->owner->pid. With this patch, the pi_state is properly guarded
against concurrent access via the requeue target hb lock.

The astute reviewer may notice that there is a window of time between when
futex_requeue() unlocks the hb locks and when futex_wait_requeue_pi() will
acquire hb2->lock. During this time the pi_state and uval are not in sync with
the underlying rtmutex owner (but the uval does indicate there are waiters, so
no atomic changes will occur in userspace). However, this is not a problem.
Should a contending thread enter lookup_pi_state() and acquire hb2->lock before
the ownership is fixed up, it will find the pi_state hung off a waiter's
(possibly the pending owner's) futex_q and block on the rtmutex. Once
futex_wait_requeue_pi() fixes up the owner, it will also move the pi_state from
the old owner's task->pi_state_list to its own.

V3: Resend of V2 with updated patch header indicating it is for RT
V2: Add DEBUG_PI_LIST support and correct locking requirements commentary
V1: Initial Patch

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@elte.hu>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: Dinakar Guniguntala <dino@in.ibm.com>
CC: John Stultz <johnstul@us.ibm.com>
---
kernel/futex.c | 17 +++++++++++++----
1 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index abce822..df30983 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1059,15 +1059,19 @@ void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
* requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
* q: the futex_q
* key: the key of the requeue target futex
+ * hb: the hash_bucket of the requeue target futex
*
* During futex_requeue, with requeue_pi=1, it is possible to acquire the
* target futex if it is uncontended or via a lock steal. Set the futex_q key
* to the requeue target futex so the waiter can detect the wakeup on the right
* futex, but remove it from the hb and NULL the rt_waiter so it can detect
- * atomic lock acquisition. Must be called with the q->lock_ptr held.
+ * atomic lock acquisition. Set the q->lock_ptr to the requeue target hb->lock
+ * to protect access to the pi_state to fixup the owner later. Must be called
+ * with both q->lock_ptr and hb->lock held.
*/
static inline
-void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key)
+void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
+ struct futex_hash_bucket *hb)
{
drop_futex_key_refs(&q->key);
get_futex_key_refs(key);
@@ -1079,6 +1083,11 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key)
WARN_ON(!q->rt_waiter);
q->rt_waiter = NULL;

+ q->lock_ptr = &hb->lock;
+#ifdef CONFIG_DEBUG_PI_LIST
+ q->list.plist.slock = &hb->lock;
+#endif
+
wake_up_state(q->task, TASK_NORMAL);
}

@@ -1137,7 +1146,7 @@ static int futex_proxy_trylock_atomic(u32 __user *pifutex,
ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
set_waiters);
if (ret == 1)
- requeue_pi_wake_futex(top_waiter, key2);
+ requeue_pi_wake_futex(top_waiter, key2, hb2);

return ret;
}
@@ -1323,7 +1332,7 @@ retry_private:
this->task, 1);
if (ret == 1) {
/* We got the lock. */
- requeue_pi_wake_futex(this, &key2);
+ requeue_pi_wake_futex(this, &key2, hb2);
continue;
} else if (ret) {
/* -EDEADLK */
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team

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