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Date: Mon, 21 Dec 2009 02:20:22 +0100 (CET) From: Andi Kleen <andi@...stfloor.org> To: paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com, ebiederm@...ssion.com Subject: [PATCH] [1/11] Add rcustring ADT for RCU protected strings 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@...ux.vnet.ibm.com Signed-off-by: Andi Kleen <ak@...ux.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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