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Subject[PATCH 4.19 46/55] sched/fair: Add tmp_alone_branch assertion
Date
From: Peter Zijlstra <peterz@infradead.org>

commit 5d299eabea5a251fbf66e8277704b874bbba92dc upstream.

The magic in list_add_leaf_cfs_rq() requires that at the end of
enqueue_task_fair():

rq->tmp_alone_branch == &rq->lead_cfs_rq_list

If this is violated, list integrity is compromised for list entries
and the tmp_alone_branch pointer might dangle.

Also, reflow list_add_leaf_cfs_rq() while there. This looses one
indentation level and generates a form that's convenient for the next
patch.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Janne Huttunen <janne.huttunen@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/sched/fair.c | 126 +++++++++++++++++++++++++++++-----------------------
1 file changed, 71 insertions(+), 55 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -284,64 +284,69 @@ static inline struct cfs_rq *group_cfs_r

static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
{
- if (!cfs_rq->on_list) {
- struct rq *rq = rq_of(cfs_rq);
- int cpu = cpu_of(rq);
+ struct rq *rq = rq_of(cfs_rq);
+ int cpu = cpu_of(rq);
+
+ if (cfs_rq->on_list)
+ return;
+
+ cfs_rq->on_list = 1;
+
+ /*
+ * Ensure we either appear before our parent (if already
+ * enqueued) or force our parent to appear after us when it is
+ * enqueued. The fact that we always enqueue bottom-up
+ * reduces this to two cases and a special case for the root
+ * cfs_rq. Furthermore, it also means that we will always reset
+ * tmp_alone_branch either when the branch is connected
+ * to a tree or when we reach the top of the tree
+ */
+ if (cfs_rq->tg->parent &&
+ cfs_rq->tg->parent->cfs_rq[cpu]->on_list) {
/*
- * Ensure we either appear before our parent (if already
- * enqueued) or force our parent to appear after us when it is
- * enqueued. The fact that we always enqueue bottom-up
- * reduces this to two cases and a special case for the root
- * cfs_rq. Furthermore, it also means that we will always reset
- * tmp_alone_branch either when the branch is connected
- * to a tree or when we reach the beg of the tree
+ * If parent is already on the list, we add the child
+ * just before. Thanks to circular linked property of
+ * the list, this means to put the child at the tail
+ * of the list that starts by parent.
*/
- if (cfs_rq->tg->parent &&
- cfs_rq->tg->parent->cfs_rq[cpu]->on_list) {
- /*
- * If parent is already on the list, we add the child
- * just before. Thanks to circular linked property of
- * the list, this means to put the child at the tail
- * of the list that starts by parent.
- */
- list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
- &(cfs_rq->tg->parent->cfs_rq[cpu]->leaf_cfs_rq_list));
- /*
- * The branch is now connected to its tree so we can
- * reset tmp_alone_branch to the beginning of the
- * list.
- */
- rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
- } else if (!cfs_rq->tg->parent) {
- /*
- * cfs rq without parent should be put
- * at the tail of the list.
- */
- list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
- &rq->leaf_cfs_rq_list);
- /*
- * We have reach the beg of a tree so we can reset
- * tmp_alone_branch to the beginning of the list.
- */
- rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
- } else {
- /*
- * The parent has not already been added so we want to
- * make sure that it will be put after us.
- * tmp_alone_branch points to the beg of the branch
- * where we will add parent.
- */
- list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
- rq->tmp_alone_branch);
- /*
- * update tmp_alone_branch to points to the new beg
- * of the branch
- */
- rq->tmp_alone_branch = &cfs_rq->leaf_cfs_rq_list;
- }
+ list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
+ &(cfs_rq->tg->parent->cfs_rq[cpu]->leaf_cfs_rq_list));
+ /*
+ * The branch is now connected to its tree so we can
+ * reset tmp_alone_branch to the beginning of the
+ * list.
+ */
+ rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
+ return;
+ }

- cfs_rq->on_list = 1;
+ if (!cfs_rq->tg->parent) {
+ /*
+ * cfs rq without parent should be put
+ * at the tail of the list.
+ */
+ list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
+ &rq->leaf_cfs_rq_list);
+ /*
+ * We have reach the top of a tree so we can reset
+ * tmp_alone_branch to the beginning of the list.
+ */
+ rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
+ return;
}
+
+ /*
+ * The parent has not already been added so we want to
+ * make sure that it will be put after us.
+ * tmp_alone_branch points to the begin of the branch
+ * where we will add parent.
+ */
+ list_add_rcu(&cfs_rq->leaf_cfs_rq_list, rq->tmp_alone_branch);
+ /*
+ * update tmp_alone_branch to points to the new begin
+ * of the branch
+ */
+ rq->tmp_alone_branch = &cfs_rq->leaf_cfs_rq_list;
}

static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
@@ -352,7 +357,12 @@ static inline void list_del_leaf_cfs_rq(
}
}

-/* Iterate through all leaf cfs_rq's on a runqueue: */
+static inline void assert_list_leaf_cfs_rq(struct rq *rq)
+{
+ SCHED_WARN_ON(rq->tmp_alone_branch != &rq->leaf_cfs_rq_list);
+}
+
+/* Iterate through all cfs_rq's on a runqueue in bottom-up order */
#define for_each_leaf_cfs_rq(rq, cfs_rq) \
list_for_each_entry_rcu(cfs_rq, &rq->leaf_cfs_rq_list, leaf_cfs_rq_list)

@@ -446,6 +456,10 @@ static inline void list_del_leaf_cfs_rq(
{
}

+static inline void assert_list_leaf_cfs_rq(struct rq *rq)
+{
+}
+
#define for_each_leaf_cfs_rq(rq, cfs_rq) \
for (cfs_rq = &rq->cfs; cfs_rq; cfs_rq = NULL)

@@ -5160,6 +5174,8 @@ enqueue_task_fair(struct rq *rq, struct
if (!se)
add_nr_running(rq, 1);

+ assert_list_leaf_cfs_rq(rq);
+
hrtick_update(rq);
}


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