Messages in this thread | | | Date | Mon, 21 Dec 2009 18:46:35 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] [1/11] Add rcustring ADT for RCU protected strings |
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On Mon, Dec 21, 2009 at 02:20:22AM +0100, Andi Kleen wrote: > > 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
Looks very interesting, possibly extremely useful!
I do have a couple of questions below, one a debugging suggestion, and the other either a bug or extreme confusion on my part.
Thanx, Paul
> 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];
My guess is that you will sooner or later need a size field, perhaps under some debug config parameter.
> +}; > + > +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;
Assuming that "str" points to the "str" field of a struct rcu_string, what prevents a grace period from elapsing at this point, freeing the "str" out from under us?
> + 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@vger.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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