sched/fair: Add tmp_alone_branch assertion

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>

1 files changed, 71 insertions, 55 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8c165f0d33b3..d6a536dec0ca 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -277,64 +277,69 @@ static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
 
 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)
@@ -345,7 +350,12 @@ static inline void list_del_leaf_cfs_rq(struct cfs_rq *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)
 
@@ -433,6 +443,10 @@ static inline void list_del_leaf_cfs_rq(struct cfs_rq *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)
 
@@ -5172,6 +5186,8 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 
         }
 
+        assert_list_leaf_cfs_rq(rq);
+
         hrtick_update(rq);
 }