Linux 4.16.11

[linux/fpc-iii.git] / drivers / xen / xenbus / xenbus_xs.c

/******************************************************************************
 * 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.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#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/reboot.h>
#include <linux/rwsem.h>
#include <linux/mutex.h>
#include <asm/xen/hypervisor.h>
#include <xen/xenbus.h>
#include <xen/xen.h>
#include "xenbus.h"

/*
 * Framework to protect suspend/resume handling against normal Xenstore
 * message handling:
 * During suspend/resume there must be no open transaction and no pending
 * Xenstore request.
 * New watch events happening in this time can be ignored by firing all watches
 * after resume.
 */

/* Lock protecting enter/exit critical region. */
static DEFINE_SPINLOCK(xs_state_lock);
/* Number of users in critical region (protected by xs_state_lock). */
static unsigned int xs_state_users;
/* Suspend handler waiting or already active (protected by xs_state_lock)? */
static int xs_suspend_active;
/* Unique Xenstore request id (protected by xs_state_lock). */
static uint32_t xs_request_id;

/* Wait queue for all callers waiting for critical region to become usable. */
static DECLARE_WAIT_QUEUE_HEAD(xs_state_enter_wq);
/* Wait queue for suspend handling waiting for critical region being empty. */
static DECLARE_WAIT_QUEUE_HEAD(xs_state_exit_wq);

/* 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);

/* Protect watch (de)register against save/restore. */
static DECLARE_RWSEM(xs_watch_rwsem);

/*
 * 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 void xs_suspend_enter(void)
{
        spin_lock(&xs_state_lock);
        xs_suspend_active++;
        spin_unlock(&xs_state_lock);
        wait_event(xs_state_exit_wq, xs_state_users == 0);
}

static void xs_suspend_exit(void)
{
        spin_lock(&xs_state_lock);
        xs_suspend_active--;
        spin_unlock(&xs_state_lock);
        wake_up_all(&xs_state_enter_wq);
}

static uint32_t xs_request_enter(struct xb_req_data *req)
{
        uint32_t rq_id;

        req->type = req->msg.type;

        spin_lock(&xs_state_lock);

        while (!xs_state_users && xs_suspend_active) {
                spin_unlock(&xs_state_lock);
                wait_event(xs_state_enter_wq, xs_suspend_active == 0);
                spin_lock(&xs_state_lock);
        }

        if (req->type == XS_TRANSACTION_START)
                xs_state_users++;
        xs_state_users++;
        rq_id = xs_request_id++;

        spin_unlock(&xs_state_lock);

        return rq_id;
}

void xs_request_exit(struct xb_req_data *req)
{
        spin_lock(&xs_state_lock);
        xs_state_users--;
        if ((req->type == XS_TRANSACTION_START && req->msg.type == XS_ERROR) ||
            req->type == XS_TRANSACTION_END)
                xs_state_users--;
        spin_unlock(&xs_state_lock);

        if (xs_suspend_active && !xs_state_users)
                wake_up(&xs_state_exit_wq);
}

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) {
                        pr_warn("xen store gave: unknown error %s\n",
                                errorstring);
                        return EINVAL;
                }
        }
        return xsd_errors[i].errnum;
}

static bool xenbus_ok(void)
{
        switch (xen_store_domain_type) {
        case XS_LOCAL:
                switch (system_state) {
                case SYSTEM_POWER_OFF:
                case SYSTEM_RESTART:
                case SYSTEM_HALT:
                        return false;
                default:
                        break;
                }
                return true;
        case XS_PV:
        case XS_HVM:
                /* FIXME: Could check that the remote domain is alive,
                 * but it is normally initial domain. */
                return true;
        default:
                break;
        }
        return false;
}

static bool test_reply(struct xb_req_data *req)
{
        if (req->state == xb_req_state_got_reply || !xenbus_ok())
                return true;

        /* Make sure to reread req->state each time. */
        barrier();

        return false;
}

static void *read_reply(struct xb_req_data *req)
{
        while (req->state != xb_req_state_got_reply) {
                wait_event(req->wq, test_reply(req));

                if (!xenbus_ok())
                        /*
                         * If we are in the process of being shut-down there is
                         * no point of trying to contact XenBus - it is either
                         * killed (xenstored application) or the other domain
                         * has been killed or is unreachable.
                         */
                        return ERR_PTR(-EIO);
                if (req->err)
                        return ERR_PTR(req->err);

        }

        return req->body;
}

static void xs_send(struct xb_req_data *req, struct xsd_sockmsg *msg)
{
        bool notify;

        req->msg = *msg;
        req->err = 0;
        req->state = xb_req_state_queued;
        init_waitqueue_head(&req->wq);

        /* Save the caller req_id and restore it later in the reply */
        req->caller_req_id = req->msg.req_id;
        req->msg.req_id = xs_request_enter(req);

        mutex_lock(&xb_write_mutex);
        list_add_tail(&req->list, &xb_write_list);
        notify = list_is_singular(&xb_write_list);
        mutex_unlock(&xb_write_mutex);

        if (notify)
                wake_up(&xb_waitq);
}

static void *xs_wait_for_reply(struct xb_req_data *req, struct xsd_sockmsg *msg)
{
        void *ret;

        ret = read_reply(req);

        xs_request_exit(req);

        msg->type = req->msg.type;
        msg->len = req->msg.len;

        mutex_lock(&xb_write_mutex);
        if (req->state == xb_req_state_queued ||
            req->state == xb_req_state_wait_reply)
                req->state = xb_req_state_aborted;
        else
                kfree(req);
        mutex_unlock(&xb_write_mutex);

        return ret;
}

static void xs_wake_up(struct xb_req_data *req)
{
        wake_up(&req->wq);
}

int xenbus_dev_request_and_reply(struct xsd_sockmsg *msg, void *par)
{
        struct xb_req_data *req;
        struct kvec *vec;

        req = kmalloc(sizeof(*req) + sizeof(*vec), GFP_KERNEL);
        if (!req)
                return -ENOMEM;

        vec = (struct kvec *)(req + 1);
        vec->iov_len = msg->len;
        vec->iov_base = msg + 1;

        req->vec = vec;
        req->num_vecs = 1;
        req->cb = xenbus_dev_queue_reply;
        req->par = par;

        xs_send(req, msg);

        return 0;
}
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 xb_req_data *req;
        struct xsd_sockmsg msg;
        void *ret = NULL;
        unsigned int i;
        int err;

        req = kmalloc(sizeof(*req), GFP_NOIO | __GFP_HIGH);
        if (!req)
                return ERR_PTR(-ENOMEM);

        req->vec = iovec;
        req->num_vecs = num_vecs;
        req->cb = xs_wake_up;

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

        xs_send(req, &msg);

        ret = xs_wait_for_reply(req, &msg);
        if (len)
                *len = msg.len;

        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) {
                pr_warn_ratelimited("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;

        id_str = xs_single(XBT_NIL, XS_TRANSACTION_START, "", NULL);
        if (IS_ERR(id_str))
                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];

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

        return xs_error(xs_single(t, XS_TRANSACTION_END, abortstr, NULL));
}
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);

/* Read an (optional) unsigned value. */
unsigned int xenbus_read_unsigned(const char *dir, const char *node,
                                  unsigned int default_val)
{
        unsigned int val;
        int ret;

        ret = xenbus_scanf(XBT_NIL, dir, node, "%u", &val);
        if (ret <= 0)
                val = default_val;

        return val;
}
EXPORT_SYMBOL_GPL(xenbus_read_unsigned);

/* 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;
}

int xs_watch_msg(struct xs_watch_event *event)
{
        if (count_strings(event->body, event->len) != 2) {
                kfree(event);
                return -EINVAL;
        }
        event->path = (const char *)event->body;
        event->token = (const char *)strchr(event->body, '\0') + 1;

        spin_lock(&watches_lock);
        event->handle = find_watch(event->token);
        if (event->handle != NULL) {
                spin_lock(&watch_events_lock);
                list_add_tail(&event->list, &watch_events);
                wake_up(&watch_events_waitq);
                spin_unlock(&watch_events_lock);
        } else
                kfree(event);
        spin_unlock(&watches_lock);

        return 0;
}

/*
 * 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;

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

        if (xen_strict_xenbus_quirk())
                return;

        if (!xenbus_read_unsigned("control",
                                  "platform-feature-xs_reset_watches", 0))
                return;

        err = xs_error(xs_single(XBT_NIL, XS_RESET_WATCHES, "", NULL));
        if (err && err != -EEXIST)
                pr_warn("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_watch_rwsem);

        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_watch_rwsem);

        return err;
}
EXPORT_SYMBOL_GPL(register_xenbus_watch);

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

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

        down_read(&xs_watch_rwsem);

        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)
                pr_warn("Failed to release watch %s: %i\n", watch->node, err);

        up_read(&xs_watch_rwsem);

        /* 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(event, tmp, &watch_events, list) {
                if (event->handle != watch)
                        continue;
                list_del(&event->list);
                kfree(event);
        }
        spin_unlock(&watch_events_lock);

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

void xs_suspend(void)
{
        xs_suspend_enter();

        down_write(&xs_watch_rwsem);
        mutex_lock(&xs_response_mutex);
}

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

        xb_init_comms();

        mutex_unlock(&xs_response_mutex);

        xs_suspend_exit();

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

        up_write(&xs_watch_rwsem);
}

void xs_suspend_cancel(void)
{
        mutex_unlock(&xs_response_mutex);
        up_write(&xs_watch_rwsem);

        xs_suspend_exit();
}

static int xenwatch_thread(void *unused)
{
        struct list_head *ent;
        struct xs_watch_event *event;

        xenwatch_pid = current->pid;

        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) {
                        event = list_entry(ent, struct xs_watch_event, list);
                        event->handle->callback(event->handle, event->path,
                                                event->token);
                        kfree(event);
                }

                mutex_unlock(&xenwatch_mutex);
        }

        return 0;
}

/*
 * Wake up all threads waiting for a xenstore reply. In case of shutdown all
 * pending replies will be marked as "aborted" in order to let the waiters
 * return in spite of xenstore possibly no longer being able to reply. This
 * will avoid blocking shutdown by a thread waiting for xenstore but being
 * necessary for shutdown processing to proceed.
 */
static int xs_reboot_notify(struct notifier_block *nb,
                            unsigned long code, void *unused)
{
        struct xb_req_data *req;

        mutex_lock(&xb_write_mutex);
        list_for_each_entry(req, &xs_reply_list, list)
                wake_up(&req->wq);
        list_for_each_entry(req, &xb_write_list, list)
                wake_up(&req->wq);
        mutex_unlock(&xb_write_mutex);
        return NOTIFY_DONE;
}

static struct notifier_block xs_reboot_nb = {
        .notifier_call = xs_reboot_notify,
};

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

        register_reboot_notifier(&xs_reboot_nb);

        /* 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);

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

        return 0;
}