// SPDX-License-Identifier: GPL-2.0

  /* kernel/rwsem.c: R/W semaphores, public implementation
   *
   * Written by David Howells (dhowells@redhat.com).
   * Derived from asm-i386/semaphore.h

   *
   * 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.
   *

   * Rwsem count bit fields re-definition and rwsem rearchitecture by
   * Waiman Long <longman@redhat.com> and
   * Peter Zijlstra <peterz@infradead.org>.

   */
  
  #include <linux/types.h>
  #include <linux/kernel.h>

  #include <linux/sched.h>

  #include <linux/sched/rt.h>
  #include <linux/sched/task.h>

  #include <linux/sched/debug.h>

  #include <linux/sched/wake_q.h>
  #include <linux/sched/signal.h>

  #include <linux/sched/clock.h>

  #include <linux/export.h>

  #include <linux/rwsem.h>

  #include <linux/atomic.h>

  #include "lock_events.h"

  #include <trace/hooks/rwsem.h>

  #include <trace/hooks/dtask.h>

  
  /*

   * The least significant 3 bits of the owner value has the following

   * meanings when set.

   *  - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers

   *  - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock.
   *  - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock.

   *

   * When the rwsem is either owned by an anonymous writer, or it is
   * reader-owned, but a spinning writer has timed out, both nonspinnable
   * bits will be set to disable optimistic spinning by readers and writers.
   * In the later case, the last unlocking reader should then check the
   * writer nonspinnable bit and clear it only to give writers preference
   * to acquire the lock via optimistic spinning, but not readers. Similar
   * action is also done in the reader slowpath.

   * When a writer acquires a rwsem, it puts its task_struct pointer
   * into the owner field. It is cleared after an unlock.
   *
   * When a reader acquires a rwsem, it will also puts its task_struct

   * pointer into the owner field with the RWSEM_READER_OWNED bit set.
   * On unlock, the owner field will largely be left untouched. So
   * for a free or reader-owned rwsem, the owner value may contain
   * information about the last reader that acquires the rwsem.

   *
   * That information may be helpful in debugging cases where the system
   * seems to hang on a reader owned rwsem especially if only one reader
   * is involved. Ideally we would like to track all the readers that own
   * a rwsem, but the overhead is simply too big.

   *
   * Reader optimistic spinning is helpful when the reader critical section
   * is short and there aren't that many readers around. It makes readers
   * relatively more preferred than writers. When a writer times out spinning
   * on a reader-owned lock and set the nospinnable bits, there are two main
   * reasons for that.
   *
   *  1) The reader critical section is long, perhaps the task sleeps after
   *     acquiring the read lock.
   *  2) There are just too many readers contending the lock causing it to
   *     take a while to service all of them.
   *
   * In the former case, long reader critical section will impede the progress
   * of writers which is usually more important for system performance. In
   * the later case, reader optimistic spinning tends to make the reader
   * groups that contain readers that acquire the lock together smaller
   * leading to more of them. That may hurt performance in some cases. In
   * other words, the setting of nonspinnable bits indicates that reader
   * optimistic spinning may not be helpful for those workloads that cause
   * it.
   *
   * Therefore, any writers that had observed the setting of the writer
   * nonspinnable bit for a given rwsem after they fail to acquire the lock
   * via optimistic spinning will set the reader nonspinnable bit once they
   * acquire the write lock. Similarly, readers that observe the setting
   * of reader nonspinnable bit at slowpath entry will set the reader
   * nonspinnable bits when they acquire the read lock via the wakeup path.
   *
   * Once the reader nonspinnable bit is on, it will only be reset when
   * a writer is able to acquire the rwsem in the fast path or somehow a
   * reader or writer in the slowpath doesn't observe the nonspinable bit.
   *
   * This is to discourage reader optmistic spinning on that particular
   * rwsem and make writers more preferred. This adaptive disabling of reader
   * optimistic spinning will alleviate the negative side effect of this
   * feature.

   */
  #define RWSEM_READER_OWNED	(1UL << 0)

  #define RWSEM_RD_NONSPINNABLE	(1UL << 1)
  #define RWSEM_WR_NONSPINNABLE	(1UL << 2)
  #define RWSEM_NONSPINNABLE	(RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE)

  #define RWSEM_OWNER_FLAGS_MASK	(RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)

  
  #ifdef CONFIG_DEBUG_RWSEMS
  # define DEBUG_RWSEMS_WARN_ON(c, sem)	do {			\
  	if (!debug_locks_silent &&				\

  	    WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty
  ",\

  		#c, atomic_long_read(&(sem)->count),		\

  		(unsigned long) sem->magic,			\

  		atomic_long_read(&(sem)->owner), (long)current,	\

  		list_empty(&(sem)->wait_list) ? "" : "not "))	\
  			debug_locks_off();			\
  	} while (0)
  #else
  # define DEBUG_RWSEMS_WARN_ON(c, sem)
  #endif
  
  /*

   * On 64-bit architectures, the bit definitions of the count are:

   *

   * Bit  0    - writer locked bit
   * Bit  1    - waiters present bit
   * Bit  2    - lock handoff bit
   * Bits 3-7  - reserved
   * Bits 8-62 - 55-bit reader count
   * Bit  63   - read fail bit
   *
   * On 32-bit architectures, the bit definitions of the count are:
   *
   * Bit  0    - writer locked bit
   * Bit  1    - waiters present bit
   * Bit  2    - lock handoff bit
   * Bits 3-7  - reserved
   * Bits 8-30 - 23-bit reader count
   * Bit  31   - read fail bit
   *
   * It is not likely that the most significant bit (read fail bit) will ever
   * be set. This guard bit is still checked anyway in the down_read() fastpath
   * just in case we need to use up more of the reader bits for other purpose
   * in the future.

   *
   * atomic_long_fetch_add() is used to obtain reader lock, whereas
   * atomic_long_cmpxchg() will be used to obtain writer lock.

   *
   * There are three places where the lock handoff bit may be set or cleared.
   * 1) rwsem_mark_wake() for readers.
   * 2) rwsem_try_write_lock() for writers.
   * 3) Error path of rwsem_down_write_slowpath().
   *
   * For all the above cases, wait_lock will be held. A writer must also
   * be the first one in the wait_list to be eligible for setting the handoff
   * bit. So concurrent setting/clearing of handoff bit is not possible.

   */
  #define RWSEM_WRITER_LOCKED	(1UL << 0)
  #define RWSEM_FLAG_WAITERS	(1UL << 1)

  #define RWSEM_FLAG_HANDOFF	(1UL << 2)

  #define RWSEM_FLAG_READFAIL	(1UL << (BITS_PER_LONG - 1))

  #define RWSEM_READER_SHIFT	8
  #define RWSEM_READER_BIAS	(1UL << RWSEM_READER_SHIFT)
  #define RWSEM_READER_MASK	(~(RWSEM_READER_BIAS - 1))
  #define RWSEM_WRITER_MASK	RWSEM_WRITER_LOCKED
  #define RWSEM_LOCK_MASK		(RWSEM_WRITER_MASK|RWSEM_READER_MASK)

  #define RWSEM_READ_FAILED_MASK	(RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\

  				 RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)

  
  /*
   * All writes to owner are protected by WRITE_ONCE() to make sure that
   * store tearing can't happen as optimistic spinners may read and use
   * the owner value concurrently without lock. Read from owner, however,
   * may not need READ_ONCE() as long as the pointer value is only used
   * for comparison and isn't being dereferenced.
   */
  static inline void rwsem_set_owner(struct rw_semaphore *sem)
  {

  	atomic_long_set(&sem->owner, (long)current);

  }
  
  static inline void rwsem_clear_owner(struct rw_semaphore *sem)
  {

  	atomic_long_set(&sem->owner, 0);
  }
  
  /*
   * Test the flags in the owner field.
   */
  static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
  {
  	return atomic_long_read(&sem->owner) & flags;

  }
  
  /*
   * The task_struct pointer of the last owning reader will be left in
   * the owner field.
   *
   * Note that the owner value just indicates the task has owned the rwsem
   * previously, it may not be the real owner or one of the real owners
   * anymore when that field is examined, so take it with a grain of salt.

   *
   * The reader non-spinnable bit is preserved.

   */
  static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
  					    struct task_struct *owner)
  {

  	unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
  		(atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE);

  	atomic_long_set(&sem->owner, val);

  }
  
  static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
  {
  	__rwsem_set_reader_owned(sem, current);
  }
  
  /*

   * Return true if the rwsem is owned by a reader.

   */

  static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)

  {

  #ifdef CONFIG_DEBUG_RWSEMS
  	/*
  	 * Check the count to see if it is write-locked.
  	 */
  	long count = atomic_long_read(&sem->count);
  
  	if (count & RWSEM_WRITER_MASK)
  		return false;
  #endif
  	return rwsem_test_oflags(sem, RWSEM_READER_OWNED);

  }
  
  #ifdef CONFIG_DEBUG_RWSEMS
  /*
   * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
   * is a task pointer in owner of a reader-owned rwsem, it will be the
   * real owner or one of the real owners. The only exception is when the
   * unlock is done by up_read_non_owner().
   */
  static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
  {

  	unsigned long val = atomic_long_read(&sem->owner);
  
  	while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
  		if (atomic_long_try_cmpxchg(&sem->owner, &val,
  					    val & RWSEM_OWNER_FLAGS_MASK))
  			return;
  	}

  }
  #else
  static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
  {
  }
  #endif
  
  /*

   * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
   * remains set. Otherwise, the operation will be aborted.
   */
  static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
  {
  	unsigned long owner = atomic_long_read(&sem->owner);
  
  	do {
  		if (!(owner & RWSEM_READER_OWNED))
  			break;
  		if (owner & RWSEM_NONSPINNABLE)
  			break;
  	} while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
  					  owner | RWSEM_NONSPINNABLE));
  }

  static inline bool rwsem_read_trylock(struct rw_semaphore *sem)
  {
  	long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
  	if (WARN_ON_ONCE(cnt < 0))
  		rwsem_set_nonspinnable(sem);
  	return !(cnt & RWSEM_READ_FAILED_MASK);
  }

  /*

   * Return just the real task structure pointer of the owner
   */
  static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
  {
  	return (struct task_struct *)
  		(atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
  }
  
  /*
   * Return the real task structure pointer of the owner and the embedded
   * flags in the owner. pflags must be non-NULL.
   */
  static inline struct task_struct *
  rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
  {
  	unsigned long owner = atomic_long_read(&sem->owner);
  
  	*pflags = owner & RWSEM_OWNER_FLAGS_MASK;
  	return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
  }
  
  /*

   * Guide to the rw_semaphore's count field.
   *
   * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
   * by a writer.
   *
   * The lock is owned by readers when
   * (1) the RWSEM_WRITER_LOCKED isn't set in count,
   * (2) some of the reader bits are set in count, and
   * (3) the owner field has RWSEM_READ_OWNED bit set.
   *
   * Having some reader bits set is not enough to guarantee a readers owned
   * lock as the readers may be in the process of backing out from the count
   * and a writer has just released the lock. So another writer may steal
   * the lock immediately after that.
   */
  
  /*
   * 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_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);

  #endif

  #ifdef CONFIG_DEBUG_RWSEMS
  	sem->magic = sem;
  #endif

  	atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
  	raw_spin_lock_init(&sem->wait_lock);
  	INIT_LIST_HEAD(&sem->wait_list);

  	atomic_long_set(&sem->owner, 0L);

  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  	osq_lock_init(&sem->osq);
  #endif

  	trace_android_vh_rwsem_init(sem);

  }

  EXPORT_SYMBOL(__init_rwsem);

  #define rwsem_first_waiter(sem) \
  	list_first_entry(&sem->wait_list, struct rwsem_waiter, list)

  
  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 */
  };

  enum writer_wait_state {
  	WRITER_NOT_FIRST,	/* Writer is not first in wait list */
  	WRITER_FIRST,		/* Writer is first in wait list     */
  	WRITER_HANDOFF		/* Writer is first & handoff needed */
  };
  
  /*
   * The typical HZ value is either 250 or 1000. So set the minimum waiting
   * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
   * queue before initiating the handoff protocol.
   */
  #define RWSEM_WAIT_TIMEOUT	DIV_ROUND_UP(HZ, 250)

  /*

   * Magic number to batch-wakeup waiting readers, even when writers are
   * also present in the queue. This both limits the amount of work the
   * waking thread must do and also prevents any potential counter overflow,
   * however unlikely.
   */
  #define MAX_READERS_WAKEUP	0x100
  
  /*

   * handle the lock release when processes blocked on it that can now run
   * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
   *   have been set.
   * - 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;
  	struct list_head wlist;

  	lockdep_assert_held(&sem->wait_lock);

  	/*
  	 * Take a peek at the queue head waiter such that we can determine
  	 * the wakeup(s) to perform.
  	 */

  	waiter = rwsem_first_waiter(sem);

  
  	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);
  			lockevent_inc(rwsem_wake_writer);
  		}
  
  		return;
  	}
  
  	/*

  	 * No reader wakeup if there are too many of them already.
  	 */
  	if (unlikely(atomic_long_read(&sem->count) < 0))
  		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) {

  		struct task_struct *owner;

  		adjustment = RWSEM_READER_BIAS;
  		oldcount = atomic_long_fetch_add(adjustment, &sem->count);
  		if (unlikely(oldcount & RWSEM_WRITER_MASK)) {

  			/*
  			 * When we've been waiting "too" long (for writers
  			 * to give up the lock), request a HANDOFF to
  			 * force the issue.
  			 */
  			if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
  			    time_after(jiffies, waiter->timeout)) {
  				adjustment -= RWSEM_FLAG_HANDOFF;
  				lockevent_inc(rwsem_rlock_handoff);
  			}
  
  			atomic_long_add(-adjustment, &sem->count);

  			return;
  		}
  		/*
  		 * Set it to reader-owned to give spinners an early
  		 * indication that readers now have the lock.

  		 * The reader nonspinnable bit seen at slowpath entry of
  		 * the reader is copied over.

  		 */

  		owner = waiter->task;
  		if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) {
  			owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE);
  			lockevent_inc(rwsem_opt_norspin);
  		}
  		__rwsem_set_reader_owned(sem, owner);

  	}
  
  	/*

  	 * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
  	 * queue. We know that the 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.
  	 *

  	 * This is an adaptation of the phase-fair R/W locks where at the
  	 * reader phase (first waiter is a reader), all readers are eligible
  	 * to acquire the lock at the same time irrespective of their order
  	 * in the queue. The writers acquire the lock according to their
  	 * order in the queue.
  	 *

  	 * We have to do wakeup in 2 passes to prevent the possibility that
  	 * the reader count may be decremented before it is incremented. It
  	 * is because the to-be-woken waiter may not have slept yet. So it
  	 * may see waiter->task got cleared, finish its critical section and
  	 * do an unlock before the reader count increment.
  	 *
  	 * 1) Collect the read-waiters in a separate list, count them and
  	 *    fully increment the reader count in rwsem.
  	 * 2) For each waiters in the new list, clear waiter->task and
  	 *    put them into wake_q to be woken up later.
  	 */

  	INIT_LIST_HEAD(&wlist);
  	list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {

  		if (waiter->type == RWSEM_WAITING_FOR_WRITE)

  			continue;

  
  		woken++;

  		list_move_tail(&waiter->list, &wlist);
  
  		/*
  		 * Limit # of readers that can be woken up per wakeup call.
  		 */
  		if (woken >= MAX_READERS_WAKEUP)
  			break;

  	}

  
  	adjustment = woken * RWSEM_READER_BIAS - adjustment;
  	lockevent_cond_inc(rwsem_wake_reader, woken);
  	if (list_empty(&sem->wait_list)) {
  		/* hit end of list above */
  		adjustment -= RWSEM_FLAG_WAITERS;
  	}

  	/*
  	 * When we've woken a reader, we no longer need to force writers
  	 * to give up the lock and we can clear HANDOFF.
  	 */
  	if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
  		adjustment -= RWSEM_FLAG_HANDOFF;

  	if (adjustment)
  		atomic_long_add(adjustment, &sem->count);
  
  	/* 2nd pass */
  	list_for_each_entry_safe(waiter, tmp, &wlist, list) {
  		struct task_struct *tsk;
  
  		tsk = waiter->task;
  		get_task_struct(tsk);
  
  		/*
  		 * Ensure calling get_task_struct() before setting the reader

  		 * waiter to nil such that rwsem_down_read_slowpath() cannot

  		 * race with do_exit() by always holding a reference count
  		 * to the task to wakeup.
  		 */
  		smp_store_release(&waiter->task, NULL);
  		/*
  		 * Ensure issuing the wakeup (either by us or someone else)
  		 * after setting the reader waiter to nil.
  		 */
  		wake_q_add_safe(wake_q, tsk);
  	}
  }
  
  /*
   * 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.

   *
   * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
   * bit is set or the lock is acquired with handoff bit cleared.

   */

  static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,

  					enum writer_wait_state wstate)

  {

  	long count, new;

  	lockdep_assert_held(&sem->wait_lock);

  	count = atomic_long_read(&sem->count);

  	do {
  		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);

  		if (has_handoff && wstate == WRITER_NOT_FIRST)
  			return false;

  		new = count;
  
  		if (count & RWSEM_LOCK_MASK) {
  			if (has_handoff || (wstate != WRITER_HANDOFF))
  				return false;
  
  			new |= RWSEM_FLAG_HANDOFF;
  		} else {
  			new |= RWSEM_WRITER_LOCKED;
  			new &= ~RWSEM_FLAG_HANDOFF;
  
  			if (list_is_singular(&sem->wait_list))
  				new &= ~RWSEM_FLAG_WAITERS;
  		}
  	} while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
  
  	/*
  	 * We have either acquired the lock with handoff bit cleared or
  	 * set the handoff bit.
  	 */
  	if (new & RWSEM_FLAG_HANDOFF)
  		return false;
  
  	rwsem_set_owner(sem);
  	return true;

  }
  
  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
  /*

   * Try to acquire read lock before the reader is put on wait queue.
   * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
   * is ongoing.
   */
  static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
  {
  	long count = atomic_long_read(&sem->count);
  
  	if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
  		return false;
  
  	count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
  	if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
  		rwsem_set_reader_owned(sem);
  		lockevent_inc(rwsem_opt_rlock);
  		return true;
  	}
  
  	/* Back out the change */
  	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
  	return false;
  }
  
  /*

   * 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 count = atomic_long_read(&sem->count);

  	while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {

  		if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,

  					count | RWSEM_WRITER_LOCKED)) {

  			rwsem_set_owner(sem);
  			lockevent_inc(rwsem_opt_wlock);
  			return true;
  		}
  	}
  	return false;
  }
  
  static inline bool owner_on_cpu(struct task_struct *owner)
  {
  	/*
  	 * As lock holder preemption issue, we both skip spinning if
  	 * task is not on cpu or its cpu is preempted
  	 */
  	return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
  }

  static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
  					   unsigned long nonspinnable)

  {
  	struct task_struct *owner;

  	unsigned long flags;

  	bool ret = true;

  	if (need_resched()) {
  		lockevent_inc(rwsem_opt_fail);

  		return false;

  	}

  	preempt_disable();

  	rcu_read_lock();

  	owner = rwsem_owner_flags(sem, &flags);

  	/*
  	 * Don't check the read-owner as the entry may be stale.
  	 */
  	if ((flags & nonspinnable) ||
  	    (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))

  		ret = false;

  	rcu_read_unlock();

  	preempt_enable();
  
  	lockevent_cond_inc(rwsem_opt_fail, !ret);

  	return ret;
  }
  
  /*

   * The rwsem_spin_on_owner() function returns the folowing 4 values
   * depending on the lock owner state.
   *   OWNER_NULL  : owner is currently NULL
   *   OWNER_WRITER: when owner changes and is a writer
   *   OWNER_READER: when owner changes and the new owner may be a reader.
   *   OWNER_NONSPINNABLE:
   *		   when optimistic spinning has to stop because either the
   *		   owner stops running, is unknown, or its timeslice has
   *		   been used up.

   */

  enum owner_state {
  	OWNER_NULL		= 1 << 0,
  	OWNER_WRITER		= 1 << 1,
  	OWNER_READER		= 1 << 2,
  	OWNER_NONSPINNABLE	= 1 << 3,
  };

  #define OWNER_SPINNABLE		(OWNER_NULL | OWNER_WRITER | OWNER_READER)

  static inline enum owner_state

  rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable)

  {

  	if (flags & nonspinnable)

  		return OWNER_NONSPINNABLE;

  	if (flags & RWSEM_READER_OWNED)

  		return OWNER_READER;

  	return owner ? OWNER_WRITER : OWNER_NULL;

  }

  static noinline enum owner_state
  rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)

  {

  	struct task_struct *new, *owner;
  	unsigned long flags, new_flags;
  	enum owner_state state;

  	owner = rwsem_owner_flags(sem, &flags);

  	state = rwsem_owner_state(owner, flags, nonspinnable);

  	if (state != OWNER_WRITER)
  		return state;

  
  	rcu_read_lock();

  	for (;;) {

  		/*
  		 * When a waiting writer set the handoff flag, it may spin
  		 * on the owner as well. Once that writer acquires the lock,
  		 * we can spin on it. So we don't need to quit even when the
  		 * handoff bit is set.
  		 */

  		new = rwsem_owner_flags(sem, &new_flags);
  		if ((new != owner) || (new_flags != flags)) {

  			state = rwsem_owner_state(new, new_flags, nonspinnable);

  			break;
  		}

  		/*
  		 * 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();

  		if (need_resched() || !owner_on_cpu(owner)) {

  			state = OWNER_NONSPINNABLE;
  			break;

  		}
  
  		cpu_relax();
  	}
  	rcu_read_unlock();

  	return state;

  }

  /*
   * Calculate reader-owned rwsem spinning threshold for writer
   *
   * The more readers own the rwsem, the longer it will take for them to
   * wind down and free the rwsem. So the empirical formula used to
   * determine the actual spinning time limit here is:
   *
   *   Spinning threshold = (10 + nr_readers/2)us
   *
   * The limit is capped to a maximum of 25us (30 readers). This is just
   * a heuristic and is subjected to change in the future.
   */
  static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
  {
  	long count = atomic_long_read(&sem->count);
  	int readers = count >> RWSEM_READER_SHIFT;
  	u64 delta;
  
  	if (readers > 30)
  		readers = 30;
  	delta = (20 + readers) * NSEC_PER_USEC / 2;
  
  	return sched_clock() + delta;
  }

  static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)

  {
  	bool taken = false;

  	int prev_owner_state = OWNER_NULL;

  	int loop = 0;
  	u64 rspin_threshold = 0;
  	unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
  					   : RWSEM_RD_NONSPINNABLE;

  
  	preempt_disable();
  
  	/* sem->wait_lock should not be held when doing optimistic spinning */

  	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 and spinning time has exceeded limit.

  	 */

  	for (;;) {

  		enum owner_state owner_state;

  		owner_state = rwsem_spin_on_owner(sem, nonspinnable);

  		if (!(owner_state & OWNER_SPINNABLE))
  			break;

  		/*
  		 * Try to acquire the lock
  		 */

  		taken = wlock ? rwsem_try_write_lock_unqueued(sem)
  			      : rwsem_try_read_lock_unqueued(sem);
  
  		if (taken)

  			break;

  
  		/*

  		 * Time-based reader-owned rwsem optimistic spinning
  		 */
  		if (wlock && (owner_state == OWNER_READER)) {
  			/*
  			 * Re-initialize rspin_threshold every time when
  			 * the owner state changes from non-reader to reader.
  			 * This allows a writer to steal the lock in between
  			 * 2 reader phases and have the threshold reset at
  			 * the beginning of the 2nd reader phase.
  			 */
  			if (prev_owner_state != OWNER_READER) {
  				if (rwsem_test_oflags(sem, nonspinnable))
  					break;
  				rspin_threshold = rwsem_rspin_threshold(sem);
  				loop = 0;
  			}
  
  			/*
  			 * Check time threshold once every 16 iterations to
  			 * avoid calling sched_clock() too frequently so
  			 * as to reduce the average latency between the times
  			 * when the lock becomes free and when the spinner
  			 * is ready to do a trylock.
  			 */
  			else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
  				rwsem_set_nonspinnable(sem);
  				lockevent_inc(rwsem_opt_nospin);
  				break;
  			}
  		}
  
  		/*

  		 * An RT task cannot do optimistic spinning if it cannot
  		 * be sure the lock holder is running or live-lock may
  		 * happen if the current task and the lock holder happen
  		 * to run in the same CPU. However, aborting optimistic
  		 * spinning while a NULL owner is detected may miss some
  		 * opportunity where spinning can continue without causing
  		 * problem.
  		 *
  		 * There are 2 possible cases where an RT task may be able
  		 * to continue spinning.
  		 *
  		 * 1) The lock owner is in the process of releasing the
  		 *    lock, sem->owner is cleared but the lock has not
  		 *    been released yet.
  		 * 2) The lock was free and owner cleared, but another
  		 *    task just comes in and acquire the lock before
  		 *    we try to get it. The new owner may be a spinnable
  		 *    writer.
  		 *
  		 * To take advantage of two scenarios listed agove, the RT
  		 * task is made to retry one more time to see if it can
  		 * acquire the lock or continue spinning on the new owning
  		 * writer. Of course, if the time lag is long enough or the
  		 * new owner is not a writer or spinnable, the RT task will
  		 * quit spinning.
  		 *
  		 * If the owner is a writer, the need_resched() check is
  		 * done inside rwsem_spin_on_owner(). If the owner is not
  		 * a writer, need_resched() check needs to be done here.

  		 */

  		if (owner_state != OWNER_WRITER) {
  			if (need_resched())
  				break;
  			if (rt_task(current) &&
  			   (prev_owner_state != OWNER_WRITER))
  				break;
  		}
  		prev_owner_state = owner_state;

  
  		/*
  		 * 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();
  	}
  	osq_unlock(&sem->osq);
  done:
  	preempt_enable();
  	lockevent_cond_inc(rwsem_opt_fail, !taken);
  	return taken;
  }

  
  /*
   * Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should
   * only be called when the reader count reaches 0.
   *
   * This give writers better chance to acquire the rwsem first before
   * readers when the rwsem was being held by readers for a relatively long
   * period of time. Race can happen that an optimistic spinner may have
   * just stolen the rwsem and set the owner, but just clearing the
   * RWSEM_WR_NONSPINNABLE bit will do no harm anyway.
   */
  static inline void clear_wr_nonspinnable(struct rw_semaphore *sem)
  {
  	if (rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE))
  		atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner);
  }

  
  /*
   * This function is called when the reader fails to acquire the lock via
   * optimistic spinning. In this case we will still attempt to do a trylock
   * when comparing the rwsem state right now with the state when entering
   * the slowpath indicates that the reader is still in a valid reader phase.
   * This happens when the following conditions are true:
   *
   * 1) The lock is currently reader owned, and
   * 2) The lock is previously not reader-owned or the last read owner changes.
   *
   * In the former case, we have transitioned from a writer phase to a
   * reader-phase while spinning. In the latter case, it means the reader
   * phase hasn't ended when we entered the optimistic spinning loop. In
   * both cases, the reader is eligible to acquire the lock. This is the
   * secondary path where a read lock is acquired optimistically.
   *
   * The reader non-spinnable bit wasn't set at time of entry or it will
   * not be here at all.
   */
  static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
  					      unsigned long last_rowner)
  {
  	unsigned long owner = atomic_long_read(&sem->owner);
  
  	if (!(owner & RWSEM_READER_OWNED))
  		return false;
  
  	if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) &&
  	    rwsem_try_read_lock_unqueued(sem)) {
  		lockevent_inc(rwsem_opt_rlock2);
  		lockevent_add(rwsem_opt_fail, -1);
  		return true;
  	}
  	return false;
  }

  #else

  static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
  					   unsigned long nonspinnable)

  {
  	return false;
  }
  
  static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)

  {
  	return false;
  }

  
  static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }

  
  static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
  					      unsigned long last_rowner)
  {
  	return false;
  }

  
  static inline int
  rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
  {
  	return 0;
  }
  #define OWNER_NULL	1

  #endif
  
  /*
   * Wait for the read lock to be granted
   */

  static struct rw_semaphore __sched *
  rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)

  {
  	long count, adjustment = -RWSEM_READER_BIAS;
  	struct rwsem_waiter waiter;
  	DEFINE_WAKE_Q(wake_q);

  	bool wake = false;

  	bool already_on_list = false;

  	/*
  	 * Save the current read-owner of rwsem, if available, and the
  	 * reader nonspinnable bit.
  	 */
  	waiter.last_rowner = atomic_long_read(&sem->owner);
  	if (!(waiter.last_rowner & RWSEM_READER_OWNED))
  		waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;

  	if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))

  		goto queue;
  
  	/*
  	 * Undo read bias from down_read() and do optimistic spinning.
  	 */
  	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
  	adjustment = 0;
  	if (rwsem_optimistic_spin(sem, false)) {

  		/* rwsem_optimistic_spin() implies ACQUIRE on success */

  		/*
  		 * Wake up other readers in the wait list if the front
  		 * waiter is a reader.
  		 */
  		if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) {
  			raw_spin_lock_irq(&sem->wait_lock);
  			if (!list_empty(&sem->wait_list))
  				rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
  						&wake_q);
  			raw_spin_unlock_irq(&sem->wait_lock);
  			wake_up_q(&wake_q);
  		}
  		return sem;

  	} else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {

  		/* rwsem_reader_phase_trylock() implies ACQUIRE on success */

  		return sem;

  	}
  
  queue:

  	waiter.task = current;
  	waiter.type = RWSEM_WAITING_FOR_READ;

  	waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;

  
  	raw_spin_lock_irq(&sem->wait_lock);
  	if (list_empty(&sem->wait_list)) {
  		/*
  		 * In case the wait queue is empty and the lock isn't owned

  		 * by a writer or has the handoff bit set, this reader can
  		 * exit the slowpath and return immediately as its
  		 * RWSEM_READER_BIAS has already been set in the count.

  		 */

  		if (adjustment && !(atomic_long_read(&sem->count) &

  		     (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {

  			/* Provide lock ACQUIRE */
  			smp_acquire__after_ctrl_dep();

  			raw_spin_unlock_irq(&sem->wait_lock);
  			rwsem_set_reader_owned(sem);
  			lockevent_inc(rwsem_rlock_fast);
  			return sem;
  		}
  		adjustment += RWSEM_FLAG_WAITERS;
  	}

  	trace_android_vh_alter_rwsem_list_add(
  					&waiter,
  					sem, &already_on_list);
  	if (!already_on_list)
  		list_add_tail(&waiter.list, &sem->wait_list);

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

  	if (adjustment)
  		count = atomic_long_add_return(adjustment, &sem->count);
  	else
  		count = atomic_long_read(&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_LOCK_MASK)) {
  		clear_wr_nonspinnable(sem);
  		wake = true;
  	}
  	if (wake || (!(count & RWSEM_WRITER_MASK) &&
  		    (adjustment & RWSEM_FLAG_WAITERS)))

  		rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

  	trace_android_vh_rwsem_wake(sem);

  	raw_spin_unlock_irq(&sem->wait_lock);
  	wake_up_q(&wake_q);
  
  	/* wait to be given the lock */

  	trace_android_vh_rwsem_read_wait_start(sem);

  	for (;;) {

  		set_current_state(state);

  		if (!smp_load_acquire(&waiter.task)) {

  			/* Matches rwsem_mark_wake()'s smp_store_release(). */

  			break;

  		}

  		if (signal_pending_state(state, current)) {
  			raw_spin_lock_irq(&sem->wait_lock);
  			if (waiter.task)
  				goto out_nolock;
  			raw_spin_unlock_irq(&sem->wait_lock);

  			/* Ordered by sem->wait_lock against rwsem_mark_wake(). */

  			break;
  		}
  		schedule();
  		lockevent_inc(rwsem_sleep_reader);
  	}
  
  	__set_current_state(TASK_RUNNING);

  	trace_android_vh_rwsem_read_wait_finish(sem);

  	lockevent_inc(rwsem_rlock);
  	return sem;

  out_nolock:
  	list_del(&waiter.list);

  	if (list_empty(&sem->wait_list)) {
  		atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
  				   &sem->count);
  	}

  	raw_spin_unlock_irq(&sem->wait_lock);
  	__set_current_state(TASK_RUNNING);

  	trace_android_vh_rwsem_read_wait_finish(sem);

  	lockevent_inc(rwsem_rlock_fail);
  	return ERR_PTR(-EINTR);
  }

  /*

   * This function is called by the a write lock owner. So the owner value
   * won't get changed by others.
   */
  static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
  						bool disable)
  {
  	if (unlikely(disable)) {
  		atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner);
  		lockevent_inc(rwsem_opt_norspin);
  	}
  }
  
  /*

   * Wait until we successfully acquire the write lock
   */

  static struct rw_semaphore *
  rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)

  {
  	long count;

  	bool disable_rspin;

  	enum writer_wait_state wstate;

  	struct rwsem_waiter waiter;
  	struct rw_semaphore *ret = sem;
  	DEFINE_WAKE_Q(wake_q);

  	bool already_on_list = false;

  
  	/* do optimistic spinning and steal lock if possible */

  	if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&

  	    rwsem_optimistic_spin(sem, true)) {
  		/* rwsem_optimistic_spin() implies ACQUIRE on success */

  		return sem;

  	}

  
  	/*

  	 * Disable reader optimistic spinning for this rwsem after
  	 * acquiring the write lock when the setting of the nonspinnable
  	 * bits are observed.
  	 */
  	disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE;
  
  	/*

  	 * Optimistic spinning failed, proceed to the slowpath
  	 * and block until we can acquire the sem.
  	 */
  	waiter.task = current;
  	waiter.type = RWSEM_WAITING_FOR_WRITE;

  	waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;

  
  	raw_spin_lock_irq(&sem->wait_lock);
  
  	/* account for this before adding a new element to the list */

  	wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;

  	trace_android_vh_alter_rwsem_list_add(
  					&waiter,
  					sem, &already_on_list);
  	if (!already_on_list)
  		list_add_tail(&waiter.list, &sem->wait_list);

  
  	/* we're now waiting on the lock */

  	if (wstate == WRITER_NOT_FIRST) {

  		count = atomic_long_read(&sem->count);
  
  		/*

  		 * If there were already threads queued before us and:
  		 *  1) there are no no active locks, wake the front
  		 *     queued process(es) as the handoff bit might be set.
  		 *  2) there are no active writers and some readers, the lock
  		 *     must be read owned; so we try to wake any read lock
  		 *     waiters that were queued ahead of us.

  		 */

  		if (count & RWSEM_WRITER_MASK)
  			goto wait;

  		rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
  					? RWSEM_WAKE_READERS
  					: RWSEM_WAKE_ANY, &wake_q);

  		if (!wake_q_empty(&wake_q)) {
  			/*
  			 * We want to minimize wait_lock hold time especially
  			 * when a large number of readers are to be woken up.
  			 */
  			raw_spin_unlock_irq(&sem->wait_lock);
  			wake_up_q(&wake_q);
  			wake_q_init(&wake_q);	/* Used again, reinit */
  			raw_spin_lock_irq(&sem->wait_lock);
  		}

  	} else {

  		atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);

  	}

  wait:

  	trace_android_vh_rwsem_wake(sem);

  	/* wait until we successfully acquire the lock */

  	trace_android_vh_rwsem_write_wait_start(sem);

  	set_current_state(state);

  	for (;;) {
  		if (rwsem_try_write_lock(sem, wstate)) {
  			/* rwsem_try_write_lock() implies ACQUIRE on success */

  			break;

  		}

  		raw_spin_unlock_irq(&sem->wait_lock);

  		/*
  		 * After setting the handoff bit and failing to acquire
  		 * the lock, attempt to spin on owner to accelerate lock
  		 * transfer. If the previous owner is a on-cpu writer and it
  		 * has just released the lock, OWNER_NULL will be returned.
  		 * In this case, we attempt to acquire the lock again
  		 * without sleeping.
  		 */

  		if (wstate == WRITER_HANDOFF &&
  		    rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)

  			goto trylock_again;

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

  		for (;;) {

  			if (signal_pending_state(state, current))
  				goto out_nolock;
  
  			schedule();
  			lockevent_inc(rwsem_sleep_writer);
  			set_current_state(state);

  			/*
  			 * If HANDOFF bit is set, unconditionally do
  			 * a trylock.
  			 */
  			if (wstate == WRITER_HANDOFF)
  				break;
  
  			if ((wstate == WRITER_NOT_FIRST) &&
  			    (rwsem_first_waiter(sem) == &waiter))
  				wstate = WRITER_FIRST;

  			count = atomic_long_read(&sem->count);

  			if (!(count & RWSEM_LOCK_MASK))
  				break;
  
  			/*
  			 * The setting of the handoff bit is deferred
  			 * until rwsem_try_write_lock() is called.
  			 */
  			if ((wstate == WRITER_FIRST) && (rt_task(current) ||
  			    time_after(jiffies, waiter.timeout))) {
  				wstate = WRITER_HANDOFF;
  				lockevent_inc(rwsem_wlock_handoff);
  				break;
  			}
  		}

  trylock_again:

  		raw_spin_lock_irq(&sem->wait_lock);
  	}
  	__set_current_state(TASK_RUNNING);

  	trace_android_vh_rwsem_write_wait_finish(sem);

  	list_del(&waiter.list);

  	rwsem_disable_reader_optspin(sem, disable_rspin);

  	raw_spin_unlock_irq(&sem->wait_lock);
  	lockevent_inc(rwsem_wlock);
  
  	return ret;
  
  out_nolock:
  	__set_current_state(TASK_RUNNING);

  	trace_android_vh_rwsem_write_wait_finish(sem);

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

  
  	if (unlikely(wstate == WRITER_HANDOFF))
  		atomic_long_add(-RWSEM_FLAG_HANDOFF,  &sem->count);

  	if (list_empty(&sem->wait_list))
  		atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
  	else

  		rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

  	raw_spin_unlock_irq(&sem->wait_lock);
  	wake_up_q(&wake_q);
  	lockevent_inc(rwsem_wlock_fail);
  
  	return ERR_PTR(-EINTR);
  }

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

  static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)

  {
  	unsigned long flags;
  	DEFINE_WAKE_Q(wake_q);
  
  	raw_spin_lock_irqsave(&sem->wait_lock, flags);
  
  	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;
  }

  
  /*
   * 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
   */

  static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)

  {
  	unsigned long flags;
  	DEFINE_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;
  }

  
  /*
   * lock for reading
   */

  static inline void __down_read(struct rw_semaphore *sem)

  {

  	if (!rwsem_read_trylock(sem)) {

  		rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);

  		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);

  	} else {
  		rwsem_set_reader_owned(sem);
  	}
  }

  static inline int __down_read_interruptible(struct rw_semaphore *sem)
  {
  	if (!rwsem_read_trylock(sem)) {
  		if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_INTERRUPTIBLE)))
  			return -EINTR;
  		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
  	} else {
  		rwsem_set_reader_owned(sem);
  	}
  	return 0;
  }

  static inline int __down_read_killable(struct rw_semaphore *sem)
  {

  	if (!rwsem_read_trylock(sem)) {

  		if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))

  			return -EINTR;

  		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);

  	} else {
  		rwsem_set_reader_owned(sem);
  	}
  	return 0;
  }
  
  static inline int __down_read_trylock(struct rw_semaphore *sem)
  {

  	long tmp;
  
  	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);

  	/*
  	 * Optimize for the case when the rwsem is not locked at all.
  	 */

  	tmp = RWSEM_UNLOCKED_VALUE;

  	do {
  		if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
  					tmp + RWSEM_READER_BIAS)) {
  			rwsem_set_reader_owned(sem);
  			return 1;
  		}
  	} while (!(tmp & RWSEM_READ_FAILED_MASK));
  	return 0;
  }
  
  /*
   * lock for writing
   */

  static inline void __down_write(struct rw_semaphore *sem)

  {

  	long tmp = RWSEM_UNLOCKED_VALUE;
  
  	if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
  						      RWSEM_WRITER_LOCKED)))
  		rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);

  	else
  		rwsem_set_owner(sem);

  }
  
  static inline int __down_write_killable(struct rw_semaphore *sem)
  {

  	long tmp = RWSEM_UNLOCKED_VALUE;
  
  	if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
  						      RWSEM_WRITER_LOCKED))) {
  		if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))

  			return -EINTR;

  	} else {
  		rwsem_set_owner(sem);

  	}

  	return 0;
  }
  
  static inline int __down_write_trylock(struct rw_semaphore *sem)
  {

  	long tmp;

  	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
  
  	tmp  = RWSEM_UNLOCKED_VALUE;

  	if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
  					    RWSEM_WRITER_LOCKED)) {

  		rwsem_set_owner(sem);
  		return true;
  	}
  	return false;
  }
  
  /*
   * unlock after reading
   */

  static inline void __up_read(struct rw_semaphore *sem)

  {
  	long tmp;

  	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);

  	DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);

  	rwsem_clear_reader_owned(sem);
  	tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);

  	DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);

  	if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==

  		      RWSEM_FLAG_WAITERS)) {
  		clear_wr_nonspinnable(sem);

  		rwsem_wake(sem, tmp);

  	}

  }
  
  /*
   * unlock after writing
   */

  static inline void __up_write(struct rw_semaphore *sem)

  {

  	long tmp;

  	DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);

  	/*
  	 * sem->owner may differ from current if the ownership is transferred
  	 * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
  	 */

  	DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
  			    !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);

  	rwsem_clear_owner(sem);

  	tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
  	if (unlikely(tmp & RWSEM_FLAG_WAITERS))

  		rwsem_wake(sem, tmp);

  }
  
  /*
   * downgrade write lock to read lock
   */
  static inline void __downgrade_write(struct rw_semaphore *sem)
  {
  	long tmp;
  
  	/*
  	 * When downgrading from exclusive to shared ownership,
  	 * anything inside the write-locked region cannot leak
  	 * into the read side. In contrast, anything in the
  	 * read-locked region is ok to be re-ordered into the
  	 * write side. As such, rely on RELEASE semantics.
  	 */

  	DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);

  	tmp = atomic_long_fetch_add_release(
  		-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
  	rwsem_set_reader_owned(sem);
  	if (tmp & RWSEM_FLAG_WAITERS)
  		rwsem_downgrade_wake(sem);
  }

  /*
   * lock for reading
   */

  void __sched down_read(struct rw_semaphore *sem)

  {
  	might_sleep();
  	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);

  	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);

  }

  EXPORT_SYMBOL(down_read);

  int __sched down_read_interruptible(struct rw_semaphore *sem)
  {
  	might_sleep();
  	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
  
  	if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_interruptible)) {
  		rwsem_release(&sem->dep_map, _RET_IP_);
  		return -EINTR;
  	}
  
  	return 0;
  }
  EXPORT_SYMBOL(down_read_interruptible);

  int __sched down_read_killable(struct rw_semaphore *sem)
  {
  	might_sleep();
  	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
  
  	if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {

  		rwsem_release(&sem->dep_map, _RET_IP_);

  		return -EINTR;
  	}

  	return 0;
  }

  EXPORT_SYMBOL(down_read_killable);

  /*
   * trylock for reading -- returns 1 if successful, 0 if contention
   */
  int down_read_trylock(struct rw_semaphore *sem)
  {
  	int ret = __down_read_trylock(sem);

  	if (ret == 1)

  		rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
  	return ret;
  }

  EXPORT_SYMBOL(down_read_trylock);
  
  /*
   * lock for writing
   */

  void __sched down_write(struct rw_semaphore *sem)

  {
  	might_sleep();
  	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);

  	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);

  }

  EXPORT_SYMBOL(down_write);
  
  /*

   * lock for writing
   */
  int __sched down_write_killable(struct rw_semaphore *sem)
  {
  	might_sleep();
  	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);

  	if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
  				  __down_write_killable)) {

  		rwsem_release(&sem->dep_map, _RET_IP_);

  		return -EINTR;
  	}

  	return 0;
  }

  EXPORT_SYMBOL(down_write_killable);
  
  /*

   * trylock for writing -- returns 1 if successful, 0 if contention
   */
  int down_write_trylock(struct rw_semaphore *sem)
  {
  	int ret = __down_write_trylock(sem);

  	if (ret == 1)

  		rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);

  	return ret;
  }

  EXPORT_SYMBOL(down_write_trylock);
  
  /*
   * release a read lock
   */
  void up_read(struct rw_semaphore *sem)
  {

  	rwsem_release(&sem->dep_map, _RET_IP_);

  	__up_read(sem);
  }

  EXPORT_SYMBOL(up_read);
  
  /*
   * release a write lock
   */
  void up_write(struct rw_semaphore *sem)
  {

  	rwsem_release(&sem->dep_map, _RET_IP_);

  	trace_android_vh_rwsem_write_finished(sem);

  	__up_write(sem);
  }

  EXPORT_SYMBOL(up_write);
  
  /*
   * downgrade write lock to read lock
   */
  void downgrade_write(struct rw_semaphore *sem)
  {

  	lock_downgrade(&sem->dep_map, _RET_IP_);

  	trace_android_vh_rwsem_write_finished(sem);

  	__downgrade_write(sem);
  }

  EXPORT_SYMBOL(downgrade_write);

  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  
  void down_read_nested(struct rw_semaphore *sem, int subclass)
  {
  	might_sleep();
  	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);

  	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);

  }

  EXPORT_SYMBOL(down_read_nested);

  int down_read_killable_nested(struct rw_semaphore *sem, int subclass)
  {
  	might_sleep();
  	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
  
  	if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
  		rwsem_release(&sem->dep_map, _RET_IP_);
  		return -EINTR;
  	}
  
  	return 0;
  }
  EXPORT_SYMBOL(down_read_killable_nested);

  void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
  {
  	might_sleep();
  	rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);

  	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
  }

  EXPORT_SYMBOL(_down_write_nest_lock);

  void down_read_non_owner(struct rw_semaphore *sem)
  {
  	might_sleep();

  	__down_read(sem);

  	__rwsem_set_reader_owned(sem, NULL);

  }

  EXPORT_SYMBOL(down_read_non_owner);

  void down_write_nested(struct rw_semaphore *sem, int subclass)
  {
  	might_sleep();
  	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);

  	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);

  }

  EXPORT_SYMBOL(down_write_nested);

  int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
  {
  	might_sleep();
  	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);

  	if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
  				  __down_write_killable)) {

  		rwsem_release(&sem->dep_map, _RET_IP_);

  		return -EINTR;
  	}

  	return 0;
  }

  EXPORT_SYMBOL(down_write_killable_nested);

  void up_read_non_owner(struct rw_semaphore *sem)
  {

  	DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);

  	__up_read(sem);
  }

  EXPORT_SYMBOL(up_read_non_owner);

  #endif