virtio: de-structify virtio_block status byte

[linux-2.6/mini2440.git] / kernel / marker.c

/*
 * Copyright (C) 2007 Mathieu Desnoyers
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/types.h>
#include <linux/jhash.h>
#include <linux/list.h>
#include <linux/rcupdate.h>
#include <linux/marker.h>
#include <linux/err.h>
#include <linux/slab.h>

extern struct marker __start___markers[];
extern struct marker __stop___markers[];

/* Set to 1 to enable marker debug output */
static const int marker_debug;

/*
 * markers_mutex nests inside module_mutex. Markers mutex protects the builtin
 * and module markers and the hash table.
 */
static DEFINE_MUTEX(markers_mutex);

/*
 * Marker hash table, containing the active markers.
 * Protected by module_mutex.
 */
#define MARKER_HASH_BITS 6
#define MARKER_TABLE_SIZE (1 << MARKER_HASH_BITS)

/*
 * Note about RCU :
 * It is used to make sure every handler has finished using its private data
 * between two consecutive operation (add or remove) on a given marker.  It is
 * also used to delay the free of multiple probes array until a quiescent state
 * is reached.
 * marker entries modifications are protected by the markers_mutex.
 */
struct marker_entry {
        struct hlist_node hlist;
        char *format;
        void (*call)(const struct marker *mdata,        /* Probe wrapper */
                void *call_private, const char *fmt, ...);
        struct marker_probe_closure single;
        struct marker_probe_closure *multi;
        int refcount;   /* Number of times armed. 0 if disarmed. */
        struct rcu_head rcu;
        void *oldptr;
        unsigned char rcu_pending:1;
        unsigned char ptype:1;
        char name[0];   /* Contains name'\0'format'\0' */
};

static struct hlist_head marker_table[MARKER_TABLE_SIZE];

/**
 * __mark_empty_function - Empty probe callback
 * @probe_private: probe private data
 * @call_private: call site private data
 * @fmt: format string
 * @...: variable argument list
 *
 * Empty callback provided as a probe to the markers. By providing this to a
 * disabled marker, we make sure the  execution flow is always valid even
 * though the function pointer change and the marker enabling are two distinct
 * operations that modifies the execution flow of preemptible code.
 */
void __mark_empty_function(void *probe_private, void *call_private,
        const char *fmt, va_list *args)
{
}
EXPORT_SYMBOL_GPL(__mark_empty_function);

/*
 * marker_probe_cb Callback that prepares the variable argument list for probes.
 * @mdata: pointer of type struct marker
 * @call_private: caller site private data
 * @fmt: format string
 * @...:  Variable argument list.
 *
 * Since we do not use "typical" pointer based RCU in the 1 argument case, we
 * need to put a full smp_rmb() in this branch. This is why we do not use
 * rcu_dereference() for the pointer read.
 */
void marker_probe_cb(const struct marker *mdata, void *call_private,
        const char *fmt, ...)
{
        va_list args;
        char ptype;

        /*
         * preempt_disable does two things : disabling preemption to make sure
         * the teardown of the callbacks can be done correctly when they are in
         * modules and they insure RCU read coherency.
         */
        preempt_disable();
        ptype = mdata->ptype;
        if (likely(!ptype)) {
                marker_probe_func *func;
                /* Must read the ptype before ptr. They are not data dependant,
                 * so we put an explicit smp_rmb() here. */
                smp_rmb();
                func = mdata->single.func;
                /* Must read the ptr before private data. They are not data
                 * dependant, so we put an explicit smp_rmb() here. */
                smp_rmb();
                va_start(args, fmt);
                func(mdata->single.probe_private, call_private, fmt, &args);
                va_end(args);
        } else {
                struct marker_probe_closure *multi;
                int i;
                /*
                 * multi points to an array, therefore accessing the array
                 * depends on reading multi. However, even in this case,
                 * we must insure that the pointer is read _before_ the array
                 * data. Same as rcu_dereference, but we need a full smp_rmb()
                 * in the fast path, so put the explicit barrier here.
                 */
                smp_read_barrier_depends();
                multi = mdata->multi;
                for (i = 0; multi[i].func; i++) {
                        va_start(args, fmt);
                        multi[i].func(multi[i].probe_private, call_private, fmt,
                                &args);
                        va_end(args);
                }
        }
        preempt_enable();
}
EXPORT_SYMBOL_GPL(marker_probe_cb);

/*
 * marker_probe_cb Callback that does not prepare the variable argument list.
 * @mdata: pointer of type struct marker
 * @call_private: caller site private data
 * @fmt: format string
 * @...:  Variable argument list.
 *
 * Should be connected to markers "MARK_NOARGS".
 */
void marker_probe_cb_noarg(const struct marker *mdata,
        void *call_private, const char *fmt, ...)
{
        va_list args;   /* not initialized */
        char ptype;

        preempt_disable();
        ptype = mdata->ptype;
        if (likely(!ptype)) {
                marker_probe_func *func;
                /* Must read the ptype before ptr. They are not data dependant,
                 * so we put an explicit smp_rmb() here. */
                smp_rmb();
                func = mdata->single.func;
                /* Must read the ptr before private data. They are not data
                 * dependant, so we put an explicit smp_rmb() here. */
                smp_rmb();
                func(mdata->single.probe_private, call_private, fmt, &args);
        } else {
                struct marker_probe_closure *multi;
                int i;
                /*
                 * multi points to an array, therefore accessing the array
                 * depends on reading multi. However, even in this case,
                 * we must insure that the pointer is read _before_ the array
                 * data. Same as rcu_dereference, but we need a full smp_rmb()
                 * in the fast path, so put the explicit barrier here.
                 */
                smp_read_barrier_depends();
                multi = mdata->multi;
                for (i = 0; multi[i].func; i++)
                        multi[i].func(multi[i].probe_private, call_private, fmt,
                                &args);
        }
        preempt_enable();
}
EXPORT_SYMBOL_GPL(marker_probe_cb_noarg);

static void free_old_closure(struct rcu_head *head)
{
        struct marker_entry *entry = container_of(head,
                struct marker_entry, rcu);
        kfree(entry->oldptr);
        /* Make sure we free the data before setting the pending flag to 0 */
        smp_wmb();
        entry->rcu_pending = 0;
}

static void debug_print_probes(struct marker_entry *entry)
{
        int i;

        if (!marker_debug)
                return;

        if (!entry->ptype) {
                printk(KERN_DEBUG "Single probe : %p %p\n",
                        entry->single.func,
                        entry->single.probe_private);
        } else {
                for (i = 0; entry->multi[i].func; i++)
                        printk(KERN_DEBUG "Multi probe %d : %p %p\n", i,
                                entry->multi[i].func,
                                entry->multi[i].probe_private);
        }
}

static struct marker_probe_closure *
marker_entry_add_probe(struct marker_entry *entry,
                marker_probe_func *probe, void *probe_private)
{
        int nr_probes = 0;
        struct marker_probe_closure *old, *new;

        WARN_ON(!probe);

        debug_print_probes(entry);
        old = entry->multi;
        if (!entry->ptype) {
                if (entry->single.func == probe &&
                                entry->single.probe_private == probe_private)
                        return ERR_PTR(-EBUSY);
                if (entry->single.func == __mark_empty_function) {
                        /* 0 -> 1 probes */
                        entry->single.func = probe;
                        entry->single.probe_private = probe_private;
                        entry->refcount = 1;
                        entry->ptype = 0;
                        debug_print_probes(entry);
                        return NULL;
                } else {
                        /* 1 -> 2 probes */
                        nr_probes = 1;
                        old = NULL;
                }
        } else {
                /* (N -> N+1), (N != 0, 1) probes */
                for (nr_probes = 0; old[nr_probes].func; nr_probes++)
                        if (old[nr_probes].func == probe
                                        && old[nr_probes].probe_private
                                                == probe_private)
                                return ERR_PTR(-EBUSY);
        }
        /* + 2 : one for new probe, one for NULL func */
        new = kzalloc((nr_probes + 2) * sizeof(struct marker_probe_closure),
                        GFP_KERNEL);
        if (new == NULL)
                return ERR_PTR(-ENOMEM);
        if (!old)
                new[0] = entry->single;
        else
                memcpy(new, old,
                        nr_probes * sizeof(struct marker_probe_closure));
        new[nr_probes].func = probe;
        new[nr_probes].probe_private = probe_private;
        entry->refcount = nr_probes + 1;
        entry->multi = new;
        entry->ptype = 1;
        debug_print_probes(entry);
        return old;
}

static struct marker_probe_closure *
marker_entry_remove_probe(struct marker_entry *entry,
                marker_probe_func *probe, void *probe_private)
{
        int nr_probes = 0, nr_del = 0, i;
        struct marker_probe_closure *old, *new;

        old = entry->multi;

        debug_print_probes(entry);
        if (!entry->ptype) {
                /* 0 -> N is an error */
                WARN_ON(entry->single.func == __mark_empty_function);
                /* 1 -> 0 probes */
                WARN_ON(probe && entry->single.func != probe);
                WARN_ON(entry->single.probe_private != probe_private);
                entry->single.func = __mark_empty_function;
                entry->refcount = 0;
                entry->ptype = 0;
                debug_print_probes(entry);
                return NULL;
        } else {
                /* (N -> M), (N > 1, M >= 0) probes */
                for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
                        if ((!probe || old[nr_probes].func == probe)
                                        && old[nr_probes].probe_private
                                                == probe_private)
                                nr_del++;
                }
        }

        if (nr_probes - nr_del == 0) {
                /* N -> 0, (N > 1) */
                entry->single.func = __mark_empty_function;
                entry->refcount = 0;
                entry->ptype = 0;
        } else if (nr_probes - nr_del == 1) {
                /* N -> 1, (N > 1) */
                for (i = 0; old[i].func; i++)
                        if ((probe && old[i].func != probe) ||
                                        old[i].probe_private != probe_private)
                                entry->single = old[i];
                entry->refcount = 1;
                entry->ptype = 0;
        } else {
                int j = 0;
                /* N -> M, (N > 1, M > 1) */
                /* + 1 for NULL */
                new = kzalloc((nr_probes - nr_del + 1)
                        * sizeof(struct marker_probe_closure), GFP_KERNEL);
                if (new == NULL)
                        return ERR_PTR(-ENOMEM);
                for (i = 0; old[i].func; i++)
                        if ((probe && old[i].func != probe) ||
                                        old[i].probe_private != probe_private)
                                new[j++] = old[i];
                entry->refcount = nr_probes - nr_del;
                entry->ptype = 1;
                entry->multi = new;
        }
        debug_print_probes(entry);
        return old;
}

/*
 * Get marker if the marker is present in the marker hash table.
 * Must be called with markers_mutex held.
 * Returns NULL if not present.
 */
static struct marker_entry *get_marker(const char *name)
{
        struct hlist_head *head;
        struct hlist_node *node;
        struct marker_entry *e;
        u32 hash = jhash(name, strlen(name), 0);

        head = &marker_table[hash & ((1 << MARKER_HASH_BITS)-1)];
        hlist_for_each_entry(e, node, head, hlist) {
                if (!strcmp(name, e->name))
                        return e;
        }
        return NULL;
}

/*
 * Add the marker to the marker hash table. Must be called with markers_mutex
 * held.
 */
static struct marker_entry *add_marker(const char *name, const char *format)
{
        struct hlist_head *head;
        struct hlist_node *node;
        struct marker_entry *e;
        size_t name_len = strlen(name) + 1;
        size_t format_len = 0;
        u32 hash = jhash(name, name_len-1, 0);

        if (format)
                format_len = strlen(format) + 1;
        head = &marker_table[hash & ((1 << MARKER_HASH_BITS)-1)];
        hlist_for_each_entry(e, node, head, hlist) {
                if (!strcmp(name, e->name)) {
                        printk(KERN_NOTICE
                                "Marker %s busy\n", name);
                        return ERR_PTR(-EBUSY); /* Already there */
                }
        }
        /*
         * Using kmalloc here to allocate a variable length element. Could
         * cause some memory fragmentation if overused.
         */
        e = kmalloc(sizeof(struct marker_entry) + name_len + format_len,
                        GFP_KERNEL);
        if (!e)
                return ERR_PTR(-ENOMEM);
        memcpy(&e->name[0], name, name_len);
        if (format) {
                e->format = &e->name[name_len];
                memcpy(e->format, format, format_len);
                if (strcmp(e->format, MARK_NOARGS) == 0)
                        e->call = marker_probe_cb_noarg;
                else
                        e->call = marker_probe_cb;
                trace_mark(core_marker_format, "name %s format %s",
                                e->name, e->format);
        } else {
                e->format = NULL;
                e->call = marker_probe_cb;
        }
        e->single.func = __mark_empty_function;
        e->single.probe_private = NULL;
        e->multi = NULL;
        e->ptype = 0;
        e->refcount = 0;
        e->rcu_pending = 0;
        hlist_add_head(&e->hlist, head);
        return e;
}

/*
 * Remove the marker from the marker hash table. Must be called with mutex_lock
 * held.
 */
static int remove_marker(const char *name)
{
        struct hlist_head *head;
        struct hlist_node *node;
        struct marker_entry *e;
        int found = 0;
        size_t len = strlen(name) + 1;
        u32 hash = jhash(name, len-1, 0);

        head = &marker_table[hash & ((1 << MARKER_HASH_BITS)-1)];
        hlist_for_each_entry(e, node, head, hlist) {
                if (!strcmp(name, e->name)) {
                        found = 1;
                        break;
                }
        }
        if (!found)
                return -ENOENT;
        if (e->single.func != __mark_empty_function)
                return -EBUSY;
        hlist_del(&e->hlist);
        /* Make sure the call_rcu has been executed */
        if (e->rcu_pending)
                rcu_barrier();
        kfree(e);
        return 0;
}

/*
 * Set the mark_entry format to the format found in the element.
 */
static int marker_set_format(struct marker_entry **entry, const char *format)
{
        struct marker_entry *e;
        size_t name_len = strlen((*entry)->name) + 1;
        size_t format_len = strlen(format) + 1;


        e = kmalloc(sizeof(struct marker_entry) + name_len + format_len,
                        GFP_KERNEL);
        if (!e)
                return -ENOMEM;
        memcpy(&e->name[0], (*entry)->name, name_len);
        e->format = &e->name[name_len];
        memcpy(e->format, format, format_len);
        if (strcmp(e->format, MARK_NOARGS) == 0)
                e->call = marker_probe_cb_noarg;
        else
                e->call = marker_probe_cb;
        e->single = (*entry)->single;
        e->multi = (*entry)->multi;
        e->ptype = (*entry)->ptype;
        e->refcount = (*entry)->refcount;
        e->rcu_pending = 0;
        hlist_add_before(&e->hlist, &(*entry)->hlist);
        hlist_del(&(*entry)->hlist);
        /* Make sure the call_rcu has been executed */
        if ((*entry)->rcu_pending)
                rcu_barrier();
        kfree(*entry);
        *entry = e;
        trace_mark(core_marker_format, "name %s format %s",
                        e->name, e->format);
        return 0;
}

/*
 * Sets the probe callback corresponding to one marker.
 */
static int set_marker(struct marker_entry **entry, struct marker *elem,
                int active)
{
        int ret;
        WARN_ON(strcmp((*entry)->name, elem->name) != 0);

        if ((*entry)->format) {
                if (strcmp((*entry)->format, elem->format) != 0) {
                        printk(KERN_NOTICE
                                "Format mismatch for probe %s "
                                "(%s), marker (%s)\n",
                                (*entry)->name,
                                (*entry)->format,
                                elem->format);
                        return -EPERM;
                }
        } else {
                ret = marker_set_format(entry, elem->format);
                if (ret)
                        return ret;
        }

        /*
         * probe_cb setup (statically known) is done here. It is
         * asynchronous with the rest of execution, therefore we only
         * pass from a "safe" callback (with argument) to an "unsafe"
         * callback (does not set arguments).
         */
        elem->call = (*entry)->call;
        /*
         * Sanity check :
         * We only update the single probe private data when the ptr is
         * set to a _non_ single probe! (0 -> 1 and N -> 1, N != 1)
         */
        WARN_ON(elem->single.func != __mark_empty_function
                && elem->single.probe_private
                != (*entry)->single.probe_private &&
                !elem->ptype);
        elem->single.probe_private = (*entry)->single.probe_private;
        /*
         * Make sure the private data is valid when we update the
         * single probe ptr.
         */
        smp_wmb();
        elem->single.func = (*entry)->single.func;
        /*
         * We also make sure that the new probe callbacks array is consistent
         * before setting a pointer to it.
         */
        rcu_assign_pointer(elem->multi, (*entry)->multi);
        /*
         * Update the function or multi probe array pointer before setting the
         * ptype.
         */
        smp_wmb();
        elem->ptype = (*entry)->ptype;
        elem->state = active;

        return 0;
}

/*
 * Disable a marker and its probe callback.
 * Note: only waiting an RCU period after setting elem->call to the empty
 * function insures that the original callback is not used anymore. This insured
 * by preempt_disable around the call site.
 */
static void disable_marker(struct marker *elem)
{
        /* leave "call" as is. It is known statically. */
        elem->state = 0;
        elem->single.func = __mark_empty_function;
        /* Update the function before setting the ptype */
        smp_wmb();
        elem->ptype = 0;        /* single probe */
        /*
         * Leave the private data and id there, because removal is racy and
         * should be done only after an RCU period. These are never used until
         * the next initialization anyway.
         */
}

/**
 * marker_update_probe_range - Update a probe range
 * @begin: beginning of the range
 * @end: end of the range
 *
 * Updates the probe callback corresponding to a range of markers.
 */
void marker_update_probe_range(struct marker *begin,
        struct marker *end)
{
        struct marker *iter;
        struct marker_entry *mark_entry;

        mutex_lock(&markers_mutex);
        for (iter = begin; iter < end; iter++) {
                mark_entry = get_marker(iter->name);
                if (mark_entry) {
                        set_marker(&mark_entry, iter,
                                        !!mark_entry->refcount);
                        /*
                         * ignore error, continue
                         */
                } else {
                        disable_marker(iter);
                }
        }
        mutex_unlock(&markers_mutex);
}

/*
 * Update probes, removing the faulty probes.
 *
 * Internal callback only changed before the first probe is connected to it.
 * Single probe private data can only be changed on 0 -> 1 and 2 -> 1
 * transitions.  All other transitions will leave the old private data valid.
 * This makes the non-atomicity of the callback/private data updates valid.
 *
 * "special case" updates :
 * 0 -> 1 callback
 * 1 -> 0 callback
 * 1 -> 2 callbacks
 * 2 -> 1 callbacks
 * Other updates all behave the same, just like the 2 -> 3 or 3 -> 2 updates.
 * Site effect : marker_set_format may delete the marker entry (creating a
 * replacement).
 */
static void marker_update_probes(void)
{
        /* Core kernel markers */
        marker_update_probe_range(__start___markers, __stop___markers);
        /* Markers in modules. */
        module_update_markers();
}

/**
 * marker_probe_register -  Connect a probe to a marker
 * @name: marker name
 * @format: format string
 * @probe: probe handler
 * @probe_private: probe private data
 *
 * private data must be a valid allocated memory address, or NULL.
 * Returns 0 if ok, error value on error.
 * The probe address must at least be aligned on the architecture pointer size.
 */
int marker_probe_register(const char *name, const char *format,
                        marker_probe_func *probe, void *probe_private)
{
        struct marker_entry *entry;
        int ret = 0;
        struct marker_probe_closure *old;

        mutex_lock(&markers_mutex);
        entry = get_marker(name);
        if (!entry) {
                entry = add_marker(name, format);
                if (IS_ERR(entry)) {
                        ret = PTR_ERR(entry);
                        goto end;
                }
        }
        /*
         * If we detect that a call_rcu is pending for this marker,
         * make sure it's executed now.
         */
        if (entry->rcu_pending)
                rcu_barrier();
        old = marker_entry_add_probe(entry, probe, probe_private);
        if (IS_ERR(old)) {
                ret = PTR_ERR(old);
                goto end;
        }
        mutex_unlock(&markers_mutex);
        marker_update_probes();         /* may update entry */
        mutex_lock(&markers_mutex);
        entry = get_marker(name);
        WARN_ON(!entry);
        entry->oldptr = old;
        entry->rcu_pending = 1;
        /* write rcu_pending before calling the RCU callback */
        smp_wmb();
#ifdef CONFIG_PREEMPT_RCU
        synchronize_sched();    /* Until we have the call_rcu_sched() */
#endif
        call_rcu(&entry->rcu, free_old_closure);
end:
        mutex_unlock(&markers_mutex);
        return ret;
}
EXPORT_SYMBOL_GPL(marker_probe_register);

/**
 * marker_probe_unregister -  Disconnect a probe from a marker
 * @name: marker name
 * @probe: probe function pointer
 * @probe_private: probe private data
 *
 * Returns the private data given to marker_probe_register, or an ERR_PTR().
 * We do not need to call a synchronize_sched to make sure the probes have
 * finished running before doing a module unload, because the module unload
 * itself uses stop_machine(), which insures that every preempt disabled section
 * have finished.
 */
int marker_probe_unregister(const char *name,
        marker_probe_func *probe, void *probe_private)
{
        struct marker_entry *entry;
        struct marker_probe_closure *old;
        int ret = -ENOENT;

        mutex_lock(&markers_mutex);
        entry = get_marker(name);
        if (!entry)
                goto end;
        if (entry->rcu_pending)
                rcu_barrier();
        old = marker_entry_remove_probe(entry, probe, probe_private);
        mutex_unlock(&markers_mutex);
        marker_update_probes();         /* may update entry */
        mutex_lock(&markers_mutex);
        entry = get_marker(name);
        if (!entry)
                goto end;
        entry->oldptr = old;
        entry->rcu_pending = 1;
        /* write rcu_pending before calling the RCU callback */
        smp_wmb();
#ifdef CONFIG_PREEMPT_RCU
        synchronize_sched();    /* Until we have the call_rcu_sched() */
#endif
        call_rcu(&entry->rcu, free_old_closure);
        remove_marker(name);    /* Ignore busy error message */
        ret = 0;
end:
        mutex_unlock(&markers_mutex);
        return ret;
}
EXPORT_SYMBOL_GPL(marker_probe_unregister);

static struct marker_entry *
get_marker_from_private_data(marker_probe_func *probe, void *probe_private)
{
        struct marker_entry *entry;
        unsigned int i;
        struct hlist_head *head;
        struct hlist_node *node;

        for (i = 0; i < MARKER_TABLE_SIZE; i++) {
                head = &marker_table[i];
                hlist_for_each_entry(entry, node, head, hlist) {
                        if (!entry->ptype) {
                                if (entry->single.func == probe
                                                && entry->single.probe_private
                                                == probe_private)
                                        return entry;
                        } else {
                                struct marker_probe_closure *closure;
                                closure = entry->multi;
                                for (i = 0; closure[i].func; i++) {
                                        if (closure[i].func == probe &&
                                                        closure[i].probe_private
                                                        == probe_private)
                                                return entry;
                                }
                        }
                }
        }
        return NULL;
}

/**
 * marker_probe_unregister_private_data -  Disconnect a probe from a marker
 * @probe: probe function
 * @probe_private: probe private data
 *
 * Unregister a probe by providing the registered private data.
 * Only removes the first marker found in hash table.
 * Return 0 on success or error value.
 * We do not need to call a synchronize_sched to make sure the probes have
 * finished running before doing a module unload, because the module unload
 * itself uses stop_machine(), which insures that every preempt disabled section
 * have finished.
 */
int marker_probe_unregister_private_data(marker_probe_func *probe,
                void *probe_private)
{
        struct marker_entry *entry;
        int ret = 0;
        struct marker_probe_closure *old;

        mutex_lock(&markers_mutex);
        entry = get_marker_from_private_data(probe, probe_private);
        if (!entry) {
                ret = -ENOENT;
                goto end;
        }
        if (entry->rcu_pending)
                rcu_barrier();
        old = marker_entry_remove_probe(entry, NULL, probe_private);
        mutex_unlock(&markers_mutex);
        marker_update_probes();         /* may update entry */
        mutex_lock(&markers_mutex);
        entry = get_marker_from_private_data(probe, probe_private);
        WARN_ON(!entry);
        entry->oldptr = old;
        entry->rcu_pending = 1;
        /* write rcu_pending before calling the RCU callback */
        smp_wmb();
#ifdef CONFIG_PREEMPT_RCU
        synchronize_sched();    /* Until we have the call_rcu_sched() */
#endif
        call_rcu(&entry->rcu, free_old_closure);
        remove_marker(entry->name);     /* Ignore busy error message */
end:
        mutex_unlock(&markers_mutex);
        return ret;
}
EXPORT_SYMBOL_GPL(marker_probe_unregister_private_data);

/**
 * marker_get_private_data - Get a marker's probe private data
 * @name: marker name
 * @probe: probe to match
 * @num: get the nth matching probe's private data
 *
 * Returns the nth private data pointer (starting from 0) matching, or an
 * ERR_PTR.
 * Returns the private data pointer, or an ERR_PTR.
 * The private data pointer should _only_ be dereferenced if the caller is the
 * owner of the data, or its content could vanish. This is mostly used to
 * confirm that a caller is the owner of a registered probe.
 */
void *marker_get_private_data(const char *name, marker_probe_func *probe,
                int num)
{
        struct hlist_head *head;
        struct hlist_node *node;
        struct marker_entry *e;
        size_t name_len = strlen(name) + 1;
        u32 hash = jhash(name, name_len-1, 0);
        int i;

        head = &marker_table[hash & ((1 << MARKER_HASH_BITS)-1)];
        hlist_for_each_entry(e, node, head, hlist) {
                if (!strcmp(name, e->name)) {
                        if (!e->ptype) {
                                if (num == 0 && e->single.func == probe)
                                        return e->single.probe_private;
                                else
                                        break;
                        } else {
                                struct marker_probe_closure *closure;
                                int match = 0;
                                closure = e->multi;
                                for (i = 0; closure[i].func; i++) {
                                        if (closure[i].func != probe)
                                                continue;
                                        if (match++ == num)
                                                return closure[i].probe_private;
                                }
                        }
                }
        }
        return ERR_PTR(-ENOENT);
}
EXPORT_SYMBOL_GPL(marker_get_private_data);