text: /* SPDX-License-Identifier: GPL-2.0 */
  #ifndef _LINUX_CLOSURE_H
  #define _LINUX_CLOSURE_H
  
  #include <linux/llist.h>
  #include <linux/sched.h>
  #include <linux/sched/task_stack.h>
  #include <linux/workqueue.h>
  
  /*
   * Closure is perhaps the most overused and abused term in computer science, but
   * since I've been unable to come up with anything better you're stuck with it
   * again.
   *
   * What are closures?
   *
   * They embed a refcount. The basic idea is they count "things that are in
   * progress" - in flight bios, some other thread that's doing something else -
   * anything you might want to wait on.
   *
   * The refcount may be manipulated with closure_get() and closure_put().
   * closure_put() is where many of the interesting things happen, when it causes
   * the refcount to go to 0.
   *
   * Closures can be used to wait on things both synchronously and asynchronously,
   * and synchronous and asynchronous use can be mixed without restriction. To
   * wait synchronously, use closure_sync() - you will sleep until your closure's
   * refcount hits 1.
   *
   * To wait asynchronously, use
   *   continue_at(cl, next_function, workqueue);
   *
   * passing it, as you might expect, the function to run when nothing is pending
   * and the workqueue to run that function out of.
   *
   * continue_at() also, critically, requires a 'return' immediately following the
   * location where this macro is referenced, to return to the calling function.
   * There's good reason for this.
   *
   * To use safely closures asynchronously, they must always have a refcount while
   * they are running owned by the thread that is running them. Otherwise, suppose
   * you submit some bios and wish to have a function run when they all complete:
   *
   * foo_endio(struct bio *bio)
   * {
   *	closure_put(cl);
   * }
   *
   * closure_init(cl);
   *
   * do_stuff();
   * closure_get(cl);
   * bio1->bi_endio = foo_endio;
   * bio_submit(bio1);
   *
   * do_more_stuff();
   * closure_get(cl);
   * bio2->bi_endio = foo_endio;
   * bio_submit(bio2);
   *
   * continue_at(cl, complete_some_read, system_wq);
   *
   * If closure's refcount started at 0, complete_some_read() could run before the
   * second bio was submitted - which is almost always not what you want! More
   * importantly, it wouldn't be possible to say whether the original thread or
   * complete_some_read()'s thread owned the closure - and whatever state it was
   * associated with!
   *
   * So, closure_init() initializes a closure's refcount to 1 - and when a
   * closure_fn is run, the refcount will be reset to 1 first.
   *
   * Then, the rule is - if you got the refcount with closure_get(), release it
   * with closure_put() (i.e, in a bio->bi_endio function). If you have a refcount
   * on a closure because you called closure_init() or you were run out of a
   * closure - _always_ use continue_at(). Doing so consistently will help
   * eliminate an entire class of particularly pernicious races.
   *
   * 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
   * closure_wait(). It will return either true or false, depending on whether the
   * closure was already on a wait list or not - a closure can only be on one wait
   * list at a time.
   *
   * Parents:
   *
   * closure_init() takes two arguments - it takes the closure to initialize, and
   * a (possibly null) parent.
   *
   * If parent is non null, the new closure will have a refcount for its lifetime;
   * a closure is considered to be "finished" when its refcount hits 0 and the
   * function to run is null. Hence
   *
   * continue_at(cl, NULL, NULL);
   *
   * returns up the (spaghetti) stack of closures, precisely like normal return
   * returns up the C stack. continue_at() with non null fn is better thought of
   * as doing a tail call.
   *
   * 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.
   */
  
  struct closure;
  struct closure_syncer;
  typedef void (closure_fn) (struct closure *);
  extern struct dentry *bcache_debug;
  
  struct closure_waitlist {
  	struct llist_head	list;
  };
  
  enum closure_state {
  	/*
  	 * CLOSURE_WAITING: Set iff the closure is on a waitlist. Must be set by
  	 * the thread that owns the closure, and cleared by the thread that's
  	 * waking up the closure.
  	 *
  	 * The rest are for debugging and don't affect behaviour:
  	 *
  	 * CLOSURE_RUNNING: Set when a closure is running (i.e. by
  	 * closure_init() and when closure_put() runs then next function), and
  	 * must be cleared before remaining hits 0. Primarily to help guard
  	 * against incorrect usage and accidentally transferring references.
  	 * continue_at() and closure_return() clear it for you, if you're doing
  	 * something unusual you can use closure_set_dead() which also helps
  	 * annotate where references are being transferred.
  	 */
  
  	CLOSURE_BITS_START	= (1U << 26),
  	CLOSURE_DESTRUCTOR	= (1U << 26),
  	CLOSURE_WAITING		= (1U << 28),
  	CLOSURE_RUNNING		= (1U << 30),
  };
  
  #define CLOSURE_GUARD_MASK					\
  	((CLOSURE_DESTRUCTOR|CLOSURE_WAITING|CLOSURE_RUNNING) << 1)
  
  #define CLOSURE_REMAINING_MASK		(CLOSURE_BITS_START - 1)
  #define CLOSURE_REMAINING_INITIALIZER	(1|CLOSURE_RUNNING)
  
  struct closure {
  	union {
  		struct {
  			struct workqueue_struct *wq;
  			struct closure_syncer	*s;
  			struct llist_node	list;
  			closure_fn		*fn;
  		};
  		struct work_struct	work;
  	};
  
  	struct closure		*parent;
  
  	atomic_t		remaining;
  
  #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
  #define CLOSURE_MAGIC_DEAD	0xc054dead
  #define CLOSURE_MAGIC_ALIVE	0xc054a11e
  
  	unsigned int		magic;
  	struct list_head	all;
  	unsigned long		ip;
  	unsigned long		waiting_on;
  #endif
  };
  
  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);
  
  /**
   * 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.
   */
  static inline void closure_sync(struct closure *cl)
  {
  	if ((atomic_read(&cl->remaining) & CLOSURE_REMAINING_MASK) != 1)
  		__closure_sync(cl);
  }
  
  #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
  
  void closure_debug_init(void);
  void closure_debug_create(struct closure *cl);
  void closure_debug_destroy(struct closure *cl);
  
  #else
  
  static inline void closure_debug_init(void) {}
  static inline void closure_debug_create(struct closure *cl) {}
  static inline void closure_debug_destroy(struct closure *cl) {}
  
  #endif
  
  static inline void closure_set_ip(struct closure *cl)
  {
  #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
  	cl->ip = _THIS_IP_;
  #endif
  }
  
  static inline void closure_set_ret_ip(struct closure *cl)
  {
  #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
  	cl->ip = _RET_IP_;
  #endif
  }
  
  static inline void closure_set_waiting(struct closure *cl, unsigned long f)
  {
  #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
  	cl->waiting_on = f;
  #endif
  }
  
  static inline void closure_set_stopped(struct closure *cl)
  {
  	atomic_sub(CLOSURE_RUNNING, &cl->remaining);
  }
  
  static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
  				  struct workqueue_struct *wq)
  {
  	closure_set_ip(cl);
  	cl->fn = fn;
  	cl->wq = wq;
  	/* between atomic_dec() in closure_put() */
  	smp_mb__before_atomic();
  }
  
  static inline void closure_queue(struct closure *cl)
  {
  	struct workqueue_struct *wq = cl->wq;
  	/**
  	 * Changes made to closure, work_struct, or a couple of other structs
  	 * may cause work.func not pointing to the right location.
  	 */
  	BUILD_BUG_ON(offsetof(struct closure, fn)
  		     != offsetof(struct work_struct, func));
  	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_wake_up - wake up all closures on a wait list,
   *		     with memory barrier
   */
  static inline void closure_wake_up(struct closure_waitlist *list)
  {
  	/* Memory barrier for the wait 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).
   *
   * 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.
   *
   * Note you are expected to immediately return after using this macro.
   */
  #define continue_at(_cl, _fn, _wq)					\
  do {									\
  	set_closure_fn(_cl, _fn, _wq);					\
  	closure_sub(_cl, CLOSURE_RUNNING + 1);				\
  } 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).
   *
   * 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);					\
  	closure_queue(_cl);						\
  } while (0)
  
  /**
   * closure_return_with_destructor - 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);				\
  	closure_sub(_cl, CLOSURE_RUNNING - CLOSURE_DESTRUCTOR + 1);	\
  } 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);
  }
  
  #endif /* _LINUX_CLOSURE_H */
  