starpu-max/src/core/dependencies/implicit_data_deps.c

/* StarPU --- Runtime system for heterogeneous multicore architectures.
 *
 * Copyright (C) 2011,2012,2016                           Inria
 * Copyright (C) 2010-2019                                Université de Bordeaux
 * Copyright (C) 2010-2013,2015-2018                      CNRS
 *
 * StarPU is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or (at
 * your option) any later version.
 *
 * StarPU 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 Lesser General Public License in COPYING.LGPL for more details.
 */

#include <starpu.h>
#include <common/config.h>
#include <core/task.h>
#include <datawizard/datawizard.h>
#include <profiling/bound.h>
#include <core/debug.h>

#if 0
# define _STARPU_DEP_DEBUG(fmt, ...) fprintf(stderr, fmt, ## __VA_ARGS__);
#else
# define _STARPU_DEP_DEBUG(fmt, ...)
#endif

static void (*write_hook)(starpu_data_handle_t);

void _starpu_implicit_data_deps_write_hook(void (*func)(starpu_data_handle_t))
{
	STARPU_ASSERT_MSG(!write_hook || write_hook == func, "only one implicit data deps hook at a time\n");
	write_hook = func;
}

static void _starpu_add_ghost_dependency(starpu_data_handle_t handle, unsigned long previous, struct starpu_task *next)
{
	struct _starpu_job *next_job = _starpu_get_job_associated_to_task(next);
	_starpu_bound_job_id_dep(handle, next_job, previous);
	STARPU_AYU_ADDDEPENDENCY(previous, handle, next_job->job_id);
}

static void _starpu_add_dependency(starpu_data_handle_t handle, struct starpu_task *previous, struct starpu_task *next)
{
	_starpu_add_ghost_dependency(handle, _starpu_get_job_associated_to_task(previous)->job_id, next);
}

/* Add pre_sync_task as new accessor among the existing ones, making it depend on the last synchronization task if any.  */
static void _starpu_add_accessor(starpu_data_handle_t handle, struct starpu_task *pre_sync_task, struct starpu_task *post_sync_task, struct _starpu_task_wrapper_dlist *post_sync_task_dependency_slot)
{
	/* Add this task to the list of readers */
	STARPU_ASSERT(!post_sync_task_dependency_slot->prev);
	STARPU_ASSERT(!post_sync_task_dependency_slot->next);
	post_sync_task_dependency_slot->task = post_sync_task;
	post_sync_task_dependency_slot->next = handle->last_submitted_accessors.next;
	post_sync_task_dependency_slot->prev = &handle->last_submitted_accessors;
	post_sync_task_dependency_slot->next->prev = post_sync_task_dependency_slot;
	handle->last_submitted_accessors.next = post_sync_task_dependency_slot;

	/* This task depends on the previous synchronization task if any */
	if (handle->last_sync_task && handle->last_sync_task != post_sync_task)
	{
		struct starpu_task *task_array[1] = {handle->last_sync_task};
		_starpu_task_declare_deps_array(pre_sync_task, 1, task_array, 0);
		_starpu_add_dependency(handle, handle->last_sync_task, pre_sync_task);
		_STARPU_DEP_DEBUG("dep %p -> %p\n", handle->last_sync_task, pre_sync_task);
	}
        else
        {
		_STARPU_DEP_DEBUG("No dep\n");
        }

	/* There was perhaps no last submitted writer but a
	 * ghost one, we should report that here, and keep the
	 * ghost writer valid */
	if (
		(
#ifdef STARPU_USE_FXT
		1
#else
		_starpu_bound_recording
#endif
		|| STARPU_AYU_EVENT
		) && handle->last_submitted_ghost_sync_id_is_valid)
	{
		_STARPU_TRACE_GHOST_TASK_DEPS(handle->last_submitted_ghost_sync_id,
			_starpu_get_job_associated_to_task(pre_sync_task));
		_starpu_add_ghost_dependency(handle, handle->last_submitted_ghost_sync_id, pre_sync_task);
		_STARPU_DEP_DEBUG("dep ID%lu -> %p\n", handle->last_submitted_ghost_sync_id, pre_sync_task);
	}

	if (!pre_sync_task->cl)
	{
		/* Add a reference to be released in _starpu_handle_job_termination */
		_starpu_spin_lock(&handle->header_lock);
		handle->busy_count++;
		_starpu_spin_unlock(&handle->header_lock);
		_starpu_get_job_associated_to_task(pre_sync_task)->implicit_dep_handle = handle;
	}
}

/* This adds a new synchronization task which depends on all the previous accessors */
static void _starpu_add_sync_task(starpu_data_handle_t handle, struct starpu_task *pre_sync_task, struct starpu_task *post_sync_task, struct starpu_task *ignored_task)
{
	/* Count the existing accessors */
	unsigned naccessors = 0;
	struct _starpu_task_wrapper_dlist *l;
	l = handle->last_submitted_accessors.next;
	while (l != &handle->last_submitted_accessors)
	{
		if (l->task == ignored_task)
		{
			/* Don't make pre_sync_task depend on post_sync_task!
			 * but still drop from the list.
			 * This happens notably when a task accesses several
			 * times to the same data.
			 */
			struct _starpu_task_wrapper_dlist *next;
			l->prev->next = l->next;
			l->next->prev = l->prev;
			l->task = NULL;
			l->prev = NULL;
			next = l->next;
			l->next = NULL;
			l = next;
		}
		else
		{
			naccessors++;
			l = l->next;
		}
	}
	_STARPU_DEP_DEBUG("%d accessors\n", naccessors);

	if (naccessors > 0)
	{
		/* Put all tasks in the list into task_array */
		struct starpu_task *task_array[naccessors];
		unsigned i = 0;
		l = handle->last_submitted_accessors.next;
		while (l != &handle->last_submitted_accessors)
		{
			STARPU_ASSERT(l->task);
			STARPU_ASSERT(l->task != ignored_task);
			task_array[i++] = l->task;
			_starpu_add_dependency(handle, l->task, pre_sync_task);
			_STARPU_DEP_DEBUG("dep %p -> %p\n", l->task, pre_sync_task);

			struct _starpu_task_wrapper_dlist *prev = l;
			l = l->next;
			prev->task = NULL;
			prev->next = NULL;
			prev->prev = NULL;
		}
		_starpu_task_declare_deps_array(pre_sync_task, naccessors, task_array, 0);
	}
#ifndef STARPU_USE_FXT
	if (_starpu_bound_recording)
#endif
	{
		/* Declare all dependencies with ghost accessors */
		struct _starpu_jobid_list *ghost_accessors_id = handle->last_submitted_ghost_accessors_id;
		while (ghost_accessors_id)
		{
			unsigned long id = ghost_accessors_id->id;
			_STARPU_TRACE_GHOST_TASK_DEPS(id,
				_starpu_get_job_associated_to_task(pre_sync_task));
			_starpu_add_ghost_dependency(handle, id, pre_sync_task);
			_STARPU_DEP_DEBUG("dep ID%lu -> %p\n", id, pre_sync_task);

			struct _starpu_jobid_list *prev = ghost_accessors_id;
			ghost_accessors_id = ghost_accessors_id->next;
			free(prev);
		}
		handle->last_submitted_ghost_accessors_id = NULL;
	}

	handle->last_submitted_accessors.next = &handle->last_submitted_accessors;
	handle->last_submitted_accessors.prev = &handle->last_submitted_accessors;
	handle->last_sync_task = post_sync_task;

	if (!post_sync_task->cl)
	{
		/* Add a reference to be released in _starpu_handle_job_termination */
		_starpu_spin_lock(&handle->header_lock);
		handle->busy_count++;
		_starpu_spin_unlock(&handle->header_lock);
		_starpu_get_job_associated_to_task(post_sync_task)->implicit_dep_handle = handle;
	}
}

/* This function adds the implicit task dependencies introduced by data
 * sequential consistency. Two tasks are provided: pre_sync and post_sync which
 * respectively indicates which task is going to depend on the previous deps
 * and on which task future deps should wait. In the case of a dependency
 * introduced by a task submission, both tasks are just the submitted task, but
 * in the case of user interactions with the DSM, these may be different tasks.
 * */
/* NB : handle->sequential_consistency_mutex must be hold by the caller;
 * returns a task, to be submitted after releasing that mutex. */
struct starpu_task *_starpu_detect_implicit_data_deps_with_handle(struct starpu_task *pre_sync_task, struct starpu_task *post_sync_task, struct _starpu_task_wrapper_dlist *post_sync_task_dependency_slot,
								  starpu_data_handle_t handle, enum starpu_data_access_mode mode, unsigned task_handle_sequential_consistency)
{
	struct starpu_task *task = NULL;

	/* Do not care about some flags */
	mode &= ~ STARPU_SSEND;
	mode &= ~ STARPU_LOCALITY;

	STARPU_ASSERT(!(mode & STARPU_SCRATCH));
        _STARPU_LOG_IN();

	if (handle->sequential_consistency && task_handle_sequential_consistency)
	{
		struct _starpu_job *pre_sync_job = _starpu_get_job_associated_to_task(pre_sync_task);
		struct _starpu_job *post_sync_job = _starpu_get_job_associated_to_task(post_sync_task);

		if (mode & STARPU_W || mode == STARPU_REDUX)
		{
			handle->initialized = 1;
			if (write_hook)
				write_hook(handle);
		}

		/* Skip tasks that are associated to a reduction phase so that
		 * they do not interfere with the application. */
		if (pre_sync_job->reduction_task || post_sync_job->reduction_task)
			return NULL;

		/* In case we are generating the DAG, we add an implicit
		 * dependency between the pre and the post sync tasks in case
		 * they are not the same. */
		if (pre_sync_task != post_sync_task
#ifndef STARPU_USE_FXT
			&& _starpu_bound_recording
#endif
		)
		{
			_STARPU_TRACE_GHOST_TASK_DEPS(pre_sync_job->job_id, post_sync_job);
			_starpu_bound_task_dep(post_sync_job, pre_sync_job);
		}

		enum starpu_data_access_mode previous_mode = handle->last_submitted_mode;

		_STARPU_DEP_DEBUG("Handle %p Tasks %p %p %x->%x\n", handle, pre_sync_task, post_sync_task, previous_mode, mode);

		/*
		 * Tasks can access the data concurrently only if they have the
		 * same access mode, which can only be either:
		 * - write with STARPU_COMMUTE
		 * - read
		 * - redux
		 *
		 * In other cases, the tasks have to depend on each other.
		 */

		if ((mode & STARPU_W && mode & STARPU_COMMUTE && previous_mode & STARPU_W && previous_mode & STARPU_COMMUTE)
		  || (mode == STARPU_R && previous_mode == STARPU_R)
		  || (mode == STARPU_REDUX && previous_mode == STARPU_REDUX))
		{
			_STARPU_DEP_DEBUG("concurrently\n");
			/* Can access concurrently with current tasks */
			_starpu_add_accessor(handle, pre_sync_task, post_sync_task, post_sync_task_dependency_slot);
		}
		else
		{
			/* Can not access concurrently, have to wait for existing accessors */
			struct _starpu_task_wrapper_dlist *l = handle->last_submitted_accessors.next;
			_STARPU_DEP_DEBUG("dependency\n");

			if ((l != &handle->last_submitted_accessors && l->next != &handle->last_submitted_accessors)
					|| (handle->last_submitted_ghost_accessors_id && handle->last_submitted_ghost_accessors_id->next))
			{
				/* Several previous accessors */

				if (mode == STARPU_W)
				{
					_STARPU_DEP_DEBUG("several predecessors, and this is a W-only task, thus can serve directly as a synchronization task.\n");
					/* Optimization: this task can not
					 * combine with others anyway, use it
					 * as synchronization task by making it
					 * wait for the previous ones. */
					_starpu_add_sync_task(handle, pre_sync_task, post_sync_task, post_sync_task);
				}
				else
				{
					_STARPU_DEP_DEBUG("several predecessors, adding sync task\n");
					/* insert an empty synchronization task
					 * which waits for the whole set,
					 * instead of creating a quadratic
					 * number of dependencies. */
					struct starpu_task *sync_task = starpu_task_create();
					STARPU_ASSERT(sync_task);
					if (previous_mode == STARPU_REDUX)
						sync_task->name = "_starpu_sync_task_redux";
					else if (mode ==  STARPU_COMMUTE || previous_mode == STARPU_COMMUTE)
						sync_task->name = "_starpu_sync_task_commute";
					else
						sync_task->name = "_starpu_sync_task";
					sync_task->cl = NULL;
					sync_task->type = post_sync_task->type;

					/* Make this task wait for the previous ones */
					_starpu_add_sync_task(handle, sync_task, sync_task, post_sync_task);
					/* And the requested task wait for this one */
					_starpu_add_accessor(handle, pre_sync_task, post_sync_task, post_sync_task_dependency_slot);

					task = sync_task;
				}
			}
			else
			{
				/* One previous accessor, make it the sync
				 * task, and start depending on it. */
				if (l != &handle->last_submitted_accessors)
				{
					_STARPU_DEP_DEBUG("One previous accessor, depending on it\n");
					handle->last_sync_task = l->task;
					l->next = NULL;
					l->prev = NULL;
					handle->last_submitted_accessors.next = &handle->last_submitted_accessors;
					handle->last_submitted_accessors.prev = &handle->last_submitted_accessors;
				}
				else if (handle->last_submitted_ghost_accessors_id)
				{
					_STARPU_DEP_DEBUG("No more currently running accessor, but a ghost id, taking it.\n");
					handle->last_submitted_ghost_sync_id = handle->last_submitted_ghost_accessors_id->id;
					handle->last_submitted_ghost_sync_id_is_valid = 1;
					free(handle->last_submitted_ghost_accessors_id);
					handle->last_submitted_ghost_accessors_id = NULL;
				}
				else
				{
					_STARPU_DEP_DEBUG("No previous accessor, no dependency\n");
				}
				_starpu_add_accessor(handle, pre_sync_task, post_sync_task, post_sync_task_dependency_slot);
			}
		}
		handle->last_submitted_mode = mode;
	}
        _STARPU_LOG_OUT();
	return task;
}

int _starpu_test_implicit_data_deps_with_handle(starpu_data_handle_t handle, enum starpu_data_access_mode mode)
{
	/* Do not care about some flags */
	mode &= ~ STARPU_SSEND;
	mode &= ~ STARPU_LOCALITY;

	STARPU_ASSERT(!(mode & STARPU_SCRATCH));

	if (handle->sequential_consistency)
	{
		if (handle->last_sync_task)
			return -EAGAIN;
		if (handle->last_submitted_accessors.next != &handle->last_submitted_accessors)
			return -EAGAIN;

		if (mode & STARPU_W || mode == STARPU_REDUX)
			handle->initialized = 1;
		handle->last_submitted_mode = mode;
	}
	return 0;
}

/* Create the implicit dependencies for a newly submitted task */
void _starpu_detect_implicit_data_deps(struct starpu_task *task)
{
	STARPU_ASSERT(task->cl);
        _STARPU_LOG_IN();

	if (!task->sequential_consistency)
		return;

	/* We don't want to enforce a sequential consistency for tasks that are
	 * not visible to the application. */
	struct _starpu_job *j = _starpu_get_job_associated_to_task(task);
	if (j->reduction_task)
		return;

	j->sequential_consistency = 1;

	unsigned nbuffers = STARPU_TASK_GET_NBUFFERS(task);
        struct _starpu_data_descr *descrs = _STARPU_JOB_GET_ORDERED_BUFFERS(j);
	struct _starpu_task_wrapper_dlist *dep_slots = _STARPU_JOB_GET_DEP_SLOTS(j);

	unsigned buffer;
	for (buffer = 0; buffer < nbuffers; buffer++)
	{
		starpu_data_handle_t handle = descrs[buffer].handle;
		enum starpu_data_access_mode mode = descrs[buffer].mode;
		struct starpu_task *new_task;

		/* Scratch memory does not introduce any deps */
		if (mode & STARPU_SCRATCH)
			continue;

		if (buffer)
		{
			starpu_data_handle_t handle_m1 = descrs[buffer-1].handle;
			enum starpu_data_access_mode mode_m1 = descrs[buffer-1].mode;
			if (handle_m1 == handle && mode_m1 == mode)
				/* We have already added dependencies for this
				 * data, skip it. This reduces the number of
				 * dependencies, and allows notify_soon to work
				 * when a task uses the same data several times
				 * (otherwise it will not be able to find out that the two
				 * dependencies will be over at the same time) */
				continue;
		}

		STARPU_PTHREAD_MUTEX_LOCK(&handle->sequential_consistency_mutex);
		unsigned index = descrs[buffer].index;
		unsigned task_handle_sequential_consistency = task->handles_sequential_consistency ? task->handles_sequential_consistency[index] : handle->sequential_consistency;
		if (!task_handle_sequential_consistency)
			j->sequential_consistency = 0;
		new_task = _starpu_detect_implicit_data_deps_with_handle(task, task, &dep_slots[buffer], handle, mode, task_handle_sequential_consistency);
		STARPU_PTHREAD_MUTEX_UNLOCK(&handle->sequential_consistency_mutex);
		if (new_task)
		{
			int ret = _starpu_task_submit_internally(new_task);
			STARPU_ASSERT(!ret);
		}
	}
        _STARPU_LOG_OUT();
}

/* This function is called when a task has been executed so that we don't
 * create dependencies to task that do not exist anymore. */
/* NB: We maintain a list of "ghost deps" in case FXT is enabled. Ghost
 * dependencies are the dependencies that are implicitely enforced by StarPU
 * even if they do not imply a real dependency. For instance in the following
 * sequence, f(Ar) g(Ar) h(Aw), we expect to have h depend on both f and g, but
 * if h is submitted after the termination of f or g, StarPU will not create a
 * dependency as this is not needed anymore. */
/* the sequential_consistency_mutex of the handle has to be already held */
void _starpu_release_data_enforce_sequential_consistency(struct starpu_task *task, struct _starpu_task_wrapper_dlist *task_dependency_slot, starpu_data_handle_t handle)
{
	STARPU_PTHREAD_MUTEX_LOCK(&handle->sequential_consistency_mutex);

	if (handle->sequential_consistency)
	{

		/* If this is the last writer, there is no point in adding
		 * extra deps to that tasks that does not exists anymore */
		if (task == handle->last_sync_task)
		{
			handle->last_sync_task = NULL;

#ifndef STARPU_USE_FXT
			if (_starpu_bound_recording)
#endif
			{
				/* Save the previous writer as the ghost last writer */
				handle->last_submitted_ghost_sync_id_is_valid = 1;
				struct _starpu_job *ghost_job = _starpu_get_job_associated_to_task(task);
				handle->last_submitted_ghost_sync_id = ghost_job->job_id;
			}
		}

		/* Same if this is one of the readers: we go through the list
		 * of readers and remove the task if it is found. */
		if (task_dependency_slot && task_dependency_slot->next)
		{
#ifdef STARPU_DEBUG
			/* Make sure we are removing ourself from the proper handle */
			struct _starpu_task_wrapper_dlist *l;
			for (l = task_dependency_slot->prev; l->task; l = l->prev)
				;
			STARPU_ASSERT(l == &handle->last_submitted_accessors);
			for (l = task_dependency_slot->next; l->task; l = l->next)
				;
			STARPU_ASSERT(l == &handle->last_submitted_accessors);
#endif
			STARPU_ASSERT(task_dependency_slot->task == task);

			task_dependency_slot->next->prev = task_dependency_slot->prev;
			task_dependency_slot->prev->next = task_dependency_slot->next;
			task_dependency_slot->task = NULL;
			task_dependency_slot->next = NULL;
			task_dependency_slot->prev = NULL;
#ifndef STARPU_USE_FXT
			if (_starpu_bound_recording)
#endif
			{
				/* Save the job id of the reader task in the ghost reader linked list list */
				struct _starpu_job *ghost_reader_job = _starpu_get_job_associated_to_task(task);
				struct _starpu_jobid_list *link;
				_STARPU_MALLOC(link, sizeof(struct _starpu_jobid_list));
				link->next = handle->last_submitted_ghost_accessors_id;
				link->id = ghost_reader_job->job_id;
				handle->last_submitted_ghost_accessors_id = link;
			}
		}
	}

	STARPU_PTHREAD_MUTEX_UNLOCK(&handle->sequential_consistency_mutex);
}

/* This is the same as _starpu_release_data_enforce_sequential_consistency, but
 * for all data of a task */
void _starpu_release_task_enforce_sequential_consistency(struct _starpu_job *j)
{
	struct starpu_task *task = j->task;

	if (!task->cl)
		return;

        struct _starpu_data_descr *descrs = _STARPU_JOB_GET_ORDERED_BUFFERS(j);
	struct _starpu_task_wrapper_dlist *slots = _STARPU_JOB_GET_DEP_SLOTS(j);

        unsigned nbuffers = STARPU_TASK_GET_NBUFFERS(task);
	unsigned index;

	/* Release all implicit dependencies */
	for (index = 0; index < nbuffers; index++)
	{
		starpu_data_handle_t handle = descrs[index].handle;
		enum starpu_data_access_mode mode = descrs[index].mode;

		if (index)
		{
			starpu_data_handle_t handle_m1 = descrs[index-1].handle;
			enum starpu_data_access_mode mode_m1 = descrs[index-1].mode;
			if (handle_m1 == handle && mode_m1 == mode)
				/* See _starpu_detect_implicit_data_deps */
				continue;
		}

		_starpu_release_data_enforce_sequential_consistency(task, &slots[index], handle);
	}

	for (index = 0; index < nbuffers; index++)
	{
		starpu_data_handle_t handle = descrs[index].handle;

		if (index && descrs[index-1].handle == descrs[index].handle)
			/* We have already released this data, skip it. This
			 * depends on ordering putting writes before reads, see
			 * _starpu_compar_handles */
			continue;

		/* Release the reference acquired in _starpu_push_task_output */
		_starpu_spin_lock(&handle->header_lock);
		STARPU_ASSERT(handle->busy_count > 0);
		handle->refcnt--;
		handle->busy_count--;
		if (!_starpu_data_check_not_busy(handle))
			_starpu_spin_unlock(&handle->header_lock);

	}
}


void _starpu_add_post_sync_tasks(struct starpu_task *post_sync_task, starpu_data_handle_t handle)
{
        _STARPU_LOG_IN();
	STARPU_PTHREAD_MUTEX_LOCK(&handle->sequential_consistency_mutex);

	if (handle->sequential_consistency)
	{
		handle->post_sync_tasks_cnt++;

		struct _starpu_task_wrapper_list *link;
		_STARPU_MALLOC(link, sizeof(struct _starpu_task_wrapper_list));
		link->task = post_sync_task;
		link->next = handle->post_sync_tasks;
		handle->post_sync_tasks = link;
	}

	STARPU_PTHREAD_MUTEX_UNLOCK(&handle->sequential_consistency_mutex);
        _STARPU_LOG_OUT();
}

void _starpu_unlock_post_sync_tasks(starpu_data_handle_t handle)
{
	struct _starpu_task_wrapper_list *post_sync_tasks = NULL;
	unsigned do_submit_tasks = 0;

	/* Here helgrind would shout that this is an unprotected access, but
	 * count can only be zero if we don't have to care about
	 * post_sync_tasks_cnt at all.  */
	if (STARPU_RUNNING_ON_VALGRIND || handle->post_sync_tasks_cnt)
	{
		STARPU_PTHREAD_MUTEX_LOCK(&handle->sequential_consistency_mutex);
		if (--handle->post_sync_tasks_cnt == 0)
		{
			/* unlock all tasks : we need not hold the lock while unlocking all these tasks */
			do_submit_tasks = 1;
			post_sync_tasks = handle->post_sync_tasks;
			handle->post_sync_tasks = NULL;
		}
		STARPU_PTHREAD_MUTEX_UNLOCK(&handle->sequential_consistency_mutex);
	}

	if (do_submit_tasks)
	{
		struct _starpu_task_wrapper_list *link = post_sync_tasks;

		while (link)
		{
			/* There is no need to depend on that task now, since it was already unlocked */
			_starpu_release_data_enforce_sequential_consistency(link->task, &_starpu_get_job_associated_to_task(link->task)->implicit_dep_slot, handle);

			int ret = _starpu_task_submit_internally(link->task);
			STARPU_ASSERT(!ret);
			struct _starpu_task_wrapper_list *tmp = link;
			link = link->next;
			free(tmp);
		}
	}
}

/* If sequential consistency mode is enabled, this function blocks until the
 * handle is available in the requested access mode. */
int _starpu_data_wait_until_available(starpu_data_handle_t handle, enum starpu_data_access_mode mode, const char *sync_name)
{
	/* If sequential consistency is enabled, wait until data is available */
	STARPU_PTHREAD_MUTEX_LOCK(&handle->sequential_consistency_mutex);
	int sequential_consistency = handle->sequential_consistency;
	if (sequential_consistency)
	{
		struct starpu_task *sync_task, *new_task;
		sync_task = starpu_task_create();
		sync_task->name = sync_name;
		sync_task->detach = 0;
		sync_task->destroy = 1;
		sync_task->type = STARPU_TASK_TYPE_INTERNAL;

		/* It is not really a RW access, but we want to make sure that
		 * all previous accesses are done */
		new_task = _starpu_detect_implicit_data_deps_with_handle(sync_task, sync_task, &_starpu_get_job_associated_to_task(sync_task)->implicit_dep_slot, handle, mode, sequential_consistency);
		STARPU_PTHREAD_MUTEX_UNLOCK(&handle->sequential_consistency_mutex);

		if (new_task)
		{
			int ret = _starpu_task_submit_internally(new_task);
			STARPU_ASSERT(!ret);
		}

		/* TODO detect if this is superflous */
		int ret = _starpu_task_submit_internally(sync_task);
		STARPU_ASSERT(!ret);
		ret = starpu_task_wait(sync_task);
		STARPU_ASSERT(ret == 0);
	}
	else
	{
		STARPU_PTHREAD_MUTEX_UNLOCK(&handle->sequential_consistency_mutex);
	}

	return 0;
}

/* This data is about to be freed, clean our stuff */
void _starpu_data_clear_implicit(starpu_data_handle_t handle)
{
	struct _starpu_jobid_list *list;

	STARPU_PTHREAD_MUTEX_LOCK(&handle->sequential_consistency_mutex);
	list = handle->last_submitted_ghost_accessors_id;
	while (list)
	{
		struct _starpu_jobid_list *next = list->next;
		free(list);
		list = next;
	}
	STARPU_PTHREAD_MUTEX_UNLOCK(&handle->sequential_consistency_mutex);
}