sync0arr.c

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/******************************************************
The wait array used in synchronization primitives

(c) 1995 Innobase Oy

Created 9/5/1995 Heikki Tuuri
*******************************************************/

#include "sync0arr.h"
#ifdef UNIV_NONINL
#include "sync0arr.ic"
#endif

#include "sync0sync.h"
#include "sync0rw.h"
#include "os0sync.h"
#include "os0file.h"
#include "srv0srv.h"

/*
			WAIT ARRAY
			==========

The wait array consists of cells each of which has an
an operating system event object created for it. The threads
waiting for a mutex, for example, can reserve a cell
in the array and suspend themselves to wait for the event
to become signaled. When using the wait array, remember to make
sure that some thread holding the synchronization object
will eventually know that there is a waiter in the array and
signal the object, to prevent infinite wait.
Why we chose to implement a wait array? First, to make
mutexes fast, we had to code our own implementation of them,
which only in usually uncommon cases resorts to using
slow operating system primitives. Then we had the choice of
assigning a unique OS event for each mutex, which would
be simpler, or using a global wait array. In some operating systems,
the global wait array solution is more efficient and flexible,
because we can do with a very small number of OS events,
say 200. In NT 3.51, allocating events seems to be a quadratic
algorithm, because 10 000 events are created fast, but
100 000 events takes a couple of minutes to create.
*/

/* A cell where an individual thread may wait suspended
until a resource is released. The suspending is implemented
using an operating system event semaphore. */
struct sync_cell_struct {
        void*           wait_object;    /* pointer to the object the
                                        thread is waiting for; if NULL
                                        the cell is free for use */
	mutex_t*	old_wait_mutex;	/* the latest wait mutex in cell */
	rw_lock_t*	old_wait_rw_lock;/* the latest wait rw-lock in cell */
        ulint		request_type;	/* lock type requested on the
        				object */
	const char*	file;		/* in debug version file where
					requested */
	ulint		line;		/* in debug version line where
					requested */
	os_thread_id_t	thread;		/* thread id of this waiting
					thread */
	ibool		waiting;	/* TRUE if the thread has already
					called sync_array_event_wait
					on this cell */
	ibool		event_set;	/* TRUE if the event is set */
        os_event_t 	event;   	/* operating system event
                                        semaphore handle */
	time_t		reservation_time;/* time when the thread reserved
					the wait cell */
};

/* NOTE: It is allowed for a thread to wait
for an event allocated for the array without owning the
protecting mutex (depending on the case: OS or database mutex), but
all changes (set or reset) to the state of the event must be made
while owning the mutex. */
struct sync_array_struct {
	ulint		n_reserved;	/* number of currently reserved
					cells in the wait array */
	ulint		n_cells;	/* number of cells in the
					wait array */
	sync_cell_t*	array;		/* pointer to wait array */
	ulint		protection;	/* this flag tells which
					mutex protects the data */
	mutex_t		mutex;		/* possible database mutex
					protecting this data structure */
	os_mutex_t	os_mutex;	/* Possible operating system mutex
					protecting the data structure.
					As this data structure is used in
					constructing the database mutex,
					to prevent infinite recursion
					in implementation, we fall back to
					an OS mutex. */
	ulint		sg_count;	/* count of how many times an
					object has been signalled */
	ulint		res_count;	/* count of cell reservations
					since creation of the array */
};

#ifdef UNIV_SYNC_DEBUG
/**********************************************************************
This function is called only in the debug version. Detects a deadlock
of one or more threads because of waits of semaphores. */
static
ibool
sync_array_detect_deadlock(
/*=======================*/
				/* out: TRUE if deadlock detected */
        sync_array_t*	arr,	/* in: wait array; NOTE! the caller must
        			own the mutex to array */
	sync_cell_t*	start,	/* in: cell where recursive search started */
	sync_cell_t*	cell,	/* in: cell to search */
	ulint		depth);	/* in: recursion depth */
#endif /* UNIV_SYNC_DEBUG */

/*********************************************************************
Gets the nth cell in array. */
static
sync_cell_t*
sync_array_get_nth_cell(
/*====================*/
				/* out: cell */
	sync_array_t*	arr,	/* in: sync array */
	ulint		n)	/* in: index */
{
	ut_a(arr);
	ut_a(n < arr->n_cells);

	return(arr->array + n);
}

/**********************************************************************
Reserves the mutex semaphore protecting a sync array. */
static
void
sync_array_enter(
/*=============*/
	sync_array_t*	arr)	/* in: sync wait array */
{
	ulint	protection;

	protection = arr->protection;

	if (protection == SYNC_ARRAY_OS_MUTEX) {
		os_mutex_enter(arr->os_mutex);
	} else if (protection == SYNC_ARRAY_MUTEX) {
		mutex_enter(&(arr->mutex));
	} else {
		ut_error;
	}
}

/**********************************************************************
Releases the mutex semaphore protecting a sync array. */
static
void
sync_array_exit(
/*============*/
	sync_array_t*	arr)	/* in: sync wait array */
{
	ulint	protection;

	protection = arr->protection;

	if (protection == SYNC_ARRAY_OS_MUTEX) {
		os_mutex_exit(arr->os_mutex);
	} else if (protection == SYNC_ARRAY_MUTEX) {
		mutex_exit(&(arr->mutex));
	} else {
		ut_error;
	}
}

/***********************************************************************
Creates a synchronization wait array. It is protected by a mutex
which is automatically reserved when the functions operating on it
are called. */

sync_array_t*
sync_array_create(
/*==============*/
				/* out, own: created wait array */
	ulint	n_cells,	/* in: number of cells in the array
				to create */
	ulint	protection)	/* in: either SYNC_ARRAY_OS_MUTEX or
				SYNC_ARRAY_MUTEX: determines the type
				of mutex protecting the data structure */
{
	sync_array_t*	arr;
	sync_cell_t*	cell_array;
	sync_cell_t*	cell;
	ulint		i;
	
	ut_a(n_cells > 0);

	/* Allocate memory for the data structures */
	arr = ut_malloc(sizeof(sync_array_t));

	cell_array = ut_malloc(sizeof(sync_cell_t) * n_cells);

	arr->n_cells = n_cells;
	arr->n_reserved = 0;
	arr->array = cell_array;
	arr->protection = protection;
	arr->sg_count = 0;
	arr->res_count = 0;

        /* Then create the mutex to protect the wait array complex */
	if (protection == SYNC_ARRAY_OS_MUTEX) {
		arr->os_mutex = os_mutex_create(NULL);
	} else if (protection == SYNC_ARRAY_MUTEX) {
		mutex_create(&(arr->mutex));
		mutex_set_level(&(arr->mutex), SYNC_NO_ORDER_CHECK);
	} else {
		ut_error;
	}

        for (i = 0; i < n_cells; i++) {
		cell = sync_array_get_nth_cell(arr, i);        	
                cell->wait_object = NULL;

                /* Create an operating system event semaphore with no name */
                cell->event = os_event_create(NULL);
		cell->event_set = FALSE; /* it is created in reset state */
	}

	return(arr);
}

/**********************************************************************
Frees the resources in a wait array. */

void
sync_array_free(
/*============*/
	sync_array_t*	arr)	/* in, own: sync wait array */
{
        ulint           i;
        sync_cell_t*   	cell;
	ulint		protection;

        ut_a(arr->n_reserved == 0);
        
	sync_array_validate(arr);
        
        for (i = 0; i < arr->n_cells; i++) {
		cell = sync_array_get_nth_cell(arr, i);        	
		os_event_free(cell->event);
        }

	protection = arr->protection;

        /* Release the mutex protecting the wait array complex */

	if (protection == SYNC_ARRAY_OS_MUTEX) {
		os_mutex_free(arr->os_mutex);
	} else if (protection == SYNC_ARRAY_MUTEX) {
		mutex_free(&(arr->mutex));
	} else {
		ut_error;
	}

	ut_free(arr->array);
	ut_free(arr);	
}

/************************************************************************
Validates the integrity of the wait array. Checks
that the number of reserved cells equals the count variable. */

void
sync_array_validate(
/*================*/
	sync_array_t*	arr)	/* in: sync wait array */
{
        ulint           i;
        sync_cell_t*   	cell;
        ulint           count           = 0;
        
        sync_array_enter(arr);

        for (i = 0; i < arr->n_cells; i++) {
		cell = sync_array_get_nth_cell(arr, i);        	

                if (cell->wait_object != NULL) {
                        count++;
                }
        }

        ut_a(count == arr->n_reserved);

        sync_array_exit(arr);
}

/***********************************************************************
Puts the cell event in set state. */
static
void
sync_cell_event_set(
/*================*/
	sync_cell_t*	cell)	/* in: array cell */
{
	os_event_set(cell->event);
	cell->event_set = TRUE;
}		

/***********************************************************************
Puts the cell event in reset state. */
static
void
sync_cell_event_reset(
/*==================*/
	sync_cell_t*	cell)	/* in: array cell */
{
	os_event_reset(cell->event);
	cell->event_set = FALSE;
}		

/**********************************************************************
Reserves a wait array cell for waiting for an object.
The event of the cell is reset to nonsignalled state. */

void
sync_array_reserve_cell(
/*====================*/
        sync_array_t*	arr,	/* in: wait array */
        void*   	object, /* in: pointer to the object to wait for */
        ulint		type,	/* in: lock request type */
	const char*	file,	/* in: file where requested */
        ulint		line,	/* in: line where requested */
        ulint*   	index)  /* out: index of the reserved cell */
{
        sync_cell_t*   	cell;
        ulint           i;
        
        ut_a(object);
        ut_a(index);

        sync_array_enter(arr);

        arr->res_count++;

	/* Reserve a new cell. */
        for (i = 0; i < arr->n_cells; i++) {
		cell = sync_array_get_nth_cell(arr, i);        	

                if (cell->wait_object == NULL) {

                        /* Make sure the event is reset */
			if (cell->event_set) {	
                        	sync_cell_event_reset(cell);
			}

			cell->reservation_time = time(NULL);
			cell->thread = os_thread_get_curr_id();

			cell->wait_object = object;

			if (type == SYNC_MUTEX) {
				cell->old_wait_mutex = object;
			} else {
				cell->old_wait_rw_lock = object;
			}
				
			cell->request_type = type;
			cell->waiting = FALSE;
			
			cell->file = file;
			cell->line = line;
			
			arr->n_reserved++;

			*index = i;

			sync_array_exit(arr);
                        
                        return;
                }
        }

        ut_error; /* No free cell found */

	return;
}

/**********************************************************************
This function should be called when a thread starts to wait on
a wait array cell. In the debug version this function checks
if the wait for a semaphore will result in a deadlock, in which
case prints info and asserts. */

void
sync_array_wait_event(
/*==================*/
        sync_array_t*	arr,	/* in: wait array */
        ulint   	index)  /* in: index of the reserved cell */
{
        sync_cell_t*   	cell;
        os_event_t 	event;
        
        ut_a(arr);

        sync_array_enter(arr);

	cell = sync_array_get_nth_cell(arr, index);        	

	ut_a(cell->wait_object);
	ut_a(!cell->waiting);
	ut_ad(os_thread_get_curr_id() == cell->thread);

       	event = cell->event;
       	cell->waiting = TRUE;

#ifdef UNIV_SYNC_DEBUG
       	
	/* We use simple enter to the mutex below, because if
	we cannot acquire it at once, mutex_enter would call
	recursively sync_array routines, leading to trouble.
	rw_lock_debug_mutex freezes the debug lists. */

	rw_lock_debug_mutex_enter();

	if (TRUE == sync_array_detect_deadlock(arr, cell, cell, 0)) {

		fputs("########################################\n", stderr);
		ut_error;
	}		

	rw_lock_debug_mutex_exit();
#endif
        sync_array_exit(arr);

        os_event_wait(event);

        sync_array_free_cell(arr, index);
}

/**********************************************************************
Reports info of a wait array cell. */
static
void
sync_array_cell_print(
/*==================*/
	FILE*		file,	/* in: file where to print */
	sync_cell_t*	cell)	/* in: sync cell */
{
	mutex_t*	mutex;
	rw_lock_t*	rwlock;
	ulint		type;

	type = cell->request_type;

	fprintf(file,
"--Thread %lu has waited at %s line %lu for %.2f seconds the semaphore:\n",
			(ulong) os_thread_pf(cell->thread), cell->file,
		       (ulong) cell->line,
		       difftime(time(NULL), cell->reservation_time));

	if (type == SYNC_MUTEX) {
		/* We use old_wait_mutex in case the cell has already
		been freed meanwhile */
		mutex = cell->old_wait_mutex;

		fprintf(file,
			"Mutex at %p created file %s line %lu, lock var %lu\n"
#ifdef UNIV_SYNC_DEBUG
			"Last time reserved in file %s line %lu, "
#endif /* UNIV_SYNC_DEBUG */
			"waiters flag %lu\n",
			mutex, mutex->cfile_name, (ulong) mutex->cline,
			(ulong) mutex->lock_word,
#ifdef UNIV_SYNC_DEBUG
			mutex->file_name, (ulong) mutex->line,
#endif /* UNIV_SYNC_DEBUG */
			(ulong) mutex->waiters);

	} else if (type == RW_LOCK_EX || type == RW_LOCK_SHARED) {

		fputs(type == RW_LOCK_EX ? "X-lock on" : "S-lock on", file);

		rwlock = cell->old_wait_rw_lock;

		fprintf(file,
			" RW-latch at %p created in file %s line %lu\n",
			rwlock, rwlock->cfile_name,
			(ulong) rwlock->cline);
		if (rwlock->writer != RW_LOCK_NOT_LOCKED) {
			fprintf(file,
			"a writer (thread id %lu) has reserved it in mode %s",
				(ulong) os_thread_pf(rwlock->writer_thread),
				rwlock->writer == RW_LOCK_EX
				? " exclusive\n"
				: " wait exclusive\n");
		}
		
		fprintf(file,
			"number of readers %lu, waiters flag %lu\n"
			"Last time read locked in file %s line %lu\n"
			"Last time write locked in file %s line %lu\n",
			(ulong) rwlock->reader_count,
			(ulong) rwlock->waiters,
			rwlock->last_s_file_name,
			(ulong) rwlock->last_s_line,