[PATCH 1/4 V2 ] futex,plist: pass the real head of the priority listto plist_del()

From: Lai Jiangshan
Date: Tue Dec 21 2010 - 04:53:46 EST



Some plist_del()s in kernel/futex.c are passed a faked head of the
priority list.

It can work because current code does not require the real head
in plist_del(). The code of plist_del() just uses the head for checking,
so it will not cause bad result even when we use a faked head.

But it is an undocumented usage:

/**
* plist_del - Remove a @node from plist.
*
* @node: &struct plist_node pointer - entry to be removed
* @head: &struct plist_head pointer - list head
*/

The document said that @head is "list head" the head of the priority list.

In futex code, several places use "plist_del(&q->list, &q->list.plist);",
they passes faked head, we fix them all.

Thank to Darren Hart for many suggests.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
---
diff --git a/kernel/futex.c b/kernel/futex.c
index 3019b92..d901f40 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -740,6 +740,23 @@ retry:
return ret;
}

+/**
+ * __unqueue_futex() - Remove the futex_q from its futex_hash_bucket
+ * @q: The futex_q to unqueue
+ *
+ * The q->lock_ptr must not be NULL and must be held by the caller.
+ */
+static void __unqueue_futex(struct futex_q *q)
+{
+ struct futex_hash_bucket *hb;
+
+ if (WARN_ON(!q->lock_ptr || !spin_is_locked(q->lock_ptr)))
+ return;
+
+ hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
+ plist_del(&q->list, &hb->chain);
+}
+
/*
* The hash bucket lock must be held when this is called.
* Afterwards, the futex_q must not be accessed.
@@ -757,7 +774,7 @@ static void wake_futex(struct futex_q *q)
*/
get_task_struct(p);

- plist_del(&q->list, &q->list.plist);
+ __unqueue_futex(q);
/*
* The waiting task can free the futex_q as soon as
* q->lock_ptr = NULL is written, without taking any locks. A
@@ -1067,7 +1084,7 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
q->key = *key;

WARN_ON(plist_node_empty(&q->list));
- plist_del(&q->list, &q->list.plist);
+ __unqueue_futex(q);

WARN_ON(!q->rt_waiter);
q->rt_waiter = NULL;
@@ -1471,7 +1488,7 @@ retry:
goto retry;
}
WARN_ON(plist_node_empty(&q->list));
- plist_del(&q->list, &q->list.plist);
+ __unqueue_futex(q);

BUG_ON(q->pi_state);

@@ -1492,7 +1509,7 @@ static void unqueue_me_pi(struct futex_q *q)
__releases(q->lock_ptr)
{
WARN_ON(plist_node_empty(&q->list));
- plist_del(&q->list, &q->list.plist);
+ __unqueue_futex(q);

BUG_ON(!q->pi_state);
free_pi_state(q->pi_state);
@@ -2133,7 +2150,7 @@ int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
* We were woken prior to requeue by a timeout or a signal.
* Unqueue the futex_q and determine which it was.
*/
- plist_del(&q->list, &q->list.plist);
+ plist_del(&q->list, &hb->chain);

/* Handle spurious wakeups gracefully */
ret = -EWOULDBLOCK;
--
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