waiting_threads.c

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/* Copyright (c) 2008, 2011, Oracle and/or its affiliates. All rights reserved.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; version 2 of the License.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */

/*
  Note that if your lock system satisfy the following condition:

    there exist four lock levels A, B, C, D, such as
      A is compatible with B
      A is not compatible with C
      D is not compatible with B

      (example A=IX, B=IS, C=S, D=X)

   you need to include lock level in the resource identifier - a
   thread waiting for lock of the type A on resource R and another
   thread waiting for lock of the type B on resource R should wait on
   different WT_RESOURCE structures, on different {lock, resource}
   pairs.  Otherwise the following is possible:

      thread1> take S-lock on R
      thread2> take IS-lock on R
      thread3> wants X-lock on R, starts waiting for threads 1 and 2 on R.
      thread3 is killed (or timeout or whatever)
      WT_RESOURCE structure for R is still in the hash, as it has two owners
      thread4> wants an IX-lock on R
      WT_RESOURCE for R is found in the hash, thread4 starts waiting on it.
      !! now thread4 is waiting for both thread1 and thread2
      !! while, in fact, IX-lock and IS-lock are compatible and
      !! thread4 should not wait for thread2.
*/

#include <waiting_threads.h>
#include <m_string.h>

/* status variables */

ulonglong wt_wait_table[WT_WAIT_STATS];
uint32 wt_wait_stats[WT_WAIT_STATS+1];
uint32 wt_cycle_stats[2][WT_CYCLE_STATS+1];
uint32 wt_success_stats;

static my_atomic_rwlock_t cycle_stats_lock, wait_stats_lock, success_stats_lock;

#ifdef SAFE_STATISTICS
#define incr(VAR, LOCK)                           \
  do {                                            \
    my_atomic_rwlock_wrlock(&(LOCK));             \
    my_atomic_add32(&(VAR), 1);                   \
    my_atomic_rwlock_wrunlock(&(LOCK));           \
  } while(0)
#else
#define incr(VAR,LOCK)  do { (VAR)++; } while(0)
#endif

static void increment_success_stats()
{
  incr(wt_success_stats, success_stats_lock);
}

static void increment_cycle_stats(uint depth, uint slot)
{
  if (depth >= WT_CYCLE_STATS)
    depth= WT_CYCLE_STATS;
  incr(wt_cycle_stats[slot][depth], cycle_stats_lock);
}

static void increment_wait_stats(ulonglong waited,int ret)
{
  uint i;
  if ((ret) == ETIMEDOUT)
    i= WT_WAIT_STATS;
  else
    for (i= 0; i < WT_WAIT_STATS && waited/10 > wt_wait_table[i]; i++) ;
  incr(wt_wait_stats[i], wait_stats_lock);
}

/*
  'lock' protects 'owners', 'state', and 'waiter_count'
  'id' is read-only

  a resource is picked up from a hash in a lock-free manner
  it's returned pinned, so it cannot be freed at once
  but it may be freed right after the pin is removed
  to free a resource it should
    1. have no owners
    2. have no waiters

  two ways to access a resource:
    1. find it in a hash
       - it's returned pinned.
        a) take a lock in exclusive mode
        b) check the state, it should be ACTIVE to be usable
        c) unpin
    2. by a direct reference
       - could only used if a resource cannot be freed
       e.g. accessing a resource by thd->waiting_for is safe,
       a resource cannot be freed as there's a thread waiting for it
*/
struct st_wt_resource {
  WT_RESOURCE_ID  id;
  uint            waiter_count;
  enum { ACTIVE, FREE } state;
#ifndef DBUG_OFF
  mysql_mutex_t  *cond_mutex; /* a mutex for the 'cond' below */
#endif
  /*
    before the 'lock' all elements are mutable, after (and including) -
    immutable in the sense that lf_hash_insert() won't memcpy() over them.
    See wt_init().
  */
#ifdef WT_RWLOCKS_USE_MUTEXES
  /*
    we need a special rwlock-like 'lock' to allow readers bypass
    waiting writers, otherwise readers can deadlock. For example:

      A waits on resource x, owned by B, B waits on resource y, owned
      by A, we have a cycle (A->x->B->y->A)
      Both A and B start deadlock detection:

        A locks x                          B locks y
        A goes deeper                      B goes deeper
        A locks y                          B locks x

      with mutexes it would deadlock. With rwlocks it won't, as long
      as both A and B are taking read locks (and they do).
      But other threads may take write locks. Assume there's
      C who wants to start waiting on x, and D who wants to start
      waiting on y.

        A read-locks x                       B read-locks y
        A goes deeper                        B goes deeper
     => C write-locks x (to add a new edge)  D write-locks y
     .. C is blocked                         D is blocked
        A read-locks y                       B read-locks x

      Now, if a read lock can bypass a pending wrote lock request, we're fine.
      If it can not, we have a deadlock.

    writer starvation is technically possible, but unlikely, because
    the contention is expected to be low.
  */
  struct {
    mysql_cond_t   cond;
    mysql_mutex_t  mutex;
    uint readers: 16;
    uint pending_writers: 15;
    uint write_locked: 1;
  } lock;
#else
  rw_lock_t lock;
#endif
  mysql_cond_t     cond; /* the corresponding mutex is provided by the caller */
  DYNAMIC_ARRAY    owners;
};

#ifdef  WT_RWLOCKS_USE_MUTEXES
static void rc_rwlock_init(WT_RESOURCE *rc)
{
  mysql_cond_init(0, &rc->lock.cond, 0);
  mysql_mutex_init(0, &rc->lock.mutex, MY_MUTEX_INIT_FAST);
}
static void rc_rwlock_destroy(WT_RESOURCE *rc)
{
  DBUG_ASSERT(rc->lock.write_locked == 0);
  DBUG_ASSERT(rc->lock.readers == 0);
  mysql_cond_destroy(&rc->lock.cond);
  mysql_mutex_destroy(&rc->lock.mutex);
}
static void rc_rdlock(WT_RESOURCE *rc)
{
  DBUG_PRINT("wt", ("TRYLOCK resid=%ld for READ", (ulong)rc->id.value));
  mysql_mutex_lock(&rc->lock.mutex);
  while (rc->lock.write_locked)
    mysql_cond_wait(&rc->lock.cond, &rc->lock.mutex);
  rc->lock.readers++;
  mysql_mutex_unlock(&rc->lock.mutex);
  DBUG_PRINT("wt", ("LOCK resid=%ld for READ", (ulong)rc->id.value));
}
static void rc_wrlock(WT_RESOURCE *rc)
{
  DBUG_PRINT("wt", ("TRYLOCK resid=%ld for WRITE", (ulong)rc->id.value));
  mysql_mutex_lock(&rc->lock.mutex);
  while (rc->lock.write_locked || rc->lock.readers)
    mysql_cond_wait(&rc->lock.cond, &rc->lock.mutex);
  rc->lock.write_locked= 1;
  mysql_mutex_unlock(&rc->lock.mutex);
  DBUG_PRINT("wt", ("LOCK resid=%ld for WRITE", (ulong)rc->id.value));
}
static void rc_unlock(WT_RESOURCE *rc)
{
  DBUG_PRINT("wt", ("UNLOCK resid=%ld", (ulong)rc->id.value));
  mysql_mutex_lock(&rc->lock.mutex);
  if (rc->lock.write_locked)
  {
    rc->lock.write_locked= 0;
    mysql_cond_broadcast(&rc->lock.cond);
  }
  else if (--rc->lock.readers == 0)
    mysql_cond_broadcast(&rc->lock.cond);
  mysql_mutex_unlock(&rc->lock.mutex);
}
#else
static void rc_rwlock_init(WT_RESOURCE *rc)
{
  my_rwlock_init(&rc->lock, 0);
}
static void rc_rwlock_destroy(WT_RESOURCE *rc)
{
  rwlock_destroy(&rc->lock);
}
static void rc_rdlock(WT_RESOURCE *rc)
{
  DBUG_PRINT("wt", ("TRYLOCK resid=%ld for READ", (ulong)rc->id.value));
  rw_rdlock(&rc->lock);
  DBUG_PRINT("wt", ("LOCK resid=%ld for READ", (ulong)rc->id.value));
}
static void rc_wrlock(WT_RESOURCE *rc)
{
  DBUG_PRINT("wt", ("TRYLOCK resid=%ld for WRITE", (ulong)rc->id.value));
  rw_wrlock(&rc->lock);
  DBUG_PRINT("wt", ("LOCK resid=%ld for WRITE", (ulong)rc->id.value));
}
static void rc_unlock(WT_RESOURCE *rc)
{
  DBUG_PRINT("wt", ("UNLOCK resid=%ld", (ulong)rc->id.value));
  rw_unlock(&rc->lock);
}
#endif

/*
  All resources are stored in a lock-free hash. Different threads
  may add new resources and perform deadlock detection concurrently.
*/
static LF_HASH      reshash;

static void wt_resource_init(uchar *arg)
{
  WT_RESOURCE *rc= (WT_RESOURCE*)(arg+LF_HASH_OVERHEAD);
  DBUG_ENTER("wt_resource_init");

  memset(rc, 0, sizeof(*rc));
  rc_rwlock_init(rc);
  mysql_cond_init(0, &rc->cond, 0);
  my_init_dynamic_array(&rc->owners, sizeof(WT_THD *), 0, 5);
  DBUG_VOID_RETURN;
}

static void wt_resource_destroy(uchar *arg)
{
  WT_RESOURCE *rc= (WT_RESOURCE*)(arg+LF_HASH_OVERHEAD);
  DBUG_ENTER("wt_resource_destroy");

  DBUG_ASSERT(rc->owners.elements == 0);
  rc_rwlock_destroy(rc);
  mysql_cond_destroy(&rc->cond);
  delete_dynamic(&rc->owners);
  DBUG_VOID_RETURN;
}

void wt_init()
{
  DBUG_ENTER("wt_init");
  DBUG_ASSERT(reshash.alloc.constructor != wt_resource_init);

  lf_hash_init(&reshash, sizeof(WT_RESOURCE), LF_HASH_UNIQUE, 0,
               sizeof_WT_RESOURCE_ID, 0, 0);
  reshash.alloc.constructor= wt_resource_init;
  reshash.alloc.destructor= wt_resource_destroy;
  /*
    Note a trick: we initialize the hash with the real element size,
    but fix it later to a shortened element size. This way
    the allocator will allocate elements correctly, but
    lf_hash_insert() will only overwrite part of the element with memcpy().
    lock, condition, and dynamic array will be intact.
  */
  reshash.element_size= offsetof(WT_RESOURCE, lock);
  memset(wt_wait_stats, 0, sizeof(wt_wait_stats));
  memset(wt_cycle_stats, 0, sizeof(wt_cycle_stats));
  wt_success_stats= 0;
  { /* initialize wt_wait_table[]. from 1 us to 1 min, log e scale */
    int i;
    double from= log(1);   /* 1 us */
    double to= log(60e6);  /* 1 min */
    for (i= 0; i < WT_WAIT_STATS; i++)
    {
      wt_wait_table[i]= (ulonglong)exp((to-from)/(WT_WAIT_STATS-1)*i+from);
      DBUG_ASSERT(i == 0 || wt_wait_table[i-1] != wt_wait_table[i]);
    }
  }
  my_atomic_rwlock_init(&cycle_stats_lock);
  my_atomic_rwlock_init(&success_stats_lock);
  my_atomic_rwlock_init(&wait_stats_lock);
  DBUG_VOID_RETURN;
}

void wt_end()
{
  DBUG_ENTER("wt_end");

  DBUG_ASSERT(reshash.count == 0);
  lf_hash_destroy(&reshash);
  my_atomic_rwlock_destroy(&cycle_stats_lock);
  my_atomic_rwlock_destroy(&success_stats_lock);
  my_atomic_rwlock_destroy(&wait_stats_lock);
  DBUG_VOID_RETURN;
}

void wt_thd_lazy_init(WT_THD *thd, const ulong *ds, const ulong *ts,
                                   const ulong *dl, const ulong *tl)
{
  DBUG_ENTER("wt_thd_lazy_init");
  thd->waiting_for= 0;
  thd->weight= 0;
  thd->deadlock_search_depth_short= ds;
  thd->timeout_short= ts;
  thd->deadlock_search_depth_long= dl;
  thd->timeout_long= tl;
  /* dynamic array is also initialized lazily - without memory allocations */
  my_init_dynamic_array(&thd->my_resources, sizeof(WT_RESOURCE *), 0, 5);
#ifndef DBUG_OFF
  thd->name= my_thread_name();
#endif
  DBUG_VOID_RETURN;
}

static int fix_thd_pins(WT_THD *thd)
{
  if (unlikely(thd->pins == 0))
  {
    thd->pins= lf_hash_get_pins(&reshash);
#ifndef DBUG_OFF
    thd->name= my_thread_name();
#endif
  }
  return thd->pins == 0;
}

void wt_thd_destroy(WT_THD *thd)
{
  DBUG_ENTER("wt_thd_destroy");

  DBUG_ASSERT(thd->my_resources.elements == 0);
  DBUG_ASSERT(thd->waiting_for == 0);

  if (thd->pins != 0)
    lf_hash_put_pins(thd->pins);

  delete_dynamic(&thd->my_resources);
  DBUG_VOID_RETURN;
}
my_bool wt_resource_id_memcmp(const void *a, const void *b)
{
  /* we use the fact that there's no padding in the middle of WT_RESOURCE_ID */
  compile_time_assert(offsetof(WT_RESOURCE_ID, type) == sizeof(ulonglong));
  return memcmp(a, b, sizeof_WT_RESOURCE_ID);
}

struct deadlock_arg {
  WT_THD * const thd;          
  uint const max_depth;        
  WT_THD *victim;              
  WT_RESOURCE *last_locked_rc; 
};

static void change_victim(WT_THD* found, struct deadlock_arg *arg)
{
  if (found->weight < arg->victim->weight)
  {
    if (arg->victim != arg->thd)
    {
      rc_unlock(arg->victim->waiting_for); /* release the previous victim */
      DBUG_ASSERT(arg->last_locked_rc == found->waiting_for);
    }
    arg->victim= found;
    arg->last_locked_rc= 0;
  }
}

static int deadlock_search(struct deadlock_arg *arg, WT_THD *blocker,
                           uint depth)
{
  WT_RESOURCE *rc, *volatile *shared_ptr= &blocker->waiting_for;
  WT_THD *cursor;
  uint i;
  int ret= WT_OK;
  DBUG_ENTER("deadlock_search");
  DBUG_PRINT("wt", ("enter: thd=%s, blocker=%s, depth=%u",
                    arg->thd->name, blocker->name, depth));

  LF_REQUIRE_PINS(1);

  arg->last_locked_rc= 0;

  if (depth > arg->max_depth)
  {
    DBUG_PRINT("wt", ("exit: WT_DEPTH_EXCEEDED (early)"));
    DBUG_RETURN(WT_DEPTH_EXCEEDED);
  }

retry:
  /*
    safe dereference as explained in lf_alloc-pin.c
    (in short: protects against lf_alloc_free() in lf_hash_delete())
  */
  do
  {
    rc= *shared_ptr;
    lf_pin(arg->thd->pins, 0, rc);
  } while (rc != *shared_ptr && LF_BACKOFF);

  if (rc == 0)
  {
    DBUG_PRINT("wt", ("exit: OK (early)"));
    DBUG_RETURN(0);
  }

  rc_rdlock(rc);
  if (rc->state != ACTIVE || *shared_ptr != rc)
  {
    /* blocker is not waiting on this resource anymore */
    rc_unlock(rc);
    lf_unpin(arg->thd->pins, 0);
    goto retry;
  }
  /* as the state is locked, we can unpin now */
  lf_unpin(arg->thd->pins, 0);

  /*
    Below is not a pure depth-first search. It's a depth-first with a
    slightest hint of breadth-first. Depth-first is:

      check(element, X):
        foreach current in element->nodes[] do:
          if current == X return error;
          check(current, X);

    while we do

      check(element, X):
        foreach current in element->nodes[] do:
          if current == X return error;
        foreach current in element->nodes[] do:
          check(current, X);

    preferring shorter deadlocks over longer ones.
  */
  for (i= 0; i < rc->owners.elements; i++)
  {
    cursor= *dynamic_element(&rc->owners, i, WT_THD**);
    /*
      We're only looking for (and detecting) cycles that include 'arg->thd'.
      That is, only deadlocks that *we* have created. For example,
        thd->A->B->thd
      (thd waits for A, A waits for B, while B is waiting for thd).
      While walking the graph we can encounter other cicles, e.g.
        thd->A->B->C->A
      This will not be detected. Instead we will walk it in circles until
      the search depth limit is reached (the latter guarantees that an
      infinite loop is impossible). We expect the thread that has created
      the cycle (one of A, B, and C) to detect its deadlock.
    */
    if (cursor == arg->thd)
    {
      ret= WT_DEADLOCK;
      increment_cycle_stats(depth, arg->max_depth ==
                                   *arg->thd->deadlock_search_depth_long);
      arg->victim= cursor;
      goto end;
    }
  }
  for (i= 0; i < rc->owners.elements; i++)
  {
    cursor= *dynamic_element(&rc->owners, i, WT_THD**);
    switch (deadlock_search(arg, cursor, depth+1)) {
    case WT_OK:
      break;
    case WT_DEPTH_EXCEEDED:
      ret= WT_DEPTH_EXCEEDED;
      break;
    case WT_DEADLOCK:
      ret= WT_DEADLOCK;
      change_victim(cursor, arg);       /* also sets arg->last_locked_rc to 0 */
      i= rc->owners.elements;           /* jump out of the loop */
      break;
    default:
      DBUG_ASSERT(0);
    }
    if (arg->last_locked_rc)
      rc_unlock(arg->last_locked_rc);
  }
end:
  /*
    Note that 'rc' is locked in this function, but it's never unlocked here.
    Instead it's saved in arg->last_locked_rc and the *caller* is
    expected to unlock it.  It's done to support different killing
    strategies. This is how it works:
    Assuming a graph

      thd->A->B->C->thd

    deadlock_search() function starts from thd, locks it (in fact it locks not
    a thd, but a resource it is waiting on, but below, for simplicity, I'll
    talk about "locking a thd"). Then it goes down recursively, locks A, and so
    on. Goes down recursively, locks B. Goes down recursively, locks C.
    Notices that C is waiting on thd. Deadlock detected. Sets arg->victim=thd.
    Returns from the last deadlock_search() call. C stays locked!
    Now it checks whether C is a more appropriate victim than 'thd'.
    If yes - arg->victim=C, otherwise C is unlocked. Returns. B stays locked.
    Now it checks whether B is a more appropriate victim than arg->victim.
    If yes - old arg->victim is unlocked and arg->victim=B,
    otherwise B is unlocked. Return.
    And so on.

    In short, a resource is locked in a frame. But it's not unlocked in the
    same frame, it's unlocked by the caller, and only after the caller checks
    that it doesn't need to use current WT_THD as a victim. If it does - the
    lock is kept and the old victim's resource is unlocked. When the recursion
    is unrolled and we are back to deadlock() function, there are only two
    locks left - on thd and on the victim.
  */
  arg->last_locked_rc= rc;
  DBUG_PRINT("wt", ("exit: %s",
                    ret == WT_DEPTH_EXCEEDED ? "WT_DEPTH_EXCEEDED" :
                    ret ? "WT_DEADLOCK" : "OK"));
  DBUG_RETURN(ret);
}

static int deadlock(WT_THD *thd, WT_THD *blocker, uint depth,
                            uint max_depth)
{
  struct deadlock_arg arg= {thd, max_depth, 0, 0};
  int ret;
  DBUG_ENTER("deadlock");
  DBUG_ASSERT(depth < 2);
  ret= deadlock_search(&arg, blocker, depth);
  if (ret == WT_DEPTH_EXCEEDED)
  {
    increment_cycle_stats(WT_CYCLE_STATS, max_depth ==
                                          *thd->deadlock_search_depth_long);
    ret= WT_OK;
  }
  /*
    if we started with depth==1, blocker was never considered for a victim
    in deadlock_search(). Do it here.
  */
  if (ret == WT_DEADLOCK && depth)
    change_victim(blocker, &arg);
  if (arg.last_locked_rc)
  {
    /*
      Special return code if there's nobody to wait for.

      depth == 0 means that we start the search from thd (thd == blocker).
      ret == WT_OK means that no cycle was found and
        arg.last_locked_rc == thd->waiting_for.
      and arg.last_locked_rc->owners.elements == 0 means that
        (applying the rule above) thd->waiting_for->owners.elements == 0,
        and thd doesn't have anybody to wait for.
    */
    if (depth == 0 && ret == WT_OK && arg.last_locked_rc->owners.elements == 0)
    {
      DBUG_ASSERT(thd == blocker);
      DBUG_ASSERT(arg.last_locked_rc == thd->waiting_for);
      ret= WT_FREE_TO_GO;
    }
    rc_unlock(arg.last_locked_rc);
  }
  /* notify the victim, if appropriate */
  if (ret == WT_DEADLOCK && arg.victim != thd)
  {
    DBUG_PRINT("wt", ("killing %s", arg.victim->name));
    arg.victim->killed= 1;
    mysql_cond_broadcast(&arg.victim->waiting_for->cond);
    rc_unlock(arg.victim->waiting_for);
    ret= WT_OK;
  }
  DBUG_RETURN(ret);
}


static int unlock_lock_and_free_resource(WT_THD *thd, WT_RESOURCE *rc)
{
  uint keylen;
  const void *key;
  DBUG_ENTER("unlock_lock_and_free_resource");

  DBUG_ASSERT(rc->state == ACTIVE);

  if (rc->owners.elements || rc->waiter_count)
  {
    DBUG_PRINT("wt", ("nothing to do, %u owners, %u waiters",
                      rc->owners.elements, rc->waiter_count));
    rc_unlock(rc);
    DBUG_RETURN(0);
  }

  if (fix_thd_pins(thd))
  {
    rc_unlock(rc);
    DBUG_RETURN(1);
  }

  /* XXX if (rc->id.type->make_key) key= rc->id.type->make_key(&rc->id, &keylen); else */
  {
    key= &rc->id;
    keylen= sizeof_WT_RESOURCE_ID;
  }

  /*
    To free the element correctly we need to:
     1. take its lock (already done).
     2. set the state to FREE
     3. release the lock
     4. remove from the hash
  */
  rc->state= FREE;
  rc_unlock(rc);
  DBUG_RETURN(lf_hash_delete(&reshash, thd->pins, key, keylen) == -1);
}


static int stop_waiting_locked(WT_THD *thd)
{
  int ret;
  WT_RESOURCE *rc= thd->waiting_for;
  DBUG_ENTER("stop_waiting_locked");

  DBUG_ASSERT(rc->waiter_count);
  DBUG_ASSERT(rc->state == ACTIVE);
  rc->waiter_count--;
  thd->waiting_for= 0;
  ret= unlock_lock_and_free_resource(thd, rc);
  DBUG_RETURN((thd->killed || ret) ? WT_DEADLOCK : WT_OK);
}

static int stop_waiting(WT_THD *thd)
{
  int ret;
  WT_RESOURCE *rc= thd->waiting_for;
  DBUG_ENTER("stop_waiting");

  if (!rc)
    DBUG_RETURN(WT_OK);
  /*
    nobody's trying to free the resource now,
    as its waiter_count is guaranteed to be non-zero
  */
  rc_wrlock(rc);
  ret= stop_waiting_locked(thd);
  DBUG_RETURN(ret);
}

int wt_thd_will_wait_for(WT_THD *thd, WT_THD *blocker,
                         const WT_RESOURCE_ID *resid)
{
  uint i;
  WT_RESOURCE *rc;
  DBUG_ENTER("wt_thd_will_wait_for");

  LF_REQUIRE_PINS(3);

  DBUG_PRINT("wt", ("enter: thd=%s, blocker=%s, resid=%lu",
                    thd->name, blocker->name, (ulong)resid->value));

  if (fix_thd_pins(thd))
    DBUG_RETURN(WT_DEADLOCK);

  if (thd->waiting_for == 0)
  {
    uint keylen;
    const void *key;
    /* XXX if (restype->make_key) key= restype->make_key(resid, &keylen); else */
    {
      key= resid;
      keylen= sizeof_WT_RESOURCE_ID;
    }

    DBUG_PRINT("wt", ("first blocker"));

retry:
    while ((rc= lf_hash_search(&reshash, thd->pins, key, keylen)) == 0)
    {
      WT_RESOURCE tmp;

      DBUG_PRINT("wt", ("failed to find rc in hash, inserting"));
      memset(&tmp, 0, sizeof(tmp));
      tmp.id= *resid;
      tmp.state= ACTIVE;

      if (lf_hash_insert(&reshash, thd->pins, &tmp) == -1) /* if OOM */
        DBUG_RETURN(WT_DEADLOCK);
      /*
        Two cases: either lf_hash_insert() failed - because another thread
        has just inserted a resource with the same id - and we need to retry.
        Or lf_hash_insert() succeeded, and then we need to repeat
        lf_hash_search() to find a real address of the newly inserted element.
        That is, we don't care what lf_hash_insert() has returned.
        And we need to repeat the loop anyway.
      */
    }
    if (rc == MY_ERRPTR)
      DBUG_RETURN(WT_DEADLOCK);

    DBUG_PRINT("wt", ("found in hash rc=%p", rc));

    rc_wrlock(rc);
    if (rc->state != ACTIVE)
    {
      DBUG_PRINT("wt", ("but it's not active, retrying"));
      /* Somebody has freed the element while we weren't looking */
      rc_unlock(rc);
      lf_hash_search_unpin(thd->pins);
      goto retry;
    }

    lf_hash_search_unpin(thd->pins); /* the element cannot go away anymore */
    thd->waiting_for= rc;
    rc->waiter_count++;
    thd->killed= 0;
  }
  else
  {
    DBUG_ASSERT(thd->waiting_for->id.type == resid->type);
    DBUG_ASSERT(resid->type->compare(&thd->waiting_for->id, resid) == 0);
    DBUG_PRINT("wt", ("adding another blocker"));

    /*
      we can safely access the resource here, it's in the hash as it has
      non-zero waiter_count
    */
    rc= thd->waiting_for;
    rc_wrlock(rc);
    DBUG_ASSERT(rc->waiter_count);
    DBUG_ASSERT(rc->state == ACTIVE);

    if (thd->killed)
    {
      stop_waiting_locked(thd);
      DBUG_RETURN(WT_DEADLOCK);
    }
  }
  /*
    Another thread could be waiting on this resource for this very 'blocker'.
    In this case we should not add it to the list for the second time.
  */
  for (i= 0; i < rc->owners.elements; i++)
    if (*dynamic_element(&rc->owners, i, WT_THD**) == blocker)
      break;
  if (i >= rc->owners.elements)
  {
    if (push_dynamic(&blocker->my_resources, (void*)&rc))
    {
      stop_waiting_locked(thd);
      DBUG_RETURN(WT_DEADLOCK); /* deadlock and OOM use the same error code */
    }
    if (push_dynamic(&rc->owners, (void*)&blocker))
    {
      pop_dynamic(&blocker->my_resources);
      stop_waiting_locked(thd);
      DBUG_RETURN(WT_DEADLOCK);
    }
  }
  rc_unlock(rc);

  if (deadlock(thd, blocker, 1, *thd->deadlock_search_depth_short) != WT_OK)
  {
    stop_waiting(thd);
    DBUG_RETURN(WT_DEADLOCK);
  }
  DBUG_RETURN(WT_OK);
}

int wt_thd_cond_timedwait(WT_THD *thd, mysql_mutex_t *mutex)
{
  int ret= WT_TIMEOUT;
  struct timespec timeout;
  ulonglong before, after, starttime;
  WT_RESOURCE *rc= thd->waiting_for;
  DBUG_ENTER("wt_thd_cond_timedwait");
  DBUG_PRINT("wt", ("enter: thd=%s, rc=%p", thd->name, rc));

#ifndef DBUG_OFF
  if (rc->cond_mutex)
    DBUG_ASSERT(rc->cond_mutex == mutex);
  else
    rc->cond_mutex= mutex;
  mysql_mutex_assert_owner(mutex);
#endif

  before= starttime= my_getsystime();

#ifdef __WIN__
  /*
    only for the sake of Windows we distinguish between
    'before' and 'starttime':

    my_getsystime() returns high-resolution value, that cannot be used for
    waiting (it doesn't follow system clock changes), but is good for time
    intervals.

    GetSystemTimeAsFileTime() follows system clock, but is low-resolution
    and will result in lousy intervals.
  */
  GetSystemTimeAsFileTime((PFILETIME)&starttime);
#endif

  rc_wrlock(rc);
  if (rc->owners.elements == 0)
    ret= WT_OK;
  rc_unlock(rc);

  set_timespec_time_nsec(timeout, starttime, (*thd->timeout_short)*1000ULL);
  if (ret == WT_TIMEOUT && !thd->killed)
    ret= mysql_cond_timedwait(&rc->cond, mutex, &timeout);
  if (ret == WT_TIMEOUT && !thd->killed)
  {
    int r= deadlock(thd, thd, 0, *thd->deadlock_search_depth_long);
    if (r == WT_FREE_TO_GO)
      ret= WT_OK;
    else if (r != WT_OK)
      ret= WT_DEADLOCK;
    else if (*thd->timeout_long > *thd->timeout_short)
    {
      set_timespec_time_nsec(timeout, starttime, (*thd->timeout_long)*1000ULL);
      if (!thd->killed)
        ret= mysql_cond_timedwait(&rc->cond, mutex, &timeout);
    }
  }
  after= my_getsystime();
  if (stop_waiting(thd) == WT_DEADLOCK) /* if we're killed */
    ret= WT_DEADLOCK;
  increment_wait_stats(after-before, ret);
  if (ret == WT_OK)
    increment_success_stats();
  DBUG_RETURN(ret);
}

void wt_thd_release(WT_THD *thd, const WT_RESOURCE_ID *resid)
{
  uint i;
  DBUG_ENTER("wt_thd_release");

  for (i= 0; i < thd->my_resources.elements; i++)
  {
    WT_RESOURCE *rc= *dynamic_element(&thd->my_resources, i, WT_RESOURCE**);
    if (!resid || (resid->type->compare(&rc->id, resid) == 0))
    {
      uint j;

      rc_wrlock(rc);
      /*
        nobody's trying to free the resource now,
        as its owners[] array is not empty (at least thd must be there)
      */
      DBUG_ASSERT(rc->state == ACTIVE);
      for (j= 0; j < rc->owners.elements; j++)
        if (*dynamic_element(&rc->owners, j, WT_THD**) == thd)
          break;
      DBUG_ASSERT(j < rc->owners.elements);
      delete_dynamic_element(&rc->owners, j);
      if (rc->owners.elements == 0)
      {
        mysql_cond_broadcast(&rc->cond);
#ifndef DBUG_OFF
        if (rc->cond_mutex)
          mysql_mutex_assert_owner(rc->cond_mutex);
#endif
      }
      unlock_lock_and_free_resource(thd, rc);
      if (resid)
      {
        delete_dynamic_element(&thd->my_resources, i);
        DBUG_VOID_RETURN;
      }
    }
  }
  if (!resid)
    reset_dynamic(&thd->my_resources);
  DBUG_VOID_RETURN;
}