bcache: kill closure locking code

Also flesh out the documentation a bit

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

2 files changed, 123 insertions, 313 deletions

diff --git a/drivers/md/bcache/closure.c b/drivers/md/bcache/closure.c
index dfff2410322e..7a228de95fd7 100644
--- a/drivers/md/bcache/closure.c
+++ b/drivers/md/bcache/closure.c
@@ -11,19 +11,6 @@
 
 #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;
@@ -42,17 +29,10 @@ static inline void closure_put_after_sub(struct closure *cl, int flags)
                         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);
 
@@ -69,19 +49,18 @@ void closure_sub(struct closure *cl, int v)
 }
 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;
@@ -106,27 +85,34 @@ void __closure_wake_up(struct closure_waitlist *wait_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.
@@ -148,46 +134,6 @@ void closure_sync(struct closure *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
 
 static LIST_HEAD(closure_list);
diff --git a/drivers/md/bcache/closure.h b/drivers/md/bcache/closure.h
index d29b773b863f..7ef7461912be 100644
--- a/drivers/md/bcache/closure.h
+++ b/drivers/md/bcache/closure.h
@@ -72,30 +72,6 @@
  * 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
@@ -121,40 +97,6 @@
  * 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;
@@ -164,12 +106,6 @@ 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 {
         /*
          * CLOSURE_WAITING: Set iff the closure is on a waitlist. Must be set by
@@ -224,8 +160,6 @@ struct closure {
 
         atomic_t                remaining;
 
-        enum closure_type       type;
-
 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
 #define CLOSURE_MAGIC_DEAD      0xc054dead
 #define CLOSURE_MAGIC_ALIVE     0xc054a11e
@@ -237,34 +171,12 @@ struct closure {
 #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
 
 void closure_debug_init(void);
@@ -293,123 +205,97 @@ static inline void closure_set_ret_ip(struct closure *cl)
 #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_end_sleep(struct closure *cl)
+{
+        __set_current_state(TASK_RUNNING);
+
+        if (atomic_read(&cl->remaining) & CLOSURE_SLEEPING)
+                atomic_sub(CLOSURE_SLEEPING, &cl->remaining);
+}
+
+static inline void __closure_start_sleep(struct closure *cl)
+{
+        closure_set_ip(cl);
+        cl->task = current;
+        set_current_state(TASK_UNINTERRUPTIBLE);
+
+        if (!(atomic_read(&cl->remaining) & CLOSURE_SLEEPING))
+                atomic_add(CLOSURE_SLEEPING, &cl->remaining);
+}
+
 static inline void closure_set_stopped(struct closure *cl)
 {
         atomic_sub(CLOSURE_RUNNING, &cl->remaining);
 }
 
-static inline bool closure_is_unlocked(struct closure *cl)
+static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
+                                  struct workqueue_struct *wq)
 {
-        return atomic_read(&cl->remaining) == -1;
+        BUG_ON(object_is_on_stack(cl));
+        closure_set_ip(cl);
+        cl->fn = fn;
+        cl->wq = wq;
+        /* between atomic_dec() in closure_put() */
+        smp_mb__before_atomic_dec();
 }
 
-static inline void do_closure_init(struct closure *cl, struct closure *parent,
-                                   bool running)
+static inline void closure_queue(struct closure *cl)
 {
-        cl->parent = parent;
-        if (parent)
-                closure_get(parent);
-
-        if (running) {
-                closure_debug_create(cl);
-                atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
+        struct workqueue_struct *wq = cl->wq;
+        if (wq) {
+                INIT_WORK(&cl->work, cl->work.func);
+                BUG_ON(!queue_work(wq, &cl->work));
         } else
-                atomic_set(&cl->remaining, -1);
-
-        closure_set_ip(cl);
+                cl->fn(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.
+/**
+ * closure_get - increment a closure's refcount
  */
-#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)
+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
+ * 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)
+static inline void closure_init(struct closure *cl, struct closure *parent)
 {
         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)
+        cl->parent = parent;
+        if (parent)
+                closure_get(parent);
 
-/**
- * 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)
+        atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
 
-static inline void __closure_end_sleep(struct closure *cl)
-{
-        __set_current_state(TASK_RUNNING);
-
-        if (atomic_read(&cl->remaining) & CLOSURE_SLEEPING)
-                atomic_sub(CLOSURE_SLEEPING, &cl->remaining);
+        closure_debug_create(cl);
+        closure_set_ip(cl);
 }
 
-static inline void __closure_start_sleep(struct closure *cl)
+static inline void closure_init_stack(struct closure *cl)
 {
-        closure_set_ip(cl);
-        cl->task = current;
-        set_current_state(TASK_UNINTERRUPTIBLE);
-
-        if (!(atomic_read(&cl->remaining) & CLOSURE_SLEEPING))
-                atomic_add(CLOSURE_SLEEPING, &cl->remaining);
+        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.
+ * closure_wake_up - wake up all closures on a wait list.
  */
 static inline void closure_wake_up(struct closure_waitlist *list)
 {
@@ -417,69 +303,19 @@ static inline void closure_wake_up(struct closure_waitlist *list)
         __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.
+/**
+ * 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 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);
-}
-
-static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
-                                  struct workqueue_struct *wq)
-{
-        BUG_ON(object_is_on_stack(cl));
-        closure_set_ip(cl);
-        cl->fn = fn;
-        cl->wq = wq;
-        /* between atomic_dec() in closure_put() */
-        smp_mb__before_atomic_dec();
-}
-
 #define continue_at(_cl, _fn, _wq)                                      \
 do {                                                                    \
         set_closure_fn(_cl, _fn, _wq);                                  \
@@ -487,8 +323,28 @@ do {									\
         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);                                  \
@@ -496,6 +352,15 @@ do {									\
         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);                         \
@@ -503,6 +368,13 @@ do {									\
         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)
@@ -511,12 +383,4 @@ static inline void closure_call(struct closure *cl, closure_fn fn,
         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 */