alistair23-linux/block/blk-ioc.c

/*
 * Functions related to io context handling
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/bootmem.h>	/* for max_pfn/max_low_pfn */
#include <linux/slab.h>

#include "blk.h"

/*
 * For io context allocations
 */
static struct kmem_cache *iocontext_cachep;

/**
 * get_io_context - increment reference count to io_context
 * @ioc: io_context to get
 *
 * Increment reference count to @ioc.
 */
void get_io_context(struct io_context *ioc)
{
	BUG_ON(atomic_long_read(&ioc->refcount) <= 0);
	atomic_long_inc(&ioc->refcount);
}
EXPORT_SYMBOL(get_io_context);

/*
 * Releasing ioc may nest into another put_io_context() leading to nested
 * fast path release.  As the ioc's can't be the same, this is okay but
 * makes lockdep whine.  Keep track of nesting and use it as subclass.
 */
#ifdef CONFIG_LOCKDEP
#define ioc_release_depth(q)		((q) ? (q)->ioc_release_depth : 0)
#define ioc_release_depth_inc(q)	(q)->ioc_release_depth++
#define ioc_release_depth_dec(q)	(q)->ioc_release_depth--
#else
#define ioc_release_depth(q)		0
#define ioc_release_depth_inc(q)	do { } while (0)
#define ioc_release_depth_dec(q)	do { } while (0)
#endif

static void icq_free_icq_rcu(struct rcu_head *head)
{
	struct io_cq *icq = container_of(head, struct io_cq, __rcu_head);

	kmem_cache_free(icq->__rcu_icq_cache, icq);
}

/*
 * Exit and free an icq.  Called with both ioc and q locked.
 */
static void ioc_exit_icq(struct io_cq *icq)
{
	struct io_context *ioc = icq->ioc;
	struct request_queue *q = icq->q;
	struct elevator_type *et = q->elevator->type;

	lockdep_assert_held(&ioc->lock);
	lockdep_assert_held(q->queue_lock);

	radix_tree_delete(&ioc->icq_tree, icq->q->id);
	hlist_del_init(&icq->ioc_node);
	list_del_init(&icq->q_node);

	/*
	 * Both setting lookup hint to and clearing it from @icq are done
	 * under queue_lock.  If it's not pointing to @icq now, it never
	 * will.  Hint assignment itself can race safely.
	 */
	if (rcu_dereference_raw(ioc->icq_hint) == icq)
		rcu_assign_pointer(ioc->icq_hint, NULL);

	if (et->ops.elevator_exit_icq_fn) {
		ioc_release_depth_inc(q);
		et->ops.elevator_exit_icq_fn(icq);
		ioc_release_depth_dec(q);
	}

	/*
	 * @icq->q might have gone away by the time RCU callback runs
	 * making it impossible to determine icq_cache.  Record it in @icq.
	 */
	icq->__rcu_icq_cache = et->icq_cache;
	call_rcu(&icq->__rcu_head, icq_free_icq_rcu);
}

/*
 * Slow path for ioc release in put_io_context().  Performs double-lock
 * dancing to unlink all icq's and then frees ioc.
 */
static void ioc_release_fn(struct work_struct *work)
{
	struct io_context *ioc = container_of(work, struct io_context,
					      release_work);
	struct request_queue *last_q = NULL;

	spin_lock_irq(&ioc->lock);

	while (!hlist_empty(&ioc->icq_list)) {
		struct io_cq *icq = hlist_entry(ioc->icq_list.first,
						struct io_cq, ioc_node);
		struct request_queue *this_q = icq->q;

		if (this_q != last_q) {
			/*
			 * Need to switch to @this_q.  Once we release
			 * @ioc->lock, it can go away along with @cic.
			 * Hold on to it.
			 */
			__blk_get_queue(this_q);

			/*
			 * blk_put_queue() might sleep thanks to kobject
			 * idiocy.  Always release both locks, put and
			 * restart.
			 */
			if (last_q) {
				spin_unlock(last_q->queue_lock);
				spin_unlock_irq(&ioc->lock);
				blk_put_queue(last_q);
			} else {
				spin_unlock_irq(&ioc->lock);
			}

			last_q = this_q;
			spin_lock_irq(this_q->queue_lock);
			spin_lock(&ioc->lock);
			continue;
		}
		ioc_exit_icq(icq);
	}

	if (last_q) {
		spin_unlock(last_q->queue_lock);
		spin_unlock_irq(&ioc->lock);
		blk_put_queue(last_q);
	} else {
		spin_unlock_irq(&ioc->lock);
	}

	kmem_cache_free(iocontext_cachep, ioc);
}

/**
 * put_io_context - put a reference of io_context
 * @ioc: io_context to put
 * @locked_q: request_queue the caller is holding queue_lock of (hint)
 *
 * Decrement reference count of @ioc and release it if the count reaches
 * zero.  If the caller is holding queue_lock of a queue, it can indicate
 * that with @locked_q.  This is an optimization hint and the caller is
 * allowed to pass in %NULL even when it's holding a queue_lock.
 */
void put_io_context(struct io_context *ioc, struct request_queue *locked_q)
{
	struct request_queue *last_q = locked_q;
	unsigned long flags;

	if (ioc == NULL)
		return;

	BUG_ON(atomic_long_read(&ioc->refcount) <= 0);
	if (locked_q)
		lockdep_assert_held(locked_q->queue_lock);

	if (!atomic_long_dec_and_test(&ioc->refcount))
		return;

	/*
	 * Destroy @ioc.  This is a bit messy because icq's are chained
	 * from both ioc and queue, and ioc->lock nests inside queue_lock.
	 * The inner ioc->lock should be held to walk our icq_list and then
	 * for each icq the outer matching queue_lock should be grabbed.
	 * ie. We need to do reverse-order double lock dancing.
	 *
	 * Another twist is that we are often called with one of the
	 * matching queue_locks held as indicated by @locked_q, which
	 * prevents performing double-lock dance for other queues.
	 *
	 * So, we do it in two stages.  The fast path uses the queue_lock
	 * the caller is holding and, if other queues need to be accessed,
	 * uses trylock to avoid introducing locking dependency.  This can
	 * handle most cases, especially if @ioc was performing IO on only
	 * single device.
	 *
	 * If trylock doesn't cut it, we defer to @ioc->release_work which
	 * can do all the double-locking dancing.
	 */
	spin_lock_irqsave_nested(&ioc->lock, flags,
				 ioc_release_depth(locked_q));

	while (!hlist_empty(&ioc->icq_list)) {
		struct io_cq *icq = hlist_entry(ioc->icq_list.first,
						struct io_cq, ioc_node);
		struct request_queue *this_q = icq->q;

		if (this_q != last_q) {
			if (last_q && last_q != locked_q)
				spin_unlock(last_q->queue_lock);
			last_q = NULL;

			if (!spin_trylock(this_q->queue_lock))
				break;
			last_q = this_q;
			continue;
		}
		ioc_exit_icq(icq);
	}

	if (last_q && last_q != locked_q)
		spin_unlock(last_q->queue_lock);

	spin_unlock_irqrestore(&ioc->lock, flags);

	/* if no icq is left, we're done; otherwise, kick release_work */
	if (hlist_empty(&ioc->icq_list))
		kmem_cache_free(iocontext_cachep, ioc);
	else
		schedule_work(&ioc->release_work);
}
EXPORT_SYMBOL(put_io_context);

/* Called by the exiting task */
void exit_io_context(struct task_struct *task)
{
	struct io_context *ioc;

	/* PF_EXITING prevents new io_context from being attached to @task */
	WARN_ON_ONCE(!(current->flags & PF_EXITING));

	task_lock(task);
	ioc = task->io_context;
	task->io_context = NULL;
	task_unlock(task);

	atomic_dec(&ioc->nr_tasks);
	put_io_context(ioc, NULL);
}

/**
 * ioc_clear_queue - break any ioc association with the specified queue
 * @q: request_queue being cleared
 *
 * Walk @q->icq_list and exit all io_cq's.  Must be called with @q locked.
 */
void ioc_clear_queue(struct request_queue *q)
{
	lockdep_assert_held(q->queue_lock);

	while (!list_empty(&q->icq_list)) {
		struct io_cq *icq = list_entry(q->icq_list.next,
					       struct io_cq, q_node);
		struct io_context *ioc = icq->ioc;

		spin_lock(&ioc->lock);
		ioc_exit_icq(icq);
		spin_unlock(&ioc->lock);
	}
}

void create_io_context_slowpath(struct task_struct *task, gfp_t gfp_flags,
				int node)
{
	struct io_context *ioc;

	ioc = kmem_cache_alloc_node(iocontext_cachep, gfp_flags | __GFP_ZERO,
				    node);
	if (unlikely(!ioc))
		return;

	/* initialize */
	atomic_long_set(&ioc->refcount, 1);
	atomic_set(&ioc->nr_tasks, 1);
	spin_lock_init(&ioc->lock);
	INIT_RADIX_TREE(&ioc->icq_tree, GFP_ATOMIC | __GFP_HIGH);
	INIT_HLIST_HEAD(&ioc->icq_list);
	INIT_WORK(&ioc->release_work, ioc_release_fn);

	/* try to install, somebody might already have beaten us to it */
	task_lock(task);
	if (!task->io_context && !(task->flags & PF_EXITING))
		task->io_context = ioc;
	else
		kmem_cache_free(iocontext_cachep, ioc);
	task_unlock(task);
}
EXPORT_SYMBOL(create_io_context_slowpath);

/**
 * get_task_io_context - get io_context of a task
 * @task: task of interest
 * @gfp_flags: allocation flags, used if allocation is necessary
 * @node: allocation node, used if allocation is necessary
 *
 * Return io_context of @task.  If it doesn't exist, it is created with
 * @gfp_flags and @node.  The returned io_context has its reference count
 * incremented.
 *
 * This function always goes through task_lock() and it's better to use
 * %current->io_context + get_io_context() for %current.
 */
struct io_context *get_task_io_context(struct task_struct *task,
				       gfp_t gfp_flags, int node)
{
	struct io_context *ioc;

	might_sleep_if(gfp_flags & __GFP_WAIT);

	do {
		task_lock(task);
		ioc = task->io_context;
		if (likely(ioc)) {
			get_io_context(ioc);
			task_unlock(task);
			return ioc;
		}
		task_unlock(task);
	} while (create_io_context(task, gfp_flags, node));

	return NULL;
}
EXPORT_SYMBOL(get_task_io_context);

/**
 * ioc_lookup_icq - lookup io_cq from ioc
 * @ioc: the associated io_context
 * @q: the associated request_queue
 *
 * Look up io_cq associated with @ioc - @q pair from @ioc.  Must be called
 * with @q->queue_lock held.
 */
struct io_cq *ioc_lookup_icq(struct io_context *ioc, struct request_queue *q)
{
	struct io_cq *icq;

	lockdep_assert_held(q->queue_lock);

	/*
	 * icq's are indexed from @ioc using radix tree and hint pointer,
	 * both of which are protected with RCU.  All removals are done
	 * holding both q and ioc locks, and we're holding q lock - if we
	 * find a icq which points to us, it's guaranteed to be valid.
	 */
	rcu_read_lock();
	icq = rcu_dereference(ioc->icq_hint);
	if (icq && icq->q == q)
		goto out;

	icq = radix_tree_lookup(&ioc->icq_tree, q->id);
	if (icq && icq->q == q)
		rcu_assign_pointer(ioc->icq_hint, icq);	/* allowed to race */
	else
		icq = NULL;
out:
	rcu_read_unlock();
	return icq;
}
EXPORT_SYMBOL(ioc_lookup_icq);

void ioc_set_changed(struct io_context *ioc, int which)
{
	struct io_cq *icq;
	struct hlist_node *n;

	hlist_for_each_entry(icq, n, &ioc->icq_list, ioc_node)
		set_bit(which, &icq->changed);
}

/**
 * ioc_ioprio_changed - notify ioprio change
 * @ioc: io_context of interest
 * @ioprio: new ioprio
 *
 * @ioc's ioprio has changed to @ioprio.  Set %ICQ_IOPRIO_CHANGED for all
 * icq's.  iosched is responsible for checking the bit and applying it on
 * request issue path.
 */
void ioc_ioprio_changed(struct io_context *ioc, int ioprio)
{
	unsigned long flags;

	spin_lock_irqsave(&ioc->lock, flags);
	ioc->ioprio = ioprio;
	ioc_set_changed(ioc, ICQ_IOPRIO_CHANGED);
	spin_unlock_irqrestore(&ioc->lock, flags);
}

/**
 * ioc_cgroup_changed - notify cgroup change
 * @ioc: io_context of interest
 *
 * @ioc's cgroup has changed.  Set %ICQ_CGROUP_CHANGED for all icq's.
 * iosched is responsible for checking the bit and applying it on request
 * issue path.
 */
void ioc_cgroup_changed(struct io_context *ioc)
{
	unsigned long flags;

	spin_lock_irqsave(&ioc->lock, flags);
	ioc_set_changed(ioc, ICQ_CGROUP_CHANGED);
	spin_unlock_irqrestore(&ioc->lock, flags);
}

static int __init blk_ioc_init(void)
{
	iocontext_cachep = kmem_cache_create("blkdev_ioc",
			sizeof(struct io_context), 0, SLAB_PANIC, NULL);
	return 0;
}
subsys_initcall(blk_ioc_init);