xenbus_xs.c

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/******************************************************************************
 * xenbus_xs.c
 *
 * This is the kernel equivalent of the "xs" library.  We don't need everything
 * and we use xenbus_comms for communication.
 *
 * Copyright (C) 2005 Rusty Russell, IBM Corporation
 *
 * 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/unistd.h>
#include <linux/errno.h>
#include <linux/types.h>
#include <linux/uio.h>
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/err.h>
#include <linux/slab.h>
#include <linux/fcntl.h>
#include <linux/kthread.h>
#include <linux/rwsem.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <asm/xen/hypervisor.h>
#include <xen/xenbus.h>
#include <xen/xen.h>
#include "xenbus_comms.h"
#include <asm/xen/hypervisor.h>

struct xs_stored_msg {
    struct list_head list;

    struct xsd_sockmsg hdr;

    union {
        /* Queued replies. */
        struct {
            char *body;
        } reply;

        /* Queued watch events. */
        struct {
            struct xenbus_watch *handle;
            char **vec;
            unsigned int vec_size;
        } watch;
    } u;
};

struct xs_handle {
    /* A list of replies. Currently only one will ever be outstanding. */
    struct list_head reply_list;
    spinlock_t reply_lock;
    wait_queue_head_t reply_waitq;

    /*
     * Mutex ordering: transaction_mutex -> watch_mutex -> request_mutex.
     * response_mutex is never taken simultaneously with the other three.
     *
     * transaction_mutex must be held before incrementing
     * transaction_count. The mutex is held when a suspend is in
     * progress to prevent new transactions starting.
     *
     * When decrementing transaction_count to zero the wait queue
     * should be woken up, the suspend code waits for count to
     * reach zero.
     */

    /* One request at a time. */
    struct mutex request_mutex;

    /* Protect xenbus reader thread against save/restore. */
    struct mutex response_mutex;

    /* Protect transactions against save/restore. */
    struct mutex transaction_mutex;
    atomic_t transaction_count;
    wait_queue_head_t transaction_wq;

    /* Protect watch (de)register against save/restore. */
    struct rw_semaphore watch_mutex;
};

static struct xs_handle xs_state;

/* List of registered watches, and a lock to protect it. */
static LIST_HEAD(watches);
static DEFINE_SPINLOCK(watches_lock);

/* List of pending watch callback events, and a lock to protect it. */
static LIST_HEAD(watch_events);
static DEFINE_SPINLOCK(watch_events_lock);

/*
 * Details of the xenwatch callback kernel thread. The thread waits on the
 * watch_events_waitq for work to do (queued on watch_events list). When it
 * wakes up it acquires the xenwatch_mutex before reading the list and
 * carrying out work.
 */
static pid_t xenwatch_pid;
static DEFINE_MUTEX(xenwatch_mutex);
static DECLARE_WAIT_QUEUE_HEAD(watch_events_waitq);

static int get_error(const char *errorstring)
{
    unsigned int i;

    for (i = 0; strcmp(errorstring, xsd_errors[i].errstring) != 0; i++) {
        if (i == ARRAY_SIZE(xsd_errors) - 1) {
            printk(KERN_WARNING
                   "XENBUS xen store gave: unknown error %s",
                   errorstring);
            return EINVAL;
        }
    }
    return xsd_errors[i].errnum;
}

static void *read_reply(enum xsd_sockmsg_type *type, unsigned int *len)
{
    struct xs_stored_msg *msg;
    char *body;

    spin_lock(&xs_state.reply_lock);

    while (list_empty(&xs_state.reply_list)) {
        spin_unlock(&xs_state.reply_lock);
        /* XXX FIXME: Avoid synchronous wait for response here. */
        wait_event(xs_state.reply_waitq,
               !list_empty(&xs_state.reply_list));
        spin_lock(&xs_state.reply_lock);
    }

    msg = list_entry(xs_state.reply_list.next,
             struct xs_stored_msg, list);
    list_del(&msg->list);

    spin_unlock(&xs_state.reply_lock);

    *type = msg->hdr.type;
    if (len)
        *len = msg->hdr.len;
    body = msg->u.reply.body;

    kfree(msg);

    return body;
}

static void transaction_start(void)
{
    mutex_lock(&xs_state.transaction_mutex);
    atomic_inc(&xs_state.transaction_count);
    mutex_unlock(&xs_state.transaction_mutex);
}

static void transaction_end(void)
{
    if (atomic_dec_and_test(&xs_state.transaction_count))
        wake_up(&xs_state.transaction_wq);
}

static void transaction_suspend(void)
{
    mutex_lock(&xs_state.transaction_mutex);
    wait_event(xs_state.transaction_wq,
           atomic_read(&xs_state.transaction_count) == 0);
}

static void transaction_resume(void)
{
    mutex_unlock(&xs_state.transaction_mutex);
}

void *xenbus_dev_request_and_reply(struct xsd_sockmsg *msg)
{
    void *ret;
    struct xsd_sockmsg req_msg = *msg;
    int err;

    if (req_msg.type == XS_TRANSACTION_START)
        transaction_start();

    mutex_lock(&xs_state.request_mutex);

    err = xb_write(msg, sizeof(*msg) + msg->len);
    if (err) {
        msg->type = XS_ERROR;
        ret = ERR_PTR(err);
    } else
        ret = read_reply(&msg->type, &msg->len);

    mutex_unlock(&xs_state.request_mutex);

    if ((msg->type == XS_TRANSACTION_END) ||
        ((req_msg.type == XS_TRANSACTION_START) &&
         (msg->type == XS_ERROR)))
        transaction_end();

    return ret;
}
EXPORT_SYMBOL(xenbus_dev_request_and_reply);

/* Send message to xs, get kmalloc'ed reply.  ERR_PTR() on error. */
static void *xs_talkv(struct xenbus_transaction t,
              enum xsd_sockmsg_type type,
              const struct kvec *iovec,
              unsigned int num_vecs,
              unsigned int *len)
{
    struct xsd_sockmsg msg;
    void *ret = NULL;
    unsigned int i;
    int err;

    msg.tx_id = t.id;
    msg.req_id = 0;
    msg.type = type;
    msg.len = 0;
    for (i = 0; i < num_vecs; i++)
        msg.len += iovec[i].iov_len;

    mutex_lock(&xs_state.request_mutex);

    err = xb_write(&msg, sizeof(msg));
    if (err) {
        mutex_unlock(&xs_state.request_mutex);
        return ERR_PTR(err);
    }

    for (i = 0; i < num_vecs; i++) {
        err = xb_write(iovec[i].iov_base, iovec[i].iov_len);
        if (err) {
            mutex_unlock(&xs_state.request_mutex);
            return ERR_PTR(err);
        }
    }

    ret = read_reply(&msg.type, len);

    mutex_unlock(&xs_state.request_mutex);

    if (IS_ERR(ret))
        return ret;

    if (msg.type == XS_ERROR) {
        err = get_error(ret);
        kfree(ret);
        return ERR_PTR(-err);
    }

    if (msg.type != type) {
        if (printk_ratelimit())
            printk(KERN_WARNING
                   "XENBUS unexpected type [%d], expected [%d]\n",
                   msg.type, type);
        kfree(ret);
        return ERR_PTR(-EINVAL);
    }
    return ret;
}

/* Simplified version of xs_talkv: single message. */
static void *xs_single(struct xenbus_transaction t,
               enum xsd_sockmsg_type type,
               const char *string,
               unsigned int *len)
{
    struct kvec iovec;

    iovec.iov_base = (void *)string;
    iovec.iov_len = strlen(string) + 1;
    return xs_talkv(t, type, &iovec, 1, len);
}

/* Many commands only need an ack, don't care what it says. */
static int xs_error(char *reply)
{
    if (IS_ERR(reply))
        return PTR_ERR(reply);
    kfree(reply);
    return 0;
}

static unsigned int count_strings(const char *strings, unsigned int len)
{
    unsigned int num;
    const char *p;

    for (p = strings, num = 0; p < strings + len; p += strlen(p) + 1)
        num++;

    return num;
}

/* Return the path to dir with /name appended. Buffer must be kfree()'ed. */
static char *join(const char *dir, const char *name)
{
    char *buffer;

    if (strlen(name) == 0)
        buffer = kasprintf(GFP_NOIO | __GFP_HIGH, "%s", dir);
    else
        buffer = kasprintf(GFP_NOIO | __GFP_HIGH, "%s/%s", dir, name);
    return (!buffer) ? ERR_PTR(-ENOMEM) : buffer;
}

static char **split(char *strings, unsigned int len, unsigned int *num)
{
    char *p, **ret;

    /* Count the strings. */
    *num = count_strings(strings, len);

    /* Transfer to one big alloc for easy freeing. */
    ret = kmalloc(*num * sizeof(char *) + len, GFP_NOIO | __GFP_HIGH);
    if (!ret) {
        kfree(strings);
        return ERR_PTR(-ENOMEM);
    }
    memcpy(&ret[*num], strings, len);
    kfree(strings);

    strings = (char *)&ret[*num];
    for (p = strings, *num = 0; p < strings + len; p += strlen(p) + 1)
        ret[(*num)++] = p;

    return ret;
}

char **xenbus_directory(struct xenbus_transaction t,
            const char *dir, const char *node, unsigned int *num)
{
    char *strings, *path;
    unsigned int len;

    path = join(dir, node);
    if (IS_ERR(path))
        return (char **)path;

    strings = xs_single(t, XS_DIRECTORY, path, &len);
    kfree(path);
    if (IS_ERR(strings))
        return (char **)strings;

    return split(strings, len, num);
}
EXPORT_SYMBOL_GPL(xenbus_directory);

/* Check if a path exists. Return 1 if it does. */
int xenbus_exists(struct xenbus_transaction t,
          const char *dir, const char *node)
{
    char **d;
    int dir_n;

    d = xenbus_directory(t, dir, node, &dir_n);
    if (IS_ERR(d))
        return 0;
    kfree(d);
    return 1;
}
EXPORT_SYMBOL_GPL(xenbus_exists);

/* Get the value of a single file.
 * Returns a kmalloced value: call free() on it after use.
 * len indicates length in bytes.
 */
void *xenbus_read(struct xenbus_transaction t,
          const char *dir, const char *node, unsigned int *len)
{
    char *path;
    void *ret;

    path = join(dir, node);
    if (IS_ERR(path))
        return (void *)path;

    ret = xs_single(t, XS_READ, path, len);
    kfree(path);
    return ret;
}
EXPORT_SYMBOL_GPL(xenbus_read);

/* Write the value of a single file.
 * Returns -err on failure.
 */
int xenbus_write(struct xenbus_transaction t,
         const char *dir, const char *node, const char *string)
{
    const char *path;
    struct kvec iovec[2];
    int ret;

    path = join(dir, node);
    if (IS_ERR(path))
        return PTR_ERR(path);

    iovec[0].iov_base = (void *)path;
    iovec[0].iov_len = strlen(path) + 1;
    iovec[1].iov_base = (void *)string;
    iovec[1].iov_len = strlen(string);

    ret = xs_error(xs_talkv(t, XS_WRITE, iovec, ARRAY_SIZE(iovec), NULL));
    kfree(path);
    return ret;
}
EXPORT_SYMBOL_GPL(xenbus_write);

/* Create a new directory. */
int xenbus_mkdir(struct xenbus_transaction t,
         const char *dir, const char *node)
{
    char *path;
    int ret;

    path = join(dir, node);
    if (IS_ERR(path))
        return PTR_ERR(path);

    ret = xs_error(xs_single(t, XS_MKDIR, path, NULL));
    kfree(path);
    return ret;
}
EXPORT_SYMBOL_GPL(xenbus_mkdir);

/* Destroy a file or directory (directories must be empty). */
int xenbus_rm(struct xenbus_transaction t, const char *dir, const char *node)
{
    char *path;
    int ret;

    path = join(dir, node);
    if (IS_ERR(path))
        return PTR_ERR(path);

    ret = xs_error(xs_single(t, XS_RM, path, NULL));
    kfree(path);
    return ret;
}
EXPORT_SYMBOL_GPL(xenbus_rm);

/* Start a transaction: changes by others will not be seen during this
 * transaction, and changes will not be visible to others until end.
 */
int xenbus_transaction_start(struct xenbus_transaction *t)
{
    char *id_str;

    transaction_start();

    id_str = xs_single(XBT_NIL, XS_TRANSACTION_START, "", NULL);
    if (IS_ERR(id_str)) {
        transaction_end();
        return PTR_ERR(id_str);
    }

    t->id = simple_strtoul(id_str, NULL, 0);
    kfree(id_str);
    return 0;
}
EXPORT_SYMBOL_GPL(xenbus_transaction_start);

/* End a transaction.
 * If abandon is true, transaction is discarded instead of committed.
 */
int xenbus_transaction_end(struct xenbus_transaction t, int abort)
{
    char abortstr[2];
    int err;

    if (abort)
        strcpy(abortstr, "F");
    else
        strcpy(abortstr, "T");

    err = xs_error(xs_single(t, XS_TRANSACTION_END, abortstr, NULL));

    transaction_end();

    return err;
}
EXPORT_SYMBOL_GPL(xenbus_transaction_end);

/* Single read and scanf: returns -errno or num scanned. */
int xenbus_scanf(struct xenbus_transaction t,
         const char *dir, const char *node, const char *fmt, ...)
{
    va_list ap;
    int ret;
    char *val;

    val = xenbus_read(t, dir, node, NULL);
    if (IS_ERR(val))
        return PTR_ERR(val);

    va_start(ap, fmt);
    ret = vsscanf(val, fmt, ap);
    va_end(ap);
    kfree(val);
    /* Distinctive errno. */
    if (ret == 0)
        return -ERANGE;
    return ret;
}
EXPORT_SYMBOL_GPL(xenbus_scanf);

/* Single printf and write: returns -errno or 0. */
int xenbus_printf(struct xenbus_transaction t,
          const char *dir, const char *node, const char *fmt, ...)
{
    va_list ap;
    int ret;
    char *buf;

    va_start(ap, fmt);
    buf = kvasprintf(GFP_NOIO | __GFP_HIGH, fmt, ap);
    va_end(ap);

    if (!buf)
        return -ENOMEM;

    ret = xenbus_write(t, dir, node, buf);

    kfree(buf);

    return ret;
}
EXPORT_SYMBOL_GPL(xenbus_printf);

/* Takes tuples of names, scanf-style args, and void **, NULL terminated. */
int xenbus_gather(struct xenbus_transaction t, const char *dir, ...)
{
    va_list ap;
    const char *name;
    int ret = 0;

    va_start(ap, dir);
    while (ret == 0 && (name = va_arg(ap, char *)) != NULL) {
        const char *fmt = va_arg(ap, char *);
        void *result = va_arg(ap, void *);
        char *p;

        p = xenbus_read(t, dir, name, NULL);
        if (IS_ERR(p)) {
            ret = PTR_ERR(p);
            break;
        }
        if (fmt) {
            if (sscanf(p, fmt, result) == 0)
                ret = -EINVAL;
            kfree(p);
        } else
            *(char **)result = p;
    }
    va_end(ap);
    return ret;
}
EXPORT_SYMBOL_GPL(xenbus_gather);

static int xs_watch(const char *path, const char *token)
{
    struct kvec iov[2];

    iov[0].iov_base = (void *)path;
    iov[0].iov_len = strlen(path) + 1;
    iov[1].iov_base = (void *)token;
    iov[1].iov_len = strlen(token) + 1;

    return xs_error(xs_talkv(XBT_NIL, XS_WATCH, iov,
                 ARRAY_SIZE(iov), NULL));
}

static int xs_unwatch(const char *path, const char *token)
{
    struct kvec iov[2];

    iov[0].iov_base = (char *)path;
    iov[0].iov_len = strlen(path) + 1;
    iov[1].iov_base = (char *)token;
    iov[1].iov_len = strlen(token) + 1;

    return xs_error(xs_talkv(XBT_NIL, XS_UNWATCH, iov,
                 ARRAY_SIZE(iov), NULL));
}

static struct xenbus_watch *find_watch(const char *token)
{
    struct xenbus_watch *i, *cmp;

    cmp = (void *)simple_strtoul(token, NULL, 16);

    list_for_each_entry(i, &watches, list)
        if (i == cmp)
            return i;

    return NULL;
}
/*
 * Certain older XenBus toolstack cannot handle reading values that are
 * not populated. Some Xen 3.4 installation are incapable of doing this
 * so if we are running on anything older than 4 do not attempt to read
 * control/platform-feature-xs_reset_watches.
 */
static bool xen_strict_xenbus_quirk(void)
{
#ifdef CONFIG_X86
    uint32_t eax, ebx, ecx, edx, base;

    base = xen_cpuid_base();
    cpuid(base + 1, &eax, &ebx, &ecx, &edx);

    if ((eax >> 16) < 4)
        return true;
#endif
    return false;

}
static void xs_reset_watches(void)
{
    int err, supported = 0;

    if (!xen_hvm_domain() || xen_initial_domain())
        return;

    if (xen_strict_xenbus_quirk())
        return;

    err = xenbus_scanf(XBT_NIL, "control",
            "platform-feature-xs_reset_watches", "%d", &supported);
    if (err != 1 || !supported)
        return;

    err = xs_error(xs_single(XBT_NIL, XS_RESET_WATCHES, "", NULL));
    if (err && err != -EEXIST)
        printk(KERN_WARNING "xs_reset_watches failed: %d\n", err);
}

/* Register callback to watch this node. */
int register_xenbus_watch(struct xenbus_watch *watch)
{
    /* Pointer in ascii is the token. */
    char token[sizeof(watch) * 2 + 1];
    int err;

    sprintf(token, "%lX", (long)watch);

    down_read(&xs_state.watch_mutex);

    spin_lock(&watches_lock);
    BUG_ON(find_watch(token));
    list_add(&watch->list, &watches);
    spin_unlock(&watches_lock);

    err = xs_watch(watch->node, token);

    if (err) {
        spin_lock(&watches_lock);
        list_del(&watch->list);
        spin_unlock(&watches_lock);
    }

    up_read(&xs_state.watch_mutex);

    return err;
}
EXPORT_SYMBOL_GPL(register_xenbus_watch);

void unregister_xenbus_watch(struct xenbus_watch *watch)
{
    struct xs_stored_msg *msg, *tmp;
    char token[sizeof(watch) * 2 + 1];
    int err;

    sprintf(token, "%lX", (long)watch);

    down_read(&xs_state.watch_mutex);

    spin_lock(&watches_lock);
    BUG_ON(!find_watch(token));
    list_del(&watch->list);
    spin_unlock(&watches_lock);

    err = xs_unwatch(watch->node, token);
    if (err)
        printk(KERN_WARNING
               "XENBUS Failed to release watch %s: %i\n",
               watch->node, err);

    up_read(&xs_state.watch_mutex);

    /* Make sure there are no callbacks running currently (unless
       its us) */
    if (current->pid != xenwatch_pid)
        mutex_lock(&xenwatch_mutex);

    /* Cancel pending watch events. */
    spin_lock(&watch_events_lock);
    list_for_each_entry_safe(msg, tmp, &watch_events, list) {
        if (msg->u.watch.handle != watch)
            continue;
        list_del(&msg->list);
        kfree(msg->u.watch.vec);
        kfree(msg);
    }
    spin_unlock(&watch_events_lock);

    if (current->pid != xenwatch_pid)
        mutex_unlock(&xenwatch_mutex);
}
EXPORT_SYMBOL_GPL(unregister_xenbus_watch);

void xs_suspend(void)
{
    transaction_suspend();
    down_write(&xs_state.watch_mutex);
    mutex_lock(&xs_state.request_mutex);
    mutex_lock(&xs_state.response_mutex);
}

void xs_resume(void)
{
    struct xenbus_watch *watch;
    char token[sizeof(watch) * 2 + 1];

    xb_init_comms();

    mutex_unlock(&xs_state.response_mutex);
    mutex_unlock(&xs_state.request_mutex);
    transaction_resume();

    /* No need for watches_lock: the watch_mutex is sufficient. */
    list_for_each_entry(watch, &watches, list) {
        sprintf(token, "%lX", (long)watch);
        xs_watch(watch->node, token);
    }

    up_write(&xs_state.watch_mutex);
}

void xs_suspend_cancel(void)
{
    mutex_unlock(&xs_state.response_mutex);
    mutex_unlock(&xs_state.request_mutex);
    up_write(&xs_state.watch_mutex);
    mutex_unlock(&xs_state.transaction_mutex);
}

static int xenwatch_thread(void *unused)
{
    struct list_head *ent;
    struct xs_stored_msg *msg;

    for (;;) {
        wait_event_interruptible(watch_events_waitq,
                     !list_empty(&watch_events));

        if (kthread_should_stop())
            break;

        mutex_lock(&xenwatch_mutex);

        spin_lock(&watch_events_lock);
        ent = watch_events.next;
        if (ent != &watch_events)
            list_del(ent);
        spin_unlock(&watch_events_lock);

        if (ent != &watch_events) {
            msg = list_entry(ent, struct xs_stored_msg, list);
            msg->u.watch.handle->callback(
                msg->u.watch.handle,
                (const char **)msg->u.watch.vec,
                msg->u.watch.vec_size);
            kfree(msg->u.watch.vec);
            kfree(msg);
        }

        mutex_unlock(&xenwatch_mutex);
    }

    return 0;
}

static int process_msg(void)
{
    struct xs_stored_msg *msg;
    char *body;
    int err;

    /*
     * We must disallow save/restore while reading a xenstore message.
     * A partial read across s/r leaves us out of sync with xenstored.
     */
    for (;;) {
        err = xb_wait_for_data_to_read();
        if (err)
            return err;
        mutex_lock(&xs_state.response_mutex);
        if (xb_data_to_read())
            break;
        /* We raced with save/restore: pending data 'disappeared'. */
        mutex_unlock(&xs_state.response_mutex);
    }


    msg = kmalloc(sizeof(*msg), GFP_NOIO | __GFP_HIGH);
    if (msg == NULL) {
        err = -ENOMEM;
        goto out;
    }

    err = xb_read(&msg->hdr, sizeof(msg->hdr));
    if (err) {
        kfree(msg);
        goto out;
    }

    if (msg->hdr.len > XENSTORE_PAYLOAD_MAX) {
        kfree(msg);
        err = -EINVAL;
        goto out;
    }

    body = kmalloc(msg->hdr.len + 1, GFP_NOIO | __GFP_HIGH);
    if (body == NULL) {
        kfree(msg);
        err = -ENOMEM;
        goto out;
    }

    err = xb_read(body, msg->hdr.len);
    if (err) {
        kfree(body);
        kfree(msg);
        goto out;
    }
    body[msg->hdr.len] = '\0';

    if (msg->hdr.type == XS_WATCH_EVENT) {
        msg->u.watch.vec = split(body, msg->hdr.len,
                     &msg->u.watch.vec_size);
        if (IS_ERR(msg->u.watch.vec)) {
            err = PTR_ERR(msg->u.watch.vec);
            kfree(msg);
            goto out;
        }

        spin_lock(&watches_lock);
        msg->u.watch.handle = find_watch(
            msg->u.watch.vec[XS_WATCH_TOKEN]);
        if (msg->u.watch.handle != NULL) {
            spin_lock(&watch_events_lock);
            list_add_tail(&msg->list, &watch_events);
            wake_up(&watch_events_waitq);
            spin_unlock(&watch_events_lock);
        } else {
            kfree(msg->u.watch.vec);
            kfree(msg);
        }
        spin_unlock(&watches_lock);
    } else {
        msg->u.reply.body = body;
        spin_lock(&xs_state.reply_lock);
        list_add_tail(&msg->list, &xs_state.reply_list);
        spin_unlock(&xs_state.reply_lock);
        wake_up(&xs_state.reply_waitq);
    }

 out:
    mutex_unlock(&xs_state.response_mutex);
    return err;
}

static int xenbus_thread(void *unused)
{
    int err;

    for (;;) {
        err = process_msg();
        if (err)
            printk(KERN_WARNING "XENBUS error %d while reading "
                   "message\n", err);
        if (kthread_should_stop())
            break;
    }

    return 0;
}

int xs_init(void)
{
    int err;
    struct task_struct *task;

    INIT_LIST_HEAD(&xs_state.reply_list);
    spin_lock_init(&xs_state.reply_lock);
    init_waitqueue_head(&xs_state.reply_waitq);

    mutex_init(&xs_state.request_mutex);
    mutex_init(&xs_state.response_mutex);
    mutex_init(&xs_state.transaction_mutex);
    init_rwsem(&xs_state.watch_mutex);
    atomic_set(&xs_state.transaction_count, 0);
    init_waitqueue_head(&xs_state.transaction_wq);

    /* Initialize the shared memory rings to talk to xenstored */
    err = xb_init_comms();
    if (err)
        return err;

    task = kthread_run(xenwatch_thread, NULL, "xenwatch");
    if (IS_ERR(task))
        return PTR_ERR(task);
    xenwatch_pid = task->pid;

    task = kthread_run(xenbus_thread, NULL, "xenbus");
    if (IS_ERR(task))
        return PTR_ERR(task);

    /* shutdown watches for kexec boot */
    xs_reset_watches();

    return 0;
}