[RFD/RFC PATCH 3/8] locking/mutex: Rework task_struct::blocked_on

[Juri Lelli <juri.lelli@redhat.com>]

From: Peter Zijlstra <peterz@infradead.org>

Track the blocked-on relation for mutexes, this allows following this
relation at schedule time. Add blocked_task to track the inverse
relation.

                ,-> task
                |     | blocked-on
                |     v
   blocked-task |   mutex
                |     | owner
                |     v
                `-- task

This patch only enables blocked-on relation, blocked-task will be
enabled in a later patch implementing proxy().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[minor changes while rebasing]
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
---
 include/linux/sched.h        | 6 ++----
 kernel/fork.c                | 6 +++---
 kernel/locking/mutex-debug.c | 7 +++----
 kernel/locking/mutex.c       | 3 +++
 4 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 977cb57d7bc9..a35e8ab3eef1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -907,10 +907,8 @@ struct task_struct {
 	struct rt_mutex_waiter		*pi_blocked_on;
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
-	/* Mutex deadlock detection: */
-	struct mutex_waiter		*blocked_on;
-#endif
+	struct task_struct 	*blocked_task;	/* task that's boosting us */
+	struct mutex 		*blocked_on;	/* lock we're blocked on */
 
 #ifdef CONFIG_TRACE_IRQFLAGS
 	unsigned int			irq_events;
diff --git a/kernel/fork.c b/kernel/fork.c
index f0b58479534f..ef27a675b0d7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1827,9 +1827,9 @@ static __latent_entropy struct task_struct *copy_process(
 	lockdep_init_task(p);
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
-	p->blocked_on = NULL; /* not blocked yet */
-#endif
+	p->blocked_task = NULL; /* nobody is boosting us yet*/
+	p->blocked_on = NULL;  /* not blocked yet */
+
 #ifdef CONFIG_BCACHE
 	p->sequential_io	= 0;
 	p->sequential_io_avg	= 0;
diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index a660d38b6c29..6605e083a3e9 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -53,8 +53,8 @@ void debug_mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 {
 	SMP_DEBUG_LOCKS_WARN_ON(!raw_spin_is_locked(&lock->wait_lock));
 
-	/* Mark the current thread as blocked on the lock: */
-	task->blocked_on = waiter;
+	/* Current thread can't be alredy blocked (since it's executing!) */
+	DEBUG_LOCKS_WARN_ON(task->blocked_on);
 }
 
 void mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
@@ -62,8 +62,7 @@ void mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 {
 	DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
 	DEBUG_LOCKS_WARN_ON(waiter->task != task);
-	DEBUG_LOCKS_WARN_ON(task->blocked_on != waiter);
-	task->blocked_on = NULL;
+	DEBUG_LOCKS_WARN_ON(task->blocked_on != lock);
 
 	list_del_init(&waiter->list);
 	waiter->task = NULL;
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index f37402cd8496..76b59b555da3 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -979,6 +979,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	}
 
 	waiter.task = current;
+	current->blocked_on = lock;
 
 	set_current_state(state);
 	for (;;) {
@@ -1047,6 +1048,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	}
 
 	mutex_remove_waiter(lock, &waiter, current);
+	current->blocked_on = NULL;
+
 	if (likely(list_empty(&lock->wait_list)))
 		__mutex_clear_flag(lock, MUTEX_FLAGS);
 
-- 
2.17.1