lockdep: lock protection locks

On Fri, 2008-08-01 at 16:26 -0700, Linus Torvalds wrote:

> On Fri, 1 Aug 2008, David Miller wrote:
> >
> > Taking more than a few locks of the same class at once is bad
> > news and it's better to find an alternative method.
>
> It's not always wrong.
>
> If you can guarantee that anybody that takes more than one lock of a
> particular class will always take a single top-level lock _first_, then
> that's all good. You can obviously screw up and take the same lock _twice_
> (which will deadlock), but at least you cannot get into ABBA situations.
>
> So maybe the right thing to do is to just teach lockdep about "lock
> protection locks". That would have solved the multi-queue issues for
> networking too - all the actual network drivers would still have taken
> just their single queue lock, but the one case that needs to take all of
> them would have taken a separate top-level lock first.
>
> Never mind that the multi-queue locks were always taken in the same order:
> it's never wrong to just have some top-level serialization, and anybody
> who needs to take <n> locks might as well do <n+1>, because they sure as
> hell aren't going to be on _any_ fastpaths.
>
> So the simplest solution really sounds like just teaching lockdep about
> that one special case. It's not "nesting" exactly, although it's obviously
> related to it.

Do as Linus suggested. The lock protection lock is called nest_lock.

Note that we still have the MAX_LOCK_DEPTH (48) limit to consider, so anything
that spills that it still up shit creek.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 21 insertions, 5 deletions

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index d3c72ad..410c336 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1372,18 +1372,32 @@ check_deadlock(struct task_struct *curr, struct held_lock *next,
 	       struct lockdep_map *next_instance, int read)
 {
 	struct held_lock *prev;
+	struct held_lock *nest = NULL;
 	int i;
 
 	for (i = 0; i < curr->lockdep_depth; i++) {
 		prev = curr->held_locks + i;
+
+		if (prev->instance == next->nest_lock)
+			nest = prev;
+
 		if (hlock_class(prev) != hlock_class(next))
 			continue;
+
 		/*
 		 * Allow read-after-read recursion of the same
 		 * lock class (i.e. read_lock(lock)+read_lock(lock)):
 		 */
 		if ((read == 2) && prev->read)
 			return 2;
+
+		/*
+		 * We're holding the nest_lock, which serializes this lock's
+		 * nesting behaviour.
+		 */
+		if (nest)
+			return 2;
+
 		return print_deadlock_bug(curr, prev, next);
 	}
 	return 1;
@@ -2507,7 +2521,7 @@ EXPORT_SYMBOL_GPL(lockdep_init_map);
  */
 static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 			  int trylock, int read, int check, int hardirqs_off,
-			  unsigned long ip)
+			  struct lockdep_map *nest_lock, unsigned long ip)
 {
 	struct task_struct *curr = current;
 	struct lock_class *class = NULL;
@@ -2566,6 +2580,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	hlock->class_idx = class - lock_classes + 1;
 	hlock->acquire_ip = ip;
 	hlock->instance = lock;
+	hlock->nest_lock = nest_lock;
 	hlock->trylock = trylock;
 	hlock->read = read;
 	hlock->check = check;
@@ -2717,7 +2732,7 @@ found_it:
 		if (!__lock_acquire(hlock->instance,
 			hlock_class(hlock)->subclass, hlock->trylock,
 				hlock->read, hlock->check, hlock->hardirqs_off,
-				hlock->acquire_ip))
+				hlock->nest_lock, hlock->acquire_ip))
 			return 0;
 	}
 
@@ -2778,7 +2793,7 @@ found_it:
 		if (!__lock_acquire(hlock->instance,
 			hlock_class(hlock)->subclass, hlock->trylock,
 				hlock->read, hlock->check, hlock->hardirqs_off,
-				hlock->acquire_ip))
+				hlock->nest_lock, hlock->acquire_ip))
 			return 0;
 	}
 
@@ -2915,7 +2930,8 @@ EXPORT_SYMBOL_GPL(lock_set_subclass);
  * and also avoid lockdep recursion:
  */
 void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
-			  int trylock, int read, int check, unsigned long ip)
+			  int trylock, int read, int check,
+			  struct lockdep_map *nest_lock, unsigned long ip)
 {
 	unsigned long flags;
 
@@ -2930,7 +2946,7 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 	current->lockdep_recursion = 1;
 	__lock_acquire(lock, subclass, trylock, read, check,
-		       irqs_disabled_flags(flags), ip);
+		       irqs_disabled_flags(flags), nest_lock, ip);
 	current->lockdep_recursion = 0;
 	raw_local_irq_restore(flags);
 }