/* rwsem.c: R/W semaphores: contention handling functions
   *
   * Written by David Howells (dhowells@redhat.com).
   * Derived from arch/i386/kernel/semaphore.c

   *
   * Writer lock-stealing by Alex Shi <alex.shi@intel.com>

   * and Michel Lespinasse <walken@google.com>

   *
   * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
   * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.

   */
  #include <linux/rwsem.h>
  #include <linux/sched.h>
  #include <linux/init.h>

  #include <linux/export.h>

  #include <linux/sched/rt.h>

  #include <linux/osq_lock.h>

  #include "rwsem.h"

  /*

   * Guide to the rw_semaphore's count field for common values.
   * (32-bit case illustrated, similar for 64-bit)
   *
   * 0x0000000X	(1) X readers active or attempting lock, no writer waiting
   *		    X = #active_readers + #readers attempting to lock
   *		    (X*ACTIVE_BIAS)
   *
   * 0x00000000	rwsem is unlocked, and no one is waiting for the lock or
   *		attempting to read lock or write lock.
   *
   * 0xffff000X	(1) X readers active or attempting lock, with waiters for lock
   *		    X = #active readers + # readers attempting lock
   *		    (X*ACTIVE_BIAS + WAITING_BIAS)
   *		(2) 1 writer attempting lock, no waiters for lock
   *		    X-1 = #active readers + #readers attempting lock
   *		    ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
   *		(3) 1 writer active, no waiters for lock
   *		    X-1 = #active readers + #readers attempting lock
   *		    ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
   *
   * 0xffff0001	(1) 1 reader active or attempting lock, waiters for lock
   *		    (WAITING_BIAS + ACTIVE_BIAS)
   *		(2) 1 writer active or attempting lock, no waiters for lock
   *		    (ACTIVE_WRITE_BIAS)
   *
   * 0xffff0000	(1) There are writers or readers queued but none active
   *		    or in the process of attempting lock.
   *		    (WAITING_BIAS)
   *		Note: writer can attempt to steal lock for this count by adding
   *		ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
   *
   * 0xfffe0001	(1) 1 writer active, or attempting lock. Waiters on queue.
   *		    (ACTIVE_WRITE_BIAS + WAITING_BIAS)
   *
   * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
   *	 the count becomes more than 0 for successful lock acquisition,
   *	 i.e. the case where there are only readers or nobody has lock.
   *	 (1st and 2nd case above).
   *
   *	 Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
   *	 checking the count becomes ACTIVE_WRITE_BIAS for successful lock
   *	 acquisition (i.e. nobody else has lock or attempts lock).  If
   *	 unsuccessful, in rwsem_down_write_failed, we'll check to see if there
   *	 are only waiters but none active (5th case above), and attempt to
   *	 steal the lock.
   *
   */
  
  /*

   * Initialize an rwsem:
   */
  void __init_rwsem(struct rw_semaphore *sem, const char *name,
  		  struct lock_class_key *key)
  {
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  	/*
  	 * Make sure we are not reinitializing a held semaphore:
  	 */
  	debug_check_no_locks_freed((void *)sem, sizeof(*sem));

  	lockdep_init_map(&sem->dep_map, name, key, 0);

  #endif

  	atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);

  	raw_spin_lock_init(&sem->wait_lock);

  	INIT_LIST_HEAD(&sem->wait_list);

  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER

  	sem->owner = NULL;

  	osq_lock_init(&sem->osq);

  #endif

  }
  
  EXPORT_SYMBOL(__init_rwsem);

  enum rwsem_waiter_type {
  	RWSEM_WAITING_FOR_WRITE,
  	RWSEM_WAITING_FOR_READ
  };

  struct rwsem_waiter {
  	struct list_head list;
  	struct task_struct *task;

  	enum rwsem_waiter_type type;

  };

  enum rwsem_wake_type {
  	RWSEM_WAKE_ANY,		/* Wake whatever's at head of wait list */
  	RWSEM_WAKE_READERS,	/* Wake readers only */
  	RWSEM_WAKE_READ_OWNED	/* Waker thread holds the read lock */
  };

  /*
   * handle the lock release when processes blocked on it that can now run
   * - if we come here from up_xxxx(), then:
   *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
   *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)

   * - there must be someone on the queue

   * - the wait_lock must be held by the caller
   * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
   *   to actually wakeup the blocked task(s) and drop the reference count,
   *   preferably when the wait_lock is released

   * - woken process blocks are discarded from the list after having task zeroed

   * - writers are only marked woken if downgrading is false

   */

  static void __rwsem_mark_wake(struct rw_semaphore *sem,
  			      enum rwsem_wake_type wake_type,
  			      struct wake_q_head *wake_q)

  {

  	struct rwsem_waiter *waiter, *tmp;
  	long oldcount, woken = 0, adjustment = 0;

  	/*
  	 * Take a peek at the queue head waiter such that we can determine
  	 * the wakeup(s) to perform.
  	 */
  	waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);

  	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {

  		if (wake_type == RWSEM_WAKE_ANY) {
  			/*
  			 * Mark writer at the front of the queue for wakeup.
  			 * Until the task is actually later awoken later by
  			 * the caller, other writers are able to steal it.
  			 * Readers, on the other hand, will block as they
  			 * will notice the queued writer.

  			 */

  			wake_q_add(wake_q, waiter->task);
  		}

  
  		return;

  	}

  	/*
  	 * Writers might steal the lock before we grant it to the next reader.

  	 * We prefer to do the first reader grant before counting readers
  	 * so we can bail out early if a writer stole the lock.

  	 */

  	if (wake_type != RWSEM_WAKE_READ_OWNED) {
  		adjustment = RWSEM_ACTIVE_READ_BIAS;
   try_reader_grant:

  		oldcount = atomic_long_fetch_add(adjustment, &sem->count);

  		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {

  			/*
  			 * If the count is still less than RWSEM_WAITING_BIAS
  			 * after removing the adjustment, it is assumed that
  			 * a writer has stolen the lock. We have to undo our
  			 * reader grant.
  			 */
  			if (atomic_long_add_return(-adjustment, &sem->count) <
  			    RWSEM_WAITING_BIAS)

  				return;

  			/* Last active locker left. Retry waking readers. */
  			goto try_reader_grant;
  		}

  		/*
  		 * It is not really necessary to set it to reader-owned here,
  		 * but it gives the spinners an early indication that the
  		 * readers now have the lock.
  		 */
  		rwsem_set_reader_owned(sem);

  	}

  	/*
  	 * Grant an infinite number of read locks to the readers at the front

  	 * of the queue. We know that woken will be at least 1 as we accounted
  	 * for above. Note we increment the 'active part' of the count by the
  	 * number of readers before waking any processes up.

  	 */

  	list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
  		struct task_struct *tsk;

  		if (waiter->type == RWSEM_WAITING_FOR_WRITE)

  			break;

  		woken++;

  		tsk = waiter->task;

  
  		wake_q_add(wake_q, tsk);

  		list_del(&waiter->list);

  		/*

  		 * Ensure that the last operation is setting the reader
  		 * waiter to nil such that rwsem_down_read_failed() cannot
  		 * race with do_exit() by always holding a reference count
  		 * to the task to wakeup.

  		 */

  		smp_store_release(&waiter->task, NULL);

  	}

  	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
  	if (list_empty(&sem->wait_list)) {
  		/* hit end of list above */
  		adjustment -= RWSEM_WAITING_BIAS;
  	}
  
  	if (adjustment)
  		atomic_long_add(adjustment, &sem->count);

  }

  /*

   * Wait for the read lock to be granted

   */

  __visible

  struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)

  {

  	long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;

  	struct rwsem_waiter waiter;

  	struct task_struct *tsk = current;

  	WAKE_Q(wake_q);

  	waiter.task = tsk;

  	waiter.type = RWSEM_WAITING_FOR_READ;

  	raw_spin_lock_irq(&sem->wait_lock);

  	if (list_empty(&sem->wait_list))
  		adjustment += RWSEM_WAITING_BIAS;

  	list_add_tail(&waiter.list, &sem->wait_list);

  	/* we're now waiting on the lock, but no longer actively locking */

  	count = atomic_long_add_return(adjustment, &sem->count);

  	/*
  	 * If there are no active locks, wake the front queued process(es).

  	 *
  	 * If there are no writers and we are first in the queue,
  	 * wake our own waiter to join the existing active readers !
  	 */
  	if (count == RWSEM_WAITING_BIAS ||
  	    (count > RWSEM_WAITING_BIAS &&
  	     adjustment != -RWSEM_ACTIVE_READ_BIAS))

  		__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

  	raw_spin_unlock_irq(&sem->wait_lock);

  	wake_up_q(&wake_q);

  
  	/* wait to be given the lock */

  	while (true) {
  		set_task_state(tsk, TASK_UNINTERRUPTIBLE);

  		if (!waiter.task)

  			break;
  		schedule();

  	}

  	__set_task_state(tsk, TASK_RUNNING);

  	return sem;
  }

  EXPORT_SYMBOL(rwsem_down_read_failed);

  /*
   * This function must be called with the sem->wait_lock held to prevent
   * race conditions between checking the rwsem wait list and setting the
   * sem->count accordingly.
   */

  static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
  {

  	/*

  	 * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.

  	 */

  	if (count != RWSEM_WAITING_BIAS)
  		return false;
  
  	/*
  	 * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
  	 * are other tasks on the wait list, we need to add on WAITING_BIAS.
  	 */
  	count = list_is_singular(&sem->wait_list) ?
  			RWSEM_ACTIVE_WRITE_BIAS :
  			RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;

  	if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
  							== RWSEM_WAITING_BIAS) {

  		rwsem_set_owner(sem);

  		return true;

  	}

  	return false;
  }

  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER

  /*

   * Try to acquire write lock before the writer has been put on wait queue.
   */
  static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
  {

  	long old, count = atomic_long_read(&sem->count);

  
  	while (true) {
  		if (!(count == 0 || count == RWSEM_WAITING_BIAS))
  			return false;

  		old = atomic_long_cmpxchg_acquire(&sem->count, count,

  				      count + RWSEM_ACTIVE_WRITE_BIAS);

  		if (old == count) {
  			rwsem_set_owner(sem);

  			return true;

  		}

  
  		count = old;
  	}
  }
  
  static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
  {
  	struct task_struct *owner;

  	bool ret = true;

  
  	if (need_resched())

  		return false;

  
  	rcu_read_lock();

  	owner = READ_ONCE(sem->owner);

  	if (!rwsem_owner_is_writer(owner)) {

  		/*

  		 * Don't spin if the rwsem is readers owned.

  		 */

  		ret = !rwsem_owner_is_reader(owner);

  		goto done;
  	}

  	ret = owner->on_cpu;
  done:
  	rcu_read_unlock();
  	return ret;

  }

  /*
   * Return true only if we can still spin on the owner field of the rwsem.
   */
  static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)

  {

  	struct task_struct *owner = READ_ONCE(sem->owner);
  
  	if (!rwsem_owner_is_writer(owner))
  		goto out;

  	rcu_read_lock();

  	while (sem->owner == owner) {
  		/*
  		 * Ensure we emit the owner->on_cpu, dereference _after_
  		 * checking sem->owner still matches owner, if that fails,
  		 * owner might point to free()d memory, if it still matches,
  		 * the rcu_read_lock() ensures the memory stays valid.
  		 */
  		barrier();
  
  		/* abort spinning when need_resched or owner is not running */
  		if (!owner->on_cpu || need_resched()) {

  			rcu_read_unlock();
  			return false;
  		}

  		cpu_relax_lowlatency();

  	}
  	rcu_read_unlock();

  out:

  	/*

  	 * If there is a new owner or the owner is not set, we continue
  	 * spinning.

  	 */

  	return !rwsem_owner_is_reader(READ_ONCE(sem->owner));

  }
  
  static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
  {

  	bool taken = false;
  
  	preempt_disable();
  
  	/* sem->wait_lock should not be held when doing optimistic spinning */
  	if (!rwsem_can_spin_on_owner(sem))
  		goto done;
  
  	if (!osq_lock(&sem->osq))
  		goto done;

  	/*
  	 * Optimistically spin on the owner field and attempt to acquire the
  	 * lock whenever the owner changes. Spinning will be stopped when:
  	 *  1) the owning writer isn't running; or
  	 *  2) readers own the lock as we can't determine if they are
  	 *     actively running or not.
  	 */
  	while (rwsem_spin_on_owner(sem)) {

  		/*

  		 * Try to acquire the lock

  		 */

  		if (rwsem_try_write_lock_unqueued(sem)) {
  			taken = true;
  			break;
  		}
  
  		/*
  		 * When there's no owner, we might have preempted between the
  		 * owner acquiring the lock and setting the owner field. If
  		 * we're an RT task that will live-lock because we won't let
  		 * the owner complete.
  		 */

  		if (!sem->owner && (need_resched() || rt_task(current)))

  			break;
  
  		/*
  		 * The cpu_relax() call is a compiler barrier which forces
  		 * everything in this loop to be re-loaded. We don't need
  		 * memory barriers as we'll eventually observe the right
  		 * values at the cost of a few extra spins.
  		 */

  		cpu_relax_lowlatency();

  	}
  	osq_unlock(&sem->osq);
  done:
  	preempt_enable();
  	return taken;
  }

  /*
   * Return true if the rwsem has active spinner
   */
  static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
  {
  	return osq_is_locked(&sem->osq);
  }

  #else
  static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
  {
  	return false;
  }

  
  static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
  {
  	return false;
  }

  #endif
  
  /*
   * Wait until we successfully acquire the write lock

   */

  static inline struct rw_semaphore *
  __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)

  {

  	long count;
  	bool waiting = true; /* any queued threads before us */

  	struct rwsem_waiter waiter;

  	struct rw_semaphore *ret = sem;

  	WAKE_Q(wake_q);

  	/* undo write bias from down_write operation, stop active locking */

  	count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);

  
  	/* do optimistic spinning and steal lock if possible */
  	if (rwsem_optimistic_spin(sem))
  		return sem;
  
  	/*
  	 * Optimistic spinning failed, proceed to the slowpath
  	 * and block until we can acquire the sem.
  	 */
  	waiter.task = current;

  	waiter.type = RWSEM_WAITING_FOR_WRITE;

  
  	raw_spin_lock_irq(&sem->wait_lock);

  
  	/* account for this before adding a new element to the list */

  	if (list_empty(&sem->wait_list))

  		waiting = false;

  	list_add_tail(&waiter.list, &sem->wait_list);
  
  	/* we're now waiting on the lock, but no longer actively locking */

  	if (waiting) {

  		count = atomic_long_read(&sem->count);

  		/*

  		 * If there were already threads queued before us and there are
  		 * no active writers, the lock must be read owned; so we try to
  		 * wake any read locks that were queued ahead of us.

  		 */

  		if (count > RWSEM_WAITING_BIAS) {
  			WAKE_Q(wake_q);

  			__rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);

  			/*
  			 * The wakeup is normally called _after_ the wait_lock
  			 * is released, but given that we are proactively waking
  			 * readers we can deal with the wake_q overhead as it is
  			 * similar to releasing and taking the wait_lock again
  			 * for attempting rwsem_try_write_lock().
  			 */
  			wake_up_q(&wake_q);
  		}

  
  	} else

  		count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);

  	/* wait until we successfully acquire the lock */

  	set_current_state(state);

  	while (true) {

  		if (rwsem_try_write_lock(count, sem))
  			break;

  		raw_spin_unlock_irq(&sem->wait_lock);

  
  		/* Block until there are no active lockers. */
  		do {

  			if (signal_pending_state(state, current))
  				goto out_nolock;

  			schedule();

  			set_current_state(state);

  		} while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);

  		raw_spin_lock_irq(&sem->wait_lock);

  	}

  	__set_current_state(TASK_RUNNING);

  	list_del(&waiter.list);
  	raw_spin_unlock_irq(&sem->wait_lock);

  	return ret;

  
  out_nolock:
  	__set_current_state(TASK_RUNNING);
  	raw_spin_lock_irq(&sem->wait_lock);
  	list_del(&waiter.list);
  	if (list_empty(&sem->wait_list))

  		atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);

  	else

  		__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

  	raw_spin_unlock_irq(&sem->wait_lock);

  	wake_up_q(&wake_q);

  
  	return ERR_PTR(-EINTR);

  }
  
  __visible struct rw_semaphore * __sched
  rwsem_down_write_failed(struct rw_semaphore *sem)
  {
  	return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);

  }

  EXPORT_SYMBOL(rwsem_down_write_failed);

  __visible struct rw_semaphore * __sched
  rwsem_down_write_failed_killable(struct rw_semaphore *sem)
  {
  	return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
  }
  EXPORT_SYMBOL(rwsem_down_write_failed_killable);

  /*
   * handle waking up a waiter on the semaphore
   * - up_read/up_write has decremented the active part of count if we come here
   */

  __visible

  struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)

  {
  	unsigned long flags;

  	WAKE_Q(wake_q);

  	/*
  	 * If a spinner is present, it is not necessary to do the wakeup.
  	 * Try to do wakeup only if the trylock succeeds to minimize
  	 * spinlock contention which may introduce too much delay in the
  	 * unlock operation.
  	 *
  	 *    spinning writer		up_write/up_read caller
  	 *    ---------------		-----------------------
  	 * [S]   osq_unlock()		[L]   osq
  	 *	 MB			      RMB
  	 * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
  	 *
  	 * Here, it is important to make sure that there won't be a missed
  	 * wakeup while the rwsem is free and the only spinning writer goes
  	 * to sleep without taking the rwsem. Even when the spinning writer
  	 * is just going to break out of the waiting loop, it will still do
  	 * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
  	 * rwsem_has_spinner() is true, it will guarantee at least one
  	 * trylock attempt on the rwsem later on.
  	 */
  	if (rwsem_has_spinner(sem)) {
  		/*
  		 * The smp_rmb() here is to make sure that the spinner
  		 * state is consulted before reading the wait_lock.
  		 */
  		smp_rmb();
  		if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
  			return sem;
  		goto locked;
  	}

  	raw_spin_lock_irqsave(&sem->wait_lock, flags);

  locked:

  	if (!list_empty(&sem->wait_list))

  		__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

  	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

  	wake_up_q(&wake_q);

  	return sem;
  }

  EXPORT_SYMBOL(rwsem_wake);

  
  /*
   * downgrade a write lock into a read lock
   * - caller incremented waiting part of count and discovered it still negative
   * - just wake up any readers at the front of the queue
   */

  __visible

  struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)

  {
  	unsigned long flags;

  	WAKE_Q(wake_q);

  	raw_spin_lock_irqsave(&sem->wait_lock, flags);

  	if (!list_empty(&sem->wait_list))

  		__rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);

  	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

  	wake_up_q(&wake_q);

  	return sem;
  }

  EXPORT_SYMBOL(rwsem_downgrade_wake);