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    Subject[PATCH] [1/11] Add rcustring ADT for RCU protected strings
    Date

    Add a little ADT for RCU protected strings. RCU is a convenient
    way to manage modifications to read-often-write-seldom strings.
    Add some helper functions to make this more straight forward.

    Used by follow-on patches to implement RCU protected sysctl strings.

    * General rules:
    * Reader has to use rcu_read_lock() and not sleep while accessing the string,
    * or alternatively get a copy with access_rcu_string()
    * Writer needs an own lock against each other.
    * Each modification should allocate a new string first and free the old
    * one with free_rcu_string()
    * In writers use rcu_assign_pointer to publicize the updated string to
    * global readers.
    * The size passed to access_rcu_string() must be the same as passed
    * to alloc_rcu_string() and be known in advance. Don't use strlen()!
    *
    * For sysctls also see proc_rcu_string() as a convenient wrapper

    Cc: paulmck@linux.vnet.ibm.com
    Signed-off-by: Andi Kleen <ak@linux.intel.com>

    ---
    include/linux/rcustring.h | 20 ++++++++
    lib/Makefile | 3 -
    lib/rcustring.c | 112 ++++++++++++++++++++++++++++++++++++++++++++++
    3 files changed, 134 insertions(+), 1 deletion(-)

    Index: linux-2.6.33-rc1-ak/include/linux/rcustring.h
    ===================================================================
    --- /dev/null
    +++ linux-2.6.33-rc1-ak/include/linux/rcustring.h
    @@ -0,0 +1,20 @@
    +#ifndef _RCUSTRING_H
    +#define _RCUSTRING_H 1
    +
    +#include <linux/gfp.h>
    +#include <linux/rcupdate.h>
    +
    +/*
    + * Simple wrapper to manage strings by RCU.
    + */
    +
    +extern char *alloc_rcu_string(int size, gfp_t gfp);
    +extern void free_rcu_string(const char *string);
    +
    +/*
    + * size must be the same as alloc_rcu_string, don't
    + * use strlen on str!
    + */
    +extern char *access_rcu_string(const char *str, int size, gfp_t gfp);
    +
    +#endif
    Index: linux-2.6.33-rc1-ak/lib/rcustring.c
    ===================================================================
    --- /dev/null
    +++ linux-2.6.33-rc1-ak/lib/rcustring.c
    @@ -0,0 +1,112 @@
    +/*
    + * Manage strings by Read-Copy-Update. This is useful for global strings
    + * that change only very rarely, but are read often.
    + *
    + * Author: Andi Kleen
    + *
    + * General rules:
    + * Reader has to use rcu_read_lock() and not sleep while accessing the string,
    + * or alternatively get a copy with access_rcu_string()
    + * Writer needs an own lock against each other.
    + * Each modification should allocate a new string first and free the old
    + * one with free_rcu_string()
    + * In writers use rcu_assign_pointer to publicize the updated string to
    + * global readers.
    + * The size passed to access_rcu_string() must be the same as passed
    + * to alloc_rcu_string() and be known in advance. Don't use strlen()!
    + *
    + * For sysctls also see proc_rcu_string() as a convenient wrapper
    + *
    + * Typical example:
    + * #define MAX_GLOBAL_SIZE ...
    + * char *global = "default";
    + *
    + * Rare writer:
    + * char *old, *new;
    + * DECLARE_MUTEX(writer_lock);
    + * mutex_lock(&writer_lock);
    + * new = alloc_rcu_string(MAX_GLOBAL_SIZE, GFP_KERNEL);
    + * if (!new) {
    + * mutex_unlock(&writer_lock);
    + * return -ENOMEM;
    + * }
    + * strlcpy(new, new_value, MAX_GLOBAL_SIZE);
    + * old = global;
    + * rcu_assign_pointer(global, new);
    + * mutex_unlock(&writer_lock);
    + * free_rcu_string(old);
    + *
    + * Sleepy reader:
    + * char *str = access_rcu_string(global, MAX_GLOBAL_SIZE, GFP_KERNEL);
    + * if (!str)
    + * return -ENOMEM;
    + * ... use str while sleeping ...
    + * kfree(string);
    + *
    + * Non sleepy reader:
    + * rcu_read_lock();
    + * ... read str ...
    + * rcu_read_unlock();
    + *
    + * Note this code could be relatively easily generalized for other kinds
    + * of non-atomic data, but this initial version only handles strings.
    + * Only need to change the strlcpy() below to memcpy()
    + */
    +#include <linux/kernel.h>
    +#include <linux/rcustring.h>
    +#include <linux/slab.h>
    +#include <linux/rcupdate.h>
    +#include <linux/module.h>
    +#include <linux/string.h>
    +
    +struct rcu_string {
    + struct rcu_head rcu;
    + char str[0];
    +};
    +
    +char *alloc_rcu_string(int size, gfp_t gfp)
    +{
    + struct rcu_string *rs = kmalloc(sizeof(struct rcu_string) + size, gfp);
    + if (!rs)
    + return NULL;
    + return rs->str;
    +}
    +EXPORT_SYMBOL(alloc_rcu_string);
    +
    +static void do_free_rcu_string(struct rcu_head *h)
    +{
    + kfree(container_of(h, struct rcu_string, rcu));
    +}
    +
    +static inline struct rcu_string *str_to_rcustr(const char *str)
    +{
    + /*
    + * Opencoded container_of because the strict type checking
    + * in normal container_of cannot deal with char str[0] vs char *str.
    + */
    + return (struct rcu_string *)(str - offsetof(struct rcu_string, str));
    +}
    +
    +void free_rcu_string(const char *str)
    +{
    + struct rcu_string *rs = str_to_rcustr(str);
    + call_rcu(&rs->rcu, do_free_rcu_string);
    +}
    +EXPORT_SYMBOL(free_rcu_string);
    +
    +/*
    + * Get a local private copy of a RCU protected string.
    + * Mostly useful to get a string that is stable while sleeping.
    + * Caller must free returned string.
    + */
    +char *access_rcu_string(const char *str, int size, gfp_t gfp)
    +{
    + char *copy = kmalloc(size, gfp);
    + if (!str)
    + return NULL;
    + rcu_read_lock();
    + strlcpy(copy, str, size);
    + rcu_read_unlock();
    + return copy;
    +}
    +EXPORT_SYMBOL(access_rcu_string);
    Index: linux-2.6.33-rc1-ak/lib/Makefile
    ===================================================================
    --- linux-2.6.33-rc1-ak.orig/lib/Makefile
    +++ linux-2.6.33-rc1-ak/lib/Makefile
    @@ -12,7 +12,8 @@ lib-y := ctype.o string.o vsprintf.o cmd
    idr.o int_sqrt.o extable.o prio_tree.o \
    sha1.o irq_regs.o reciprocal_div.o argv_split.o \
    proportions.o prio_heap.o ratelimit.o show_mem.o \
    - is_single_threaded.o plist.o decompress.o flex_array.o
    + is_single_threaded.o plist.o decompress.o flex_array.o \
    + rcustring.o

    lib-$(CONFIG_MMU) += ioremap.o
    lib-$(CONFIG_SMP) += cpumask.o

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      Last update: 2009-12-21 02:25    [W:0.029 / U:59.712 seconds]
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