xen/blkback: fix xenbus_transaction_start() hang caused by double xenbus_transaction_...

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / drivers / block / xen-blkback / blkback.c

/******************************************************************************
 *
 * Back-end of the driver for virtual block devices. This portion of the
 * driver exports a 'unified' block-device interface that can be accessed
 * by any operating system that implements a compatible front end. A
 * reference front-end implementation can be found in:
 *  drivers/block/xen-blkfront.c
 *
 * Copyright (c) 2003-2004, Keir Fraser & Steve Hand
 * Copyright (c) 2005, Christopher Clark
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License version 2
 * as published by the Free Software Foundation; or, when distributed
 * separately from the Linux kernel or incorporated into other
 * software packages, subject to the following license:
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this source file (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use, copy, modify,
 * merge, publish, distribute, sublicense, and/or sell copies of the Software,
 * and to permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */

#include <linux/spinlock.h>
#include <linux/kthread.h>
#include <linux/list.h>
#include <linux/delay.h>
#include <linux/freezer.h>

#include <xen/events.h>
#include <xen/page.h>
#include <asm/xen/hypervisor.h>
#include <asm/xen/hypercall.h>
#include "common.h"

/*
 * These are rather arbitrary. They are fairly large because adjacent requests
 * pulled from a communication ring are quite likely to end up being part of
 * the same scatter/gather request at the disc.
 *
 * ** TRY INCREASING 'xen_blkif_reqs' IF WRITE SPEEDS SEEM TOO LOW **
 *
 * This will increase the chances of being able to write whole tracks.
 * 64 should be enough to keep us competitive with Linux.
 */
static int xen_blkif_reqs = 64;
module_param_named(reqs, xen_blkif_reqs, int, 0);
MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate");

/* Run-time switchable: /sys/module/blkback/parameters/ */
static unsigned int log_stats;
module_param(log_stats, int, 0644);

/*
 * Each outstanding request that we've passed to the lower device layers has a
 * 'pending_req' allocated to it. Each buffer_head that completes decrements
 * the pendcnt towards zero. When it hits zero, the specified domain has a
 * response queued for it, with the saved 'id' passed back.
 */
struct pending_req {
        struct xen_blkif        *blkif;
        u64                     id;
        int                     nr_pages;
        atomic_t                pendcnt;
        unsigned short          operation;
        int                     status;
        struct list_head        free_list;
};

#define BLKBACK_INVALID_HANDLE (~0)

struct xen_blkbk {
        struct pending_req      *pending_reqs;
        /* List of all 'pending_req' available */
        struct list_head        pending_free;
        /* And its spinlock. */
        spinlock_t              pending_free_lock;
        wait_queue_head_t       pending_free_wq;
        /* The list of all pages that are available. */
        struct page             **pending_pages;
        /* And the grant handles that are available. */
        grant_handle_t          *pending_grant_handles;
};

static struct xen_blkbk *blkbk;

/*
 * Little helpful macro to figure out the index and virtual address of the
 * pending_pages[..]. For each 'pending_req' we have have up to
 * BLKIF_MAX_SEGMENTS_PER_REQUEST (11) pages. The seg would be from 0 through
 * 10 and would index in the pending_pages[..].
 */
static inline int vaddr_pagenr(struct pending_req *req, int seg)
{
        return (req - blkbk->pending_reqs) *
                BLKIF_MAX_SEGMENTS_PER_REQUEST + seg;
}

#define pending_page(req, seg) pending_pages[vaddr_pagenr(req, seg)]

static inline unsigned long vaddr(struct pending_req *req, int seg)
{
        unsigned long pfn = page_to_pfn(blkbk->pending_page(req, seg));
        return (unsigned long)pfn_to_kaddr(pfn);
}

#define pending_handle(_req, _seg) \
        (blkbk->pending_grant_handles[vaddr_pagenr(_req, _seg)])


static int do_block_io_op(struct xen_blkif *blkif);
static int dispatch_rw_block_io(struct xen_blkif *blkif,
                                struct blkif_request *req,
                                struct pending_req *pending_req);
static void make_response(struct xen_blkif *blkif, u64 id,
                          unsigned short op, int st);

/*
 * Retrieve from the 'pending_reqs' a free pending_req structure to be used.
 */
static struct pending_req *alloc_req(void)
{
        struct pending_req *req = NULL;
        unsigned long flags;

        spin_lock_irqsave(&blkbk->pending_free_lock, flags);
        if (!list_empty(&blkbk->pending_free)) {
                req = list_entry(blkbk->pending_free.next, struct pending_req,
                                 free_list);
                list_del(&req->free_list);
        }
        spin_unlock_irqrestore(&blkbk->pending_free_lock, flags);
        return req;
}

/*
 * Return the 'pending_req' structure back to the freepool. We also
 * wake up the thread if it was waiting for a free page.
 */
static void free_req(struct pending_req *req)
{
        unsigned long flags;
        int was_empty;

        spin_lock_irqsave(&blkbk->pending_free_lock, flags);
        was_empty = list_empty(&blkbk->pending_free);
        list_add(&req->free_list, &blkbk->pending_free);
        spin_unlock_irqrestore(&blkbk->pending_free_lock, flags);
        if (was_empty)
                wake_up(&blkbk->pending_free_wq);
}

/*
 * Routines for managing virtual block devices (vbds).
 */
static int xen_vbd_translate(struct phys_req *req, struct xen_blkif *blkif,
                             int operation)
{
        struct xen_vbd *vbd = &blkif->vbd;
        int rc = -EACCES;

        if ((operation != READ) && vbd->readonly)
                goto out;

        if (unlikely((req->sector_number + req->nr_sects) > vbd_sz(vbd)))
                goto out;

        req->dev  = vbd->pdevice;
        req->bdev = vbd->bdev;
        rc = 0;

 out:
        return rc;
}

static void xen_vbd_resize(struct xen_blkif *blkif)
{
        struct xen_vbd *vbd = &blkif->vbd;
        struct xenbus_transaction xbt;
        int err;
        struct xenbus_device *dev = xen_blkbk_xenbus(blkif->be);
        unsigned long long new_size = vbd_sz(vbd);

        pr_info(DRV_PFX "VBD Resize: Domid: %d, Device: (%d, %d)\n",
                blkif->domid, MAJOR(vbd->pdevice), MINOR(vbd->pdevice));
        pr_info(DRV_PFX "VBD Resize: new size %llu\n", new_size);
        vbd->size = new_size;
again:
        err = xenbus_transaction_start(&xbt);
        if (err) {
                pr_warn(DRV_PFX "Error starting transaction");
                return;
        }
        err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
                            (unsigned long long)vbd_sz(vbd));
        if (err) {
                pr_warn(DRV_PFX "Error writing new size");
                goto abort;
        }
        /*
         * Write the current state; we will use this to synchronize
         * the front-end. If the current state is "connected" the
         * front-end will get the new size information online.
         */
        err = xenbus_printf(xbt, dev->nodename, "state", "%d", dev->state);
        if (err) {
                pr_warn(DRV_PFX "Error writing the state");
                goto abort;
        }

        err = xenbus_transaction_end(xbt, 0);
        if (err == -EAGAIN)
                goto again;
        if (err)
                pr_warn(DRV_PFX "Error ending transaction");
        return;
abort:
        xenbus_transaction_end(xbt, 1);
}

/*
 * Notification from the guest OS.
 */
static void blkif_notify_work(struct xen_blkif *blkif)
{
        blkif->waiting_reqs = 1;
        wake_up(&blkif->wq);
}

irqreturn_t xen_blkif_be_int(int irq, void *dev_id)
{
        blkif_notify_work(dev_id);
        return IRQ_HANDLED;
}

/*
 * SCHEDULER FUNCTIONS
 */

static void print_stats(struct xen_blkif *blkif)
{
        pr_info("xen-blkback (%s): oo %3d  |  rd %4d  |  wr %4d  |  f %4d\n",
                 current->comm, blkif->st_oo_req,
                 blkif->st_rd_req, blkif->st_wr_req, blkif->st_f_req);
        blkif->st_print = jiffies + msecs_to_jiffies(10 * 1000);
        blkif->st_rd_req = 0;
        blkif->st_wr_req = 0;
        blkif->st_oo_req = 0;
}

int xen_blkif_schedule(void *arg)
{
        struct xen_blkif *blkif = arg;
        struct xen_vbd *vbd = &blkif->vbd;

        xen_blkif_get(blkif);

        while (!kthread_should_stop()) {
                if (try_to_freeze())
                        continue;
                if (unlikely(vbd->size != vbd_sz(vbd)))
                        xen_vbd_resize(blkif);

                wait_event_interruptible(
                        blkif->wq,
                        blkif->waiting_reqs || kthread_should_stop());
                wait_event_interruptible(
                        blkbk->pending_free_wq,
                        !list_empty(&blkbk->pending_free) ||
                        kthread_should_stop());

                blkif->waiting_reqs = 0;
                smp_mb(); /* clear flag *before* checking for work */

                if (do_block_io_op(blkif))
                        blkif->waiting_reqs = 1;

                if (log_stats && time_after(jiffies, blkif->st_print))
                        print_stats(blkif);
        }

        if (log_stats)
                print_stats(blkif);

        blkif->xenblkd = NULL;
        xen_blkif_put(blkif);

        return 0;
}

struct seg_buf {
        unsigned long buf;
        unsigned int nsec;
};
/*
 * Unmap the grant references, and also remove the M2P over-rides
 * used in the 'pending_req'.
 */
static void xen_blkbk_unmap(struct pending_req *req)
{
        struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        unsigned int i, invcount = 0;
        grant_handle_t handle;
        int ret;

        for (i = 0; i < req->nr_pages; i++) {
                handle = pending_handle(req, i);
                if (handle == BLKBACK_INVALID_HANDLE)
                        continue;
                gnttab_set_unmap_op(&unmap[invcount], vaddr(req, i),
                                    GNTMAP_host_map, handle);
                pending_handle(req, i) = BLKBACK_INVALID_HANDLE;
                invcount++;
        }

        ret = HYPERVISOR_grant_table_op(
                GNTTABOP_unmap_grant_ref, unmap, invcount);
        BUG_ON(ret);
        /*
         * Note, we use invcount, so nr->pages, so we can't index
         * using vaddr(req, i).
         */
        for (i = 0; i < invcount; i++) {
                ret = m2p_remove_override(
                        virt_to_page(unmap[i].host_addr), false);
                if (ret) {
                        pr_alert(DRV_PFX "Failed to remove M2P override for %lx\n",
                                 (unsigned long)unmap[i].host_addr);
                        continue;
                }
        }
}

static int xen_blkbk_map(struct blkif_request *req,
                         struct pending_req *pending_req,
                         struct seg_buf seg[])
{
        struct gnttab_map_grant_ref map[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        int i;
        int nseg = req->nr_segments;
        int ret = 0;

        /*
         * Fill out preq.nr_sects with proper amount of sectors, and setup
         * assign map[..] with the PFN of the page in our domain with the
         * corresponding grant reference for each page.
         */
        for (i = 0; i < nseg; i++) {
                uint32_t flags;

                flags = GNTMAP_host_map;
                if (pending_req->operation != BLKIF_OP_READ)
                        flags |= GNTMAP_readonly;
                gnttab_set_map_op(&map[i], vaddr(pending_req, i), flags,
                                  req->u.rw.seg[i].gref,
                                  pending_req->blkif->domid);
        }

        ret = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, map, nseg);
        BUG_ON(ret);

        /*
         * Now swizzle the MFN in our domain with the MFN from the other domain
         * so that when we access vaddr(pending_req,i) it has the contents of
         * the page from the other domain.
         */
        for (i = 0; i < nseg; i++) {
                if (unlikely(map[i].status != 0)) {
                        pr_debug(DRV_PFX "invalid buffer -- could not remap it\n");
                        map[i].handle = BLKBACK_INVALID_HANDLE;
                        ret |= 1;
                }

                pending_handle(pending_req, i) = map[i].handle;

                if (ret)
                        continue;

                ret = m2p_add_override(PFN_DOWN(map[i].dev_bus_addr),
                        blkbk->pending_page(pending_req, i), false);
                if (ret) {
                        pr_alert(DRV_PFX "Failed to install M2P override for %lx (ret: %d)\n",
                                 (unsigned long)map[i].dev_bus_addr, ret);
                        /* We could switch over to GNTTABOP_copy */
                        continue;
                }

                seg[i].buf  = map[i].dev_bus_addr |
                        (req->u.rw.seg[i].first_sect << 9);
        }
        return ret;
}

/*
 * Completion callback on the bio's. Called as bh->b_end_io()
 */

static void __end_block_io_op(struct pending_req *pending_req, int error)
{
        /* An error fails the entire request. */
        if ((pending_req->operation == BLKIF_OP_FLUSH_DISKCACHE) &&
            (error == -EOPNOTSUPP)) {
                pr_debug(DRV_PFX "flush diskcache op failed, not supported\n");
                xen_blkbk_flush_diskcache(XBT_NIL, pending_req->blkif->be, 0);
                pending_req->status = BLKIF_RSP_EOPNOTSUPP;
        } else if (error) {
                pr_debug(DRV_PFX "Buffer not up-to-date at end of operation,"
                         " error=%d\n", error);
                pending_req->status = BLKIF_RSP_ERROR;
        }

        /*
         * If all of the bio's have completed it is time to unmap
         * the grant references associated with 'request' and provide
         * the proper response on the ring.
         */
        if (atomic_dec_and_test(&pending_req->pendcnt)) {
                xen_blkbk_unmap(pending_req);
                make_response(pending_req->blkif, pending_req->id,
                              pending_req->operation, pending_req->status);
                xen_blkif_put(pending_req->blkif);
                free_req(pending_req);
        }
}

/*
 * bio callback.
 */
static void end_block_io_op(struct bio *bio, int error)
{
        __end_block_io_op(bio->bi_private, error);
        bio_put(bio);
}



/*
 * Function to copy the from the ring buffer the 'struct blkif_request'
 * (which has the sectors we want, number of them, grant references, etc),
 * and transmute  it to the block API to hand it over to the proper block disk.
 */
static int do_block_io_op(struct xen_blkif *blkif)
{
        union blkif_back_rings *blk_rings = &blkif->blk_rings;
        struct blkif_request req;
        struct pending_req *pending_req;
        RING_IDX rc, rp;
        int more_to_do = 0;

        rc = blk_rings->common.req_cons;
        rp = blk_rings->common.sring->req_prod;
        rmb(); /* Ensure we see queued requests up to 'rp'. */

        while (rc != rp) {

                if (RING_REQUEST_CONS_OVERFLOW(&blk_rings->common, rc))
                        break;

                if (kthread_should_stop()) {
                        more_to_do = 1;
                        break;
                }

                pending_req = alloc_req();
                if (NULL == pending_req) {
                        blkif->st_oo_req++;
                        more_to_do = 1;
                        break;
                }

                switch (blkif->blk_protocol) {
                case BLKIF_PROTOCOL_NATIVE:
                        memcpy(&req, RING_GET_REQUEST(&blk_rings->native, rc), sizeof(req));
                        break;
                case BLKIF_PROTOCOL_X86_32:
                        blkif_get_x86_32_req(&req, RING_GET_REQUEST(&blk_rings->x86_32, rc));
                        break;
                case BLKIF_PROTOCOL_X86_64:
                        blkif_get_x86_64_req(&req, RING_GET_REQUEST(&blk_rings->x86_64, rc));
                        break;
                default:
                        BUG();
                }
                blk_rings->common.req_cons = ++rc; /* before make_response() */

                /* Apply all sanity checks to /private copy/ of request. */
                barrier();

                if (dispatch_rw_block_io(blkif, &req, pending_req))
                        break;

                /* Yield point for this unbounded loop. */
                cond_resched();
        }

        return more_to_do;
}

/*
 * Transmutation of the 'struct blkif_request' to a proper 'struct bio'
 * and call the 'submit_bio' to pass it to the underlying storage.
 */
static int dispatch_rw_block_io(struct xen_blkif *blkif,
                                struct blkif_request *req,
                                struct pending_req *pending_req)
{
        struct phys_req preq;
        struct seg_buf seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        unsigned int nseg;
        struct bio *bio = NULL;
        struct bio *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        int i, nbio = 0;
        int operation;
        struct blk_plug plug;

        switch (req->operation) {
        case BLKIF_OP_READ:
                blkif->st_rd_req++;
                operation = READ;
                break;
        case BLKIF_OP_WRITE:
                blkif->st_wr_req++;
                operation = WRITE_ODIRECT;
                break;
        case BLKIF_OP_FLUSH_DISKCACHE:
                blkif->st_f_req++;
                operation = WRITE_FLUSH;
                /*
                 * The frontend likes to set this to -1, which xen_vbd_translate
                 * is alergic too.
                 */
                req->u.rw.sector_number = 0;
                break;
        case BLKIF_OP_WRITE_BARRIER:
        default:
                operation = 0; /* make gcc happy */
                goto fail_response;
                break;
        }

        /* Check that the number of segments is sane. */
        nseg = req->nr_segments;
        if (unlikely(nseg == 0 && operation != WRITE_FLUSH) ||
            unlikely(nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
                pr_debug(DRV_PFX "Bad number of segments in request (%d)\n",
                         nseg);
                /* Haven't submitted any bio's yet. */
                goto fail_response;
        }

        preq.dev           = req->handle;
        preq.sector_number = req->u.rw.sector_number;
        preq.nr_sects      = 0;

        pending_req->blkif     = blkif;
        pending_req->id        = req->id;
        pending_req->operation = req->operation;
        pending_req->status    = BLKIF_RSP_OKAY;
        pending_req->nr_pages  = nseg;

        for (i = 0; i < nseg; i++) {
                seg[i].nsec = req->u.rw.seg[i].last_sect -
                        req->u.rw.seg[i].first_sect + 1;
                if ((req->u.rw.seg[i].last_sect >= (PAGE_SIZE >> 9)) ||
                    (req->u.rw.seg[i].last_sect < req->u.rw.seg[i].first_sect))
                        goto fail_response;
                preq.nr_sects += seg[i].nsec;

        }

        if (xen_vbd_translate(&preq, blkif, operation) != 0) {
                pr_debug(DRV_PFX "access denied: %s of [%llu,%llu] on dev=%04x\n",
                         operation == READ ? "read" : "write",
                         preq.sector_number,
                         preq.sector_number + preq.nr_sects, preq.dev);
                goto fail_response;
        }

        /*
         * This check _MUST_ be done after xen_vbd_translate as the preq.bdev
         * is set there.
         */
        for (i = 0; i < nseg; i++) {
                if (((int)preq.sector_number|(int)seg[i].nsec) &
                    ((bdev_logical_block_size(preq.bdev) >> 9) - 1)) {
                        pr_debug(DRV_PFX "Misaligned I/O request from domain %d",
                                 blkif->domid);
                        goto fail_response;
                }
        }

        /*
         * If we have failed at this point, we need to undo the M2P override,
         * set gnttab_set_unmap_op on all of the grant references and perform
         * the hypercall to unmap the grants - that is all done in
         * xen_blkbk_unmap.
         */
        if (xen_blkbk_map(req, pending_req, seg))
                goto fail_flush;

        /* This corresponding xen_blkif_put is done in __end_block_io_op */
        xen_blkif_get(blkif);

        for (i = 0; i < nseg; i++) {
                while ((bio == NULL) ||
                       (bio_add_page(bio,
                                     blkbk->pending_page(pending_req, i),
                                     seg[i].nsec << 9,
                                     seg[i].buf & ~PAGE_MASK) == 0)) {

                        bio = bio_alloc(GFP_KERNEL, nseg-i);
                        if (unlikely(bio == NULL))
                                goto fail_put_bio;

                        biolist[nbio++] = bio;
                        bio->bi_bdev    = preq.bdev;
                        bio->bi_private = pending_req;
                        bio->bi_end_io  = end_block_io_op;
                        bio->bi_sector  = preq.sector_number;
                }

                preq.sector_number += seg[i].nsec;
        }

        /* This will be hit if the operation was a flush. */
        if (!bio) {
                BUG_ON(operation != WRITE_FLUSH);

                bio = bio_alloc(GFP_KERNEL, 0);
                if (unlikely(bio == NULL))
                        goto fail_put_bio;

                biolist[nbio++] = bio;
                bio->bi_bdev    = preq.bdev;
                bio->bi_private = pending_req;
                bio->bi_end_io  = end_block_io_op;
        }

        /*
         * We set it one so that the last submit_bio does not have to call
         * atomic_inc.
         */
        atomic_set(&pending_req->pendcnt, nbio);

        /* Get a reference count for the disk queue and start sending I/O */
        blk_start_plug(&plug);

        for (i = 0; i < nbio; i++)
                submit_bio(operation, biolist[i]);

        /* Let the I/Os go.. */
        blk_finish_plug(&plug);

        if (operation == READ)
                blkif->st_rd_sect += preq.nr_sects;
        else if (operation == WRITE || operation == WRITE_FLUSH)
                blkif->st_wr_sect += preq.nr_sects;

        return 0;

 fail_flush:
        xen_blkbk_unmap(pending_req);
 fail_response:
        /* Haven't submitted any bio's yet. */
        make_response(blkif, req->id, req->operation, BLKIF_RSP_ERROR);
        free_req(pending_req);
        msleep(1); /* back off a bit */
        return -EIO;

 fail_put_bio:
        for (i = 0; i < nbio; i++)
                bio_put(biolist[i]);
        __end_block_io_op(pending_req, -EINVAL);
        msleep(1); /* back off a bit */
        return -EIO;
}



/*
 * Put a response on the ring on how the operation fared.
 */
static void make_response(struct xen_blkif *blkif, u64 id,
                          unsigned short op, int st)
{
        struct blkif_response  resp;
        unsigned long     flags;
        union blkif_back_rings *blk_rings = &blkif->blk_rings;
        int more_to_do = 0;
        int notify;

        resp.id        = id;
        resp.operation = op;
        resp.status    = st;

        spin_lock_irqsave(&blkif->blk_ring_lock, flags);
        /* Place on the response ring for the relevant domain. */
        switch (blkif->blk_protocol) {
        case BLKIF_PROTOCOL_NATIVE:
                memcpy(RING_GET_RESPONSE(&blk_rings->native, blk_rings->native.rsp_prod_pvt),
                       &resp, sizeof(resp));
                break;
        case BLKIF_PROTOCOL_X86_32:
                memcpy(RING_GET_RESPONSE(&blk_rings->x86_32, blk_rings->x86_32.rsp_prod_pvt),
                       &resp, sizeof(resp));
                break;
        case BLKIF_PROTOCOL_X86_64:
                memcpy(RING_GET_RESPONSE(&blk_rings->x86_64, blk_rings->x86_64.rsp_prod_pvt),
                       &resp, sizeof(resp));
                break;
        default:
                BUG();
        }
        blk_rings->common.rsp_prod_pvt++;
        RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&blk_rings->common, notify);
        if (blk_rings->common.rsp_prod_pvt == blk_rings->common.req_cons) {
                /*
                 * Tail check for pending requests. Allows frontend to avoid
                 * notifications if requests are already in flight (lower
                 * overheads and promotes batching).
                 */
                RING_FINAL_CHECK_FOR_REQUESTS(&blk_rings->common, more_to_do);

        } else if (RING_HAS_UNCONSUMED_REQUESTS(&blk_rings->common)) {
                more_to_do = 1;
        }

        spin_unlock_irqrestore(&blkif->blk_ring_lock, flags);

        if (more_to_do)
                blkif_notify_work(blkif);
        if (notify)
                notify_remote_via_irq(blkif->irq);
}

static int __init xen_blkif_init(void)
{
        int i, mmap_pages;
        int rc = 0;

        if (!xen_pv_domain())
                return -ENODEV;

        blkbk = kzalloc(sizeof(struct xen_blkbk), GFP_KERNEL);
        if (!blkbk) {
                pr_alert(DRV_PFX "%s: out of memory!\n", __func__);
                return -ENOMEM;
        }

        mmap_pages = xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST;

        blkbk->pending_reqs          = kmalloc(sizeof(blkbk->pending_reqs[0]) *
                                        xen_blkif_reqs, GFP_KERNEL);
        blkbk->pending_grant_handles = kzalloc(sizeof(blkbk->pending_grant_handles[0]) *
                                        mmap_pages, GFP_KERNEL);
        blkbk->pending_pages         = kzalloc(sizeof(blkbk->pending_pages[0]) *
                                        mmap_pages, GFP_KERNEL);

        if (!blkbk->pending_reqs || !blkbk->pending_grant_handles ||
            !blkbk->pending_pages) {
                rc = -ENOMEM;
                goto out_of_memory;
        }

        for (i = 0; i < mmap_pages; i++) {
                blkbk->pending_grant_handles[i] = BLKBACK_INVALID_HANDLE;
                blkbk->pending_pages[i] = alloc_page(GFP_KERNEL);
                if (blkbk->pending_pages[i] == NULL) {
                        rc = -ENOMEM;
                        goto out_of_memory;
                }
        }
        rc = xen_blkif_interface_init();
        if (rc)
                goto failed_init;

        memset(blkbk->pending_reqs, 0, sizeof(blkbk->pending_reqs));

        INIT_LIST_HEAD(&blkbk->pending_free);
        spin_lock_init(&blkbk->pending_free_lock);
        init_waitqueue_head(&blkbk->pending_free_wq);

        for (i = 0; i < xen_blkif_reqs; i++)
                list_add_tail(&blkbk->pending_reqs[i].free_list,
                              &blkbk->pending_free);

        rc = xen_blkif_xenbus_init();
        if (rc)
                goto failed_init;

        return 0;

 out_of_memory:
        pr_alert(DRV_PFX "%s: out of memory\n", __func__);
 failed_init:
        kfree(blkbk->pending_reqs);
        kfree(blkbk->pending_grant_handles);
        for (i = 0; i < mmap_pages; i++) {
                if (blkbk->pending_pages[i])
                        __free_page(blkbk->pending_pages[i]);
        }
        kfree(blkbk->pending_pages);
        kfree(blkbk);
        blkbk = NULL;
        return rc;
}

module_init(xen_blkif_init);

MODULE_LICENSE("Dual BSD/GPL");