locking/lockdep: Add new check to lock_downgrade()

Commit:

  f8319483f57f ("locking/lockdep: Provide a type check for lock_is_held")

didn't fully cover rwsems as downgrade_write() was left out.

Introduce lock_downgrade() and use it to add new checks.

See-also: http://marc.info/?l=linux-kernel&m=148581164003149&w=2
Originally-written-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1486053497-9948-3-git-send-email-hooanon05g@gmail.com
[ Rewrote the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/lockdep.h

@@ -361,6 +361,8 @@ static inline void lock_set_subclass(struct lockdep_map *lock,
 	lock_set_class(lock, lock->name, lock->key, subclass, ip);
 }
 
+extern void lock_downgrade(struct lockdep_map *lock, unsigned long ip);
+
 extern void lockdep_set_current_reclaim_state(gfp_t gfp_mask);
 extern void lockdep_clear_current_reclaim_state(void);
 extern void lockdep_trace_alloc(gfp_t mask);
@@ -411,6 +413,7 @@ static inline void lockdep_on(void)
 
 # define lock_acquire(l, s, t, r, c, n, i)	do { } while (0)
 # define lock_release(l, n, i)			do { } while (0)
+# define lock_downgrade(l, i)			do { } while (0)
 # define lock_set_class(l, n, k, s, i)		do { } while (0)
 # define lock_set_subclass(l, s, i)		do { } while (0)
 # define lockdep_set_current_reclaim_state(g)	do { } while (0)

kernel/locking/lockdep.c

@@ -3533,6 +3533,44 @@ __lock_set_class(struct lockdep_map *lock, const char *name,
 	return 1;
 }
 
+static int __lock_downgrade(struct lockdep_map *lock, unsigned long ip)
+{
+	struct task_struct *curr = current;
+	struct held_lock *hlock;
+	unsigned int depth;
+	int i;
+
+	depth = curr->lockdep_depth;
+	/*
+	 * This function is about (re)setting the class of a held lock,
+	 * yet we're not actually holding any locks. Naughty user!
+	 */
+	if (DEBUG_LOCKS_WARN_ON(!depth))
+		return 0;
+
+	hlock = find_held_lock(curr, lock, depth, &i);
+	if (!hlock)
+		return print_unlock_imbalance_bug(curr, lock, ip);
+
+	curr->lockdep_depth = i;
+	curr->curr_chain_key = hlock->prev_chain_key;
+
+	WARN(hlock->read, "downgrading a read lock");
+	hlock->read = 1;
+	hlock->acquire_ip = ip;
+
+	if (reacquire_held_locks(curr, depth, i))
+		return 0;
+
+	/*
+	 * I took it apart and put it back together again, except now I have
+	 * these 'spare' parts.. where shall I put them.
+	 */
+	if (DEBUG_LOCKS_WARN_ON(curr->lockdep_depth != depth))
+		return 0;
+	return 1;
+}
+
 /*
  * Remove the lock to the list of currently held locks - this gets
  * called on mutex_unlock()/spin_unlock*() (or on a failed
@@ -3759,6 +3797,23 @@ void lock_set_class(struct lockdep_map *lock, const char *name,
 }
 EXPORT_SYMBOL_GPL(lock_set_class);
 
+void lock_downgrade(struct lockdep_map *lock, unsigned long ip)
+{
+	unsigned long flags;
+
+	if (unlikely(current->lockdep_recursion))
+		return;
+
+	raw_local_irq_save(flags);
+	current->lockdep_recursion = 1;
+	check_flags(flags);
+	if (__lock_downgrade(lock, ip))
+		check_chain_key(current);
+	current->lockdep_recursion = 0;
+	raw_local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(lock_downgrade);
+
 /*
  * We are not always called with irqs disabled - do that here,
  * and also avoid lockdep recursion:

kernel/locking/rwsem.c

@@ -124,10 +124,8 @@ EXPORT_SYMBOL(up_write);
  */
 void downgrade_write(struct rw_semaphore *sem)
 {
-	/*
-	 * lockdep: a downgraded write will live on as a write
-	 * dependency.
-	 */
+	lock_downgrade(&sem->dep_map, _RET_IP_);
+
 	rwsem_set_reader_owned(sem);
 	__downgrade_write(sem);
 }