futex,plist: Pass the real head of the priority list to plist_del()

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

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

But it is 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 says that the @head is the "list head" head of the priority list.

In futex code, several places use "plist_del(&q->list, &q->list.plist);",
they pass a fake head. We need to fix them all.

Thanks to Darren Hart for many suggestions.

Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by:  Lai Jiangshan <laijs@cn.fujitsu.com>
LKML-Reference: <4D11984A.5030203@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/futex.c b/kernel/futex.c
index b766d28..6feeea4 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -775,6 +775,24 @@ 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)
+                       || plist_node_empty(&q->list)))
+               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.
@@ -792,7 +810,7 @@ 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
@@ -1100,8 +1118,7 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
        get_futex_key_refs(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;
@@ -1504,8 +1521,7 @@ retry:
                        spin_unlock(lock_ptr);
                        goto retry;
                }
-               WARN_ON(plist_node_empty(&q->list));
-               plist_del(&q->list, &q->list.plist);
+               __unqueue_futex(q);
 
                BUG_ON(q->pi_state);
 
@@ -1525,8 +1541,7 @@ retry:
 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);
@@ -2167,7 +2182,7 @@ 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;