bcache: kill closure locking code

Also flesh out the documentation a bit

Signed-off-by: Kent Overstreet <[e-mail]>

drivers/md/bcache/closure.c

···
 
 #include "closure.h"
 
-#define CL_FIELD(type, field)					\
-	case TYPE_ ## type:					\
-	return &container_of(cl, struct type, cl)->field
-
-static struct closure_waitlist *closure_waitlist(struct closure *cl)
-{
-	switch (cl->type) {
-		CL_FIELD(closure_with_waitlist, wait);
-	default:
-		return NULL;
-	}
-}
-
 static inline void closure_put_after_sub(struct closure *cl, int flags)
 {
 	int r = flags & CLOSURE_REMAINING_MASK;
···
 			closure_queue(cl);
 		} else {
 			struct closure *parent = cl->parent;
-			struct closure_waitlist *wait = closure_waitlist(cl);
 			closure_fn *destructor = cl->fn;
 
 			closure_debug_destroy(cl);
-
-			smp_mb();
-			atomic_set(&cl->remaining, -1);
-
-			if (wait)
-				closure_wake_up(wait);
 
 			if (destructor)
 				destructor(cl);
···
 }
 EXPORT_SYMBOL(closure_sub);
 
+/**
+ * closure_put - decrement a closure's refcount
+ */
 void closure_put(struct closure *cl)
 {
 	closure_put_after_sub(cl, atomic_dec_return(&cl->remaining));
 }
 EXPORT_SYMBOL(closure_put);
 
-static void set_waiting(struct closure *cl, unsigned long f)
-{
-#ifdef CONFIG_BCACHE_CLOSURES_DEBUG
-	cl->waiting_on = f;
-#endif
-}
-
+/**
+ * closure_wake_up - wake up all closures on a wait list, without memory barrier
+ */
 void __closure_wake_up(struct closure_waitlist *wait_list)
 {
 	struct llist_node *list;
···
 		cl = container_of(reverse, struct closure, list);
 		reverse = llist_next(reverse);
 
-		set_waiting(cl, 0);
+		closure_set_waiting(cl, 0);
 		closure_sub(cl, CLOSURE_WAITING + 1);
 	}
 }
 EXPORT_SYMBOL(__closure_wake_up);
 
-bool closure_wait(struct closure_waitlist *list, struct closure *cl)
+/**
+ * closure_wait - add a closure to a waitlist
+ *
+ * @waitlist will own a ref on @cl, which will be released when
+ * closure_wake_up() is called on @waitlist.
+ *
+ */
+bool closure_wait(struct closure_waitlist *waitlist, struct closure *cl)
 {
 	if (atomic_read(&cl->remaining) & CLOSURE_WAITING)
 		return false;
 
-	set_waiting(cl, _RET_IP_);
+	closure_set_waiting(cl, _RET_IP_);
 	atomic_add(CLOSURE_WAITING + 1, &cl->remaining);
-	llist_add(&cl->list, &list->list);
+	llist_add(&cl->list, &waitlist->list);
 
 	return true;
 }
 EXPORT_SYMBOL(closure_wait);
 
 /**
- * closure_sync() - sleep until a closure a closure has nothing left to wait on
+ * closure_sync - sleep until a closure a closure has nothing left to wait on
  *
  * Sleeps until the refcount hits 1 - the thread that's running the closure owns
  * the last refcount.
···
 	__closure_end_sleep(cl);
 }
 EXPORT_SYMBOL(closure_sync);
-
-/**
- * closure_trylock() - try to acquire the closure, without waiting
- * @cl:		closure to lock
- *
- * Returns true if the closure was succesfully locked.
- */
-bool closure_trylock(struct closure *cl, struct closure *parent)
-{
-	if (atomic_cmpxchg(&cl->remaining, -1,
-			   CLOSURE_REMAINING_INITIALIZER) != -1)
-		return false;
-
-	smp_mb();
-
-	cl->parent = parent;
-	if (parent)
-		closure_get(parent);
-
-	closure_set_ret_ip(cl);
-	closure_debug_create(cl);
-	return true;
-}
-EXPORT_SYMBOL(closure_trylock);
-
-void __closure_lock(struct closure *cl, struct closure *parent,
-		    struct closure_waitlist *wait_list)
-{
-	struct closure wait;
-	closure_init_stack(&wait);
-
-	while (1) {
-		if (closure_trylock(cl, parent))
-			return;
-
-		closure_wait_event(wait_list, &wait,
-				   atomic_read(&cl->remaining) == -1);
-	}
-}
-EXPORT_SYMBOL(__closure_lock);
 
 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
 

drivers/md/bcache/closure.h

···
  * closure - _always_ use continue_at(). Doing so consistently will help
  * eliminate an entire class of particularly pernicious races.
  *
- * For a closure to wait on an arbitrary event, we need to introduce waitlists:
- *
- * struct closure_waitlist list;
- * closure_wait_event(list, cl, condition);
- * closure_wake_up(wait_list);
- *
- * These work analagously to wait_event() and wake_up() - except that instead of
- * operating on the current thread (for wait_event()) and lists of threads, they
- * operate on an explicit closure and lists of closures.
- *
- * Because it's a closure we can now wait either synchronously or
- * asynchronously. closure_wait_event() returns the current value of the
- * condition, and if it returned false continue_at() or closure_sync() can be
- * used to wait for it to become true.
- *
- * It's useful for waiting on things when you can't sleep in the context in
- * which you must check the condition (perhaps a spinlock held, or you might be
- * beneath generic_make_request() - in which case you can't sleep on IO).
- *
- * closure_wait_event() will wait either synchronously or asynchronously,
- * depending on whether the closure is in blocking mode or not. You can pick a
- * mode explicitly with closure_wait_event_sync() and
- * closure_wait_event_async(), which do just what you might expect.
- *
  * Lastly, you might have a wait list dedicated to a specific event, and have no
  * need for specifying the condition - you just want to wait until someone runs
  * closure_wake_up() on the appropriate wait list. In that case, just use
···
  * All this implies that a closure should typically be embedded in a particular
  * struct (which its refcount will normally control the lifetime of), and that
  * struct can very much be thought of as a stack frame.
- *
- * Locking:
- *
- * Closures are based on work items but they can be thought of as more like
- * threads - in that like threads and unlike work items they have a well
- * defined lifetime; they are created (with closure_init()) and eventually
- * complete after a continue_at(cl, NULL, NULL).
- *
- * Suppose you've got some larger structure with a closure embedded in it that's
- * used for periodically doing garbage collection. You only want one garbage
- * collection happening at a time, so the natural thing to do is protect it with
- * a lock. However, it's difficult to use a lock protecting a closure correctly
- * because the unlock should come after the last continue_to() (additionally, if
- * you're using the closure asynchronously a mutex won't work since a mutex has
- * to be unlocked by the same process that locked it).
- *
- * So to make it less error prone and more efficient, we also have the ability
- * to use closures as locks:
- *
- * closure_init_unlocked();
- * closure_trylock();
- *
- * That's all we need for trylock() - the last closure_put() implicitly unlocks
- * it for you.  But for closure_lock(), we also need a wait list:
- *
- * struct closure_with_waitlist frobnicator_cl;
- *
- * closure_init_unlocked(&frobnicator_cl);
- * closure_lock(&frobnicator_cl);
- *
- * A closure_with_waitlist embeds a closure and a wait list - much like struct
- * delayed_work embeds a work item and a timer_list. The important thing is, use
- * it exactly like you would a regular closure and closure_put() will magically
- * handle everything for you.
  */
 
 struct closure;
···
 
 struct closure_waitlist {
 	struct llist_head	list;
-};
-
-enum closure_type {
-	TYPE_closure				= 0,
-	TYPE_closure_with_waitlist		= 1,
-	MAX_CLOSURE_TYPE			= 1,
 };
 
 enum closure_state {
···
 
 	atomic_t		remaining;
 
-	enum closure_type	type;
-
 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
 #define CLOSURE_MAGIC_DEAD	0xc054dead
 #define CLOSURE_MAGIC_ALIVE	0xc054a11e
···
 #endif
 };
 
-struct closure_with_waitlist {
-	struct closure		cl;
-	struct closure_waitlist	wait;
-};
-
-extern unsigned invalid_closure_type(void);
-
-#define __CLOSURE_TYPE(cl, _t)						\
-	  __builtin_types_compatible_p(typeof(cl), struct _t)		\
-		? TYPE_ ## _t :						\
-
-#define __closure_type(cl)						\
-(									\
-	__CLOSURE_TYPE(cl, closure)					\
-	__CLOSURE_TYPE(cl, closure_with_waitlist)			\
-	invalid_closure_type()						\
-)
-
 void closure_sub(struct closure *cl, int v);
 void closure_put(struct closure *cl);
 void __closure_wake_up(struct closure_waitlist *list);
 bool closure_wait(struct closure_waitlist *list, struct closure *cl);
 void closure_sync(struct closure *cl);
-
-bool closure_trylock(struct closure *cl, struct closure *parent);
-void __closure_lock(struct closure *cl, struct closure *parent,
-		    struct closure_waitlist *wait_list);
 
 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
 
···
 #endif
 }
 
-static inline void closure_get(struct closure *cl)
+static inline void closure_set_waiting(struct closure *cl, unsigned long f)
 {
 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
-	BUG_ON((atomic_inc_return(&cl->remaining) &
-		CLOSURE_REMAINING_MASK) <= 1);
-#else
-	atomic_inc(&cl->remaining);
+	cl->waiting_on = f;
 #endif
 }
-
-static inline void closure_set_stopped(struct closure *cl)
-{
-	atomic_sub(CLOSURE_RUNNING, &cl->remaining);
-}
-
-static inline bool closure_is_unlocked(struct closure *cl)
-{
-	return atomic_read(&cl->remaining) == -1;
-}
-
-static inline void do_closure_init(struct closure *cl, struct closure *parent,
-				   bool running)
-{
-	cl->parent = parent;
-	if (parent)
-		closure_get(parent);
-
-	if (running) {
-		closure_debug_create(cl);
-		atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
-	} else
-		atomic_set(&cl->remaining, -1);
-
-	closure_set_ip(cl);
-}
-
-/*
- * Hack to get at the embedded closure if there is one, by doing an unsafe cast:
- * the result of __closure_type() is thrown away, it's used merely for type
- * checking.
- */
-#define __to_internal_closure(cl)				\
-({								\
-	BUILD_BUG_ON(__closure_type(*cl) > MAX_CLOSURE_TYPE);	\
-	(struct closure *) cl;					\
-})
-
-#define closure_init_type(cl, parent, running)			\
-do {								\
-	struct closure *_cl = __to_internal_closure(cl);	\
-	_cl->type = __closure_type(*(cl));			\
-	do_closure_init(_cl, parent, running);			\
-} while (0)
-
-/**
- * closure_init() - Initialize a closure, setting the refcount to 1
- * @cl:		closure to initialize
- * @parent:	parent of the new closure. cl will take a refcount on it for its
- *		lifetime; may be NULL.
- */
-#define closure_init(cl, parent)				\
-	closure_init_type(cl, parent, true)
-
-static inline void closure_init_stack(struct closure *cl)
-{
-	memset(cl, 0, sizeof(struct closure));
-	atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER|CLOSURE_STACK);
-}
-
-/**
- * closure_init_unlocked() - Initialize a closure but leave it unlocked.
- * @cl:		closure to initialize
- *
- * For when the closure will be used as a lock. The closure may not be used
- * until after a closure_lock() or closure_trylock().
- */
-#define closure_init_unlocked(cl)				\
-do {								\
-	memset((cl), 0, sizeof(*(cl)));				\
-	closure_init_type(cl, NULL, false);			\
-} while (0)
-
-/**
- * closure_lock() - lock and initialize a closure.
- * @cl:		the closure to lock
- * @parent:	the new parent for this closure
- *
- * The closure must be of one of the types that has a waitlist (otherwise we
- * wouldn't be able to sleep on contention).
- *
- * @parent has exactly the same meaning as in closure_init(); if non null, the
- * closure will take a reference on @parent which will be released when it is
- * unlocked.
- */
-#define closure_lock(cl, parent)				\
-	__closure_lock(__to_internal_closure(cl), parent, &(cl)->wait)
 
 static inline void __closure_end_sleep(struct closure *cl)
 {
···
 		atomic_add(CLOSURE_SLEEPING, &cl->remaining);
 }
 
-/**
- * closure_wake_up() - wake up all closures on a wait list.
- */
-static inline void closure_wake_up(struct closure_waitlist *list)
+static inline void closure_set_stopped(struct closure *cl)
 {
-	smp_mb();
-	__closure_wake_up(list);
-}
-
-/*
- * Wait on an event, synchronously or asynchronously - analogous to wait_event()
- * but for closures.
- *
- * The loop is oddly structured so as to avoid a race; we must check the
- * condition again after we've added ourself to the waitlist. We know if we were
- * already on the waitlist because closure_wait() returns false; thus, we only
- * schedule or break if closure_wait() returns false. If it returns true, we
- * just loop again - rechecking the condition.
- *
- * The __closure_wake_up() is necessary because we may race with the event
- * becoming true; i.e. we see event false -> wait -> recheck condition, but the
- * thread that made the event true may have called closure_wake_up() before we
- * added ourself to the wait list.
- *
- * We have to call closure_sync() at the end instead of just
- * __closure_end_sleep() because a different thread might've called
- * closure_wake_up() before us and gotten preempted before they dropped the
- * refcount on our closure. If this was a stack allocated closure, that would be
- * bad.
- */
-#define closure_wait_event(list, cl, condition)				\
-({									\
-	typeof(condition) ret;						\
-									\
-	while (1) {							\
-		ret = (condition);					\
-		if (ret) {						\
-			__closure_wake_up(list);			\
-			closure_sync(cl);				\
-			break;						\
-		}							\
-									\
-		__closure_start_sleep(cl);				\
-									\
-		if (!closure_wait(list, cl))				\
-			schedule();					\
-	}								\
-									\
-	ret;								\
-})
-
-static inline void closure_queue(struct closure *cl)
-{
-	struct workqueue_struct *wq = cl->wq;
-	if (wq) {
-		INIT_WORK(&cl->work, cl->work.func);
-		BUG_ON(!queue_work(wq, &cl->work));
-	} else
-		cl->fn(cl);
+	atomic_sub(CLOSURE_RUNNING, &cl->remaining);
 }
 
 static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
···
 	smp_mb__before_atomic_dec();
 }
 
+static inline void closure_queue(struct closure *cl)
+{
+	struct workqueue_struct *wq = cl->wq;
+	if (wq) {
+		INIT_WORK(&cl->work, cl->work.func);
+		BUG_ON(!queue_work(wq, &cl->work));
+	} else
+		cl->fn(cl);
+}
+
+/**
+ * closure_get - increment a closure's refcount
+ */
+static inline void closure_get(struct closure *cl)
+{
+#ifdef CONFIG_BCACHE_CLOSURES_DEBUG
+	BUG_ON((atomic_inc_return(&cl->remaining) &
+		CLOSURE_REMAINING_MASK) <= 1);
+#else
+	atomic_inc(&cl->remaining);
+#endif
+}
+
+/**
+ * closure_init - Initialize a closure, setting the refcount to 1
+ * @cl:		closure to initialize
+ * @parent:	parent of the new closure. cl will take a refcount on it for its
+ *		lifetime; may be NULL.
+ */
+static inline void closure_init(struct closure *cl, struct closure *parent)
+{
+	memset(cl, 0, sizeof(struct closure));
+	cl->parent = parent;
+	if (parent)
+		closure_get(parent);
+
+	atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
+
+	closure_debug_create(cl);
+	closure_set_ip(cl);
+}
+
+static inline void closure_init_stack(struct closure *cl)
+{
+	memset(cl, 0, sizeof(struct closure));
+	atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER|CLOSURE_STACK);
+}
+
+/**
+ * closure_wake_up - wake up all closures on a wait list.
+ */
+static inline void closure_wake_up(struct closure_waitlist *list)
+{
+	smp_mb();
+	__closure_wake_up(list);
+}
+
+/**
+ * continue_at - jump to another function with barrier
+ *
+ * After @cl is no longer waiting on anything (i.e. all outstanding refs have
+ * been dropped with closure_put()), it will resume execution at @fn running out
+ * of @wq (or, if @wq is NULL, @fn will be called by closure_put() directly).
+ *
+ * NOTE: This macro expands to a return in the calling function!
+ *
+ * This is because after calling continue_at() you no longer have a ref on @cl,
+ * and whatever @cl owns may be freed out from under you - a running closure fn
+ * has a ref on its own closure which continue_at() drops.
+ */
 #define continue_at(_cl, _fn, _wq)					\
 do {									\
 	set_closure_fn(_cl, _fn, _wq);					\
···
 	return;								\
 } while (0)
 
+/**
+ * closure_return - finish execution of a closure
+ *
+ * This is used to indicate that @cl is finished: when all outstanding refs on
+ * @cl have been dropped @cl's ref on its parent closure (as passed to
+ * closure_init()) will be dropped, if one was specified - thus this can be
+ * thought of as returning to the parent closure.
+ */
 #define closure_return(_cl)	continue_at((_cl), NULL, NULL)
 
+/**
+ * continue_at_nobarrier - jump to another function without barrier
+ *
+ * Causes @fn to be executed out of @cl, in @wq context (or called directly if
+ * @wq is NULL).
+ *
+ * NOTE: like continue_at(), this macro expands to a return in the caller!
+ *
+ * The ref the caller of continue_at_nobarrier() had on @cl is now owned by @fn,
+ * thus it's not safe to touch anything protected by @cl after a
+ * continue_at_nobarrier().
+ */
 #define continue_at_nobarrier(_cl, _fn, _wq)				\
 do {									\
 	set_closure_fn(_cl, _fn, _wq);					\
···
 	return;								\
 } while (0)
 
+/**
+ * closure_return - finish execution of a closure, with destructor
+ *
+ * Works like closure_return(), except @destructor will be called when all
+ * outstanding refs on @cl have been dropped; @destructor may be used to safely
+ * free the memory occupied by @cl, and it is called with the ref on the parent
+ * closure still held - so @destructor could safely return an item to a
+ * freelist protected by @cl's parent.
+ */
 #define closure_return_with_destructor(_cl, _destructor)		\
 do {									\
 	set_closure_fn(_cl, _destructor, NULL);				\
···
 	return;								\
 } while (0)
 
+/**
+ * closure_call - execute @fn out of a new, uninitialized closure
+ *
+ * Typically used when running out of one closure, and we want to run @fn
+ * asynchronously out of a new closure - @parent will then wait for @cl to
+ * finish.
+ */
 static inline void closure_call(struct closure *cl, closure_fn fn,
 				struct workqueue_struct *wq,
 				struct closure *parent)
 {
 	closure_init(cl, parent);
 	continue_at_nobarrier(cl, fn, wq);
-}
-
-static inline void closure_trylock_call(struct closure *cl, closure_fn fn,
-					struct workqueue_struct *wq,
-					struct closure *parent)
-{
-	if (closure_trylock(cl, parent))
-		continue_at_nobarrier(cl, fn, wq);
 }
 
 #endif /* _LINUX_CLOSURE_H */