blk-ioc.c

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/*
 * 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;

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);

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

    if (icq->flags & ICQ_EXITED)
        return;

    if (et->ops.elevator_exit_icq_fn)
        et->ops.elevator_exit_icq_fn(icq);

    icq->flags |= ICQ_EXITED;
}

/* Release an icq.  Called with both ioc and q locked. */
static void ioc_destroy_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);

    ioc_exit_icq(icq);

    /*
     * @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);
    unsigned long flags;

    /*
     * Exiting icq may call into put_io_context() through elevator
     * which will trigger lockdep warning.  The ioc's are guaranteed to
     * be different, use a different locking subclass here.  Use
     * irqsave variant as there's no spin_lock_irq_nested().
     */
    spin_lock_irqsave_nested(&ioc->lock, flags, 1);

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

        if (spin_trylock(q->queue_lock)) {
            ioc_destroy_icq(icq);
            spin_unlock(q->queue_lock);
        } else {
            spin_unlock_irqrestore(&ioc->lock, flags);
            cpu_relax();
            spin_lock_irqsave_nested(&ioc->lock, flags, 1);
        }
    }

    spin_unlock_irqrestore(&ioc->lock, flags);

    kmem_cache_free(iocontext_cachep, ioc);
}

void put_io_context(struct io_context *ioc)
{
    unsigned long flags;
    bool free_ioc = false;

    if (ioc == NULL)
        return;

    BUG_ON(atomic_long_read(&ioc->refcount) <= 0);

    /*
     * Releasing ioc requires reverse order double locking and we may
     * already be holding a queue_lock.  Do it asynchronously from wq.
     */
    if (atomic_long_dec_and_test(&ioc->refcount)) {
        spin_lock_irqsave(&ioc->lock, flags);
        if (!hlist_empty(&ioc->icq_list))
            schedule_work(&ioc->release_work);
        else
            free_ioc = true;
        spin_unlock_irqrestore(&ioc->lock, flags);
    }

    if (free_ioc)
        kmem_cache_free(iocontext_cachep, ioc);
}
EXPORT_SYMBOL(put_io_context);

void put_io_context_active(struct io_context *ioc)
{
    struct hlist_node *n;
    unsigned long flags;
    struct io_cq *icq;

    if (!atomic_dec_and_test(&ioc->active_ref)) {
        put_io_context(ioc);
        return;
    }

    /*
     * Need ioc lock to walk icq_list and q lock to exit icq.  Perform
     * reverse double locking.  Read comment in ioc_release_fn() for
     * explanation on the nested locking annotation.
     */
retry:
    spin_lock_irqsave_nested(&ioc->lock, flags, 1);
    hlist_for_each_entry(icq, n, &ioc->icq_list, ioc_node) {
        if (icq->flags & ICQ_EXITED)
            continue;
        if (spin_trylock(icq->q->queue_lock)) {
            ioc_exit_icq(icq);
            spin_unlock(icq->q->queue_lock);
        } else {
            spin_unlock_irqrestore(&ioc->lock, flags);
            cpu_relax();
            goto retry;
        }
    }
    spin_unlock_irqrestore(&ioc->lock, flags);

    put_io_context(ioc);
}

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

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

    atomic_dec(&ioc->nr_tasks);
    put_io_context_active(ioc);
}

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_destroy_icq(icq);
        spin_unlock(&ioc->lock);
    }
}

int create_task_io_context(struct task_struct *task, gfp_t gfp_flags, int node)
{
    struct io_context *ioc;
    int ret;

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

    /* initialize */
    atomic_long_set(&ioc->refcount, 1);
    atomic_set(&ioc->nr_tasks, 1);
    atomic_set(&ioc->active_ref, 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.  ioc shouldn't be installed if someone else
     * already did or @task, which isn't %current, is exiting.  Note
     * that we need to allow ioc creation on exiting %current as exit
     * path may issue IOs from e.g. exit_files().  The exit path is
     * responsible for not issuing IO after exit_io_context().
     */
    task_lock(task);
    if (!task->io_context &&
        (task == current || !(task->flags & PF_EXITING)))
        task->io_context = ioc;
    else
        kmem_cache_free(iocontext_cachep, ioc);

    ret = task->io_context ? 0 : -EBUSY;

    task_unlock(task);

    return ret;
}

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_task_io_context(task, gfp_flags, node));

    return NULL;
}
EXPORT_SYMBOL(get_task_io_context);

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);

struct io_cq *ioc_create_icq(struct io_context *ioc, struct request_queue *q,
                 gfp_t gfp_mask)
{
    struct elevator_type *et = q->elevator->type;
    struct io_cq *icq;

    /* allocate stuff */
    icq = kmem_cache_alloc_node(et->icq_cache, gfp_mask | __GFP_ZERO,
                    q->node);
    if (!icq)
        return NULL;

    if (radix_tree_preload(gfp_mask) < 0) {
        kmem_cache_free(et->icq_cache, icq);
        return NULL;
    }

    icq->ioc = ioc;
    icq->q = q;
    INIT_LIST_HEAD(&icq->q_node);
    INIT_HLIST_NODE(&icq->ioc_node);

    /* lock both q and ioc and try to link @icq */
    spin_lock_irq(q->queue_lock);
    spin_lock(&ioc->lock);

    if (likely(!radix_tree_insert(&ioc->icq_tree, q->id, icq))) {
        hlist_add_head(&icq->ioc_node, &ioc->icq_list);
        list_add(&icq->q_node, &q->icq_list);
        if (et->ops.elevator_init_icq_fn)
            et->ops.elevator_init_icq_fn(icq);
    } else {
        kmem_cache_free(et->icq_cache, icq);
        icq = ioc_lookup_icq(ioc, q);
        if (!icq)
            printk(KERN_ERR "cfq: icq link failed!\n");
    }

    spin_unlock(&ioc->lock);
    spin_unlock_irq(q->queue_lock);
    radix_tree_preload_end();
    return icq;
}

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);