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SubjectAnother idea for simplifying locking in kernel/module.c
	Here is a way to replace all of the specialized "stop CPU"
locking code in kernel/module.c with an rw_semaphore by using
down_read_trylock in try_module_get() and down_write when beginning to
unload the module.

The following UNTESTED patch, a net deletion of 136 lines,
illustrates the basic idea. I've only verified that the resulting
kernel/module.c compiles. I'm sorry that I can't test it right now as
I'm in the midst of working on something else.

By the way, I think this change could also help eliminate the
module->state variable, which I believe currently can change while
a reader could be trying to read it.

Adam J. Richter __ ______________ 575 Oroville Road
adam@yggdrasil.com \ / Milpitas, California 95035
+1 408 309-6081 | g g d r a s i l United States of America
"Free Software For The Rest Of Us."

--- linux-2.5.54/include/linux/module.h 2003-01-01 19:22:53.000000000 -0800
+++ linux/include/linux/module.h 2003-01-06 17:17:17.000000000 -0800
@@ -13,12 +13,13 @@
#include <linux/stat.h>
#include <linux/compiler.h>
#include <linux/cache.h>
#include <linux/kmod.h>
#include <linux/elf.h>
#include <linux/stringify.h>
+#include <linux/rwsem.h>

#include <asm/module.h>
#include <asm/uaccess.h> /* For struct exception_table_entry */

/* Not Yet Implemented */
#define MODULE_LICENSE(name)
@@ -147,12 +148,14 @@
struct mod_arch_specific arch;

/* Am I unsafe to unload? */
int unsafe;

#ifdef CONFIG_MODULE_UNLOAD
+ struct rw_semaphore unload_rwsem;
+
/* Reference counts */
struct module_ref ref[NR_CPUS];

/* What modules depend on me? */
struct list_head modules_which_use_me;

@@ -198,15 +201,16 @@
static inline int try_module_get(struct module *module)
{
int ret = 1;

if (module) {
unsigned int cpu = get_cpu();
- if (likely(module_is_live(module)))
+ if (down_read_trylock(&module->unload_rwsem)) {
local_inc(&module->ref[cpu].count);
- else
+ up_read(&module->unload_rwsem);
+ } else
ret = 0;
put_cpu();
}
return ret;
}

--- linux-2.5.54/kernel/module.c 2003-01-01 19:22:35.000000000 -0800
+++ linux/kernel/module.c 2003-01-06 17:29:09.000000000 -0800
@@ -18,6 +18,7 @@
*/
#include <linux/config.h>
#include <linux/module.h>
+#include <linux/module_rwsem.h>
#include <linux/moduleloader.h>
#include <linux/init.h>
#include <linux/slab.h>
@@ -126,6 +127,10 @@
{
unsigned int i;

+ init_rwsem(&mod->unload_rwsem);
+ down_read(&mod->unload_rwsem);
+ /* Prevent unloading of module until module_init() completes. */
+
INIT_LIST_HEAD(&mod->modules_which_use_me);
for (i = 0; i < NR_CPUS; i++)
atomic_set(&mod->ref[i].count, 0);
@@ -195,154 +200,6 @@
}
}

-#ifdef CONFIG_SMP
-/* Thread to stop each CPU in user context. */
-enum stopref_state {
- STOPREF_WAIT,
- STOPREF_PREPARE,
- STOPREF_DISABLE_IRQ,
- STOPREF_EXIT,
-};
-
-static enum stopref_state stopref_state;
-static unsigned int stopref_num_threads;
-static atomic_t stopref_thread_ack;
-
-static int stopref(void *cpu)
-{
- int irqs_disabled = 0;
- int prepared = 0;
-
- sprintf(current->comm, "kmodule%lu\n", (unsigned long)cpu);
-
- /* Highest priority we can manage, and move to right CPU. */
-#if 0 /* FIXME */
- struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
- setscheduler(current->pid, SCHED_FIFO, &param);
-#endif
- set_cpus_allowed(current, 1UL << (unsigned long)cpu);
-
- /* Ack: we are alive */
- atomic_inc(&stopref_thread_ack);
-
- /* Simple state machine */
- while (stopref_state != STOPREF_EXIT) {
- if (stopref_state == STOPREF_DISABLE_IRQ && !irqs_disabled) {
- local_irq_disable();
- irqs_disabled = 1;
- /* Ack: irqs disabled. */
- atomic_inc(&stopref_thread_ack);
- } else if (stopref_state == STOPREF_PREPARE && !prepared) {
- /* Everyone is in place, hold CPU. */
- preempt_disable();
- prepared = 1;
- atomic_inc(&stopref_thread_ack);
- }
- if (irqs_disabled || prepared)
- cpu_relax();
- else
- yield();
- }
-
- /* Ack: we are exiting. */
- atomic_inc(&stopref_thread_ack);
-
- if (irqs_disabled)
- local_irq_enable();
- if (prepared)
- preempt_enable();
-
- return 0;
-}
-
-/* Change the thread state */
-static void stopref_set_state(enum stopref_state state, int sleep)
-{
- atomic_set(&stopref_thread_ack, 0);
- wmb();
- stopref_state = state;
- while (atomic_read(&stopref_thread_ack) != stopref_num_threads) {
- if (sleep)
- yield();
- else
- cpu_relax();
- }
-}
-
-/* Stop the machine. Disables irqs. */
-static int stop_refcounts(void)
-{
- unsigned int i, cpu;
- unsigned long old_allowed;
- int ret = 0;
-
- /* One thread per cpu. We'll do our own. */
- cpu = smp_processor_id();
-
- /* FIXME: racy with set_cpus_allowed. */
- old_allowed = current->cpus_allowed;
- set_cpus_allowed(current, 1UL << (unsigned long)cpu);
-
- atomic_set(&stopref_thread_ack, 0);
- stopref_num_threads = 0;
- stopref_state = STOPREF_WAIT;
-
- /* No CPUs can come up or down during this. */
- down(&cpucontrol);
-
- for (i = 0; i < NR_CPUS; i++) {
- if (i == cpu || !cpu_online(i))
- continue;
- ret = kernel_thread(stopref, (void *)(long)i, CLONE_KERNEL);
- if (ret < 0)
- break;
- stopref_num_threads++;
- }
-
- /* Wait for them all to come to life. */
- while (atomic_read(&stopref_thread_ack) != stopref_num_threads)
- yield();
-
- /* If some failed, kill them all. */
- if (ret < 0) {
- stopref_set_state(STOPREF_EXIT, 1);
- up(&cpucontrol);
- return ret;
- }
-
- /* Don't schedule us away at this point, please. */
- preempt_disable();
-
- /* Now they are all scheduled, make them hold the CPUs, ready. */
- stopref_set_state(STOPREF_PREPARE, 0);
-
- /* Make them disable irqs. */
- stopref_set_state(STOPREF_DISABLE_IRQ, 0);
-
- local_irq_disable();
- return 0;
-}
-
-/* Restart the machine. Re-enables irqs. */
-static void restart_refcounts(void)
-{
- stopref_set_state(STOPREF_EXIT, 0);
- local_irq_enable();
- preempt_enable();
- up(&cpucontrol);
-}
-#else /* ...!SMP */
-static inline int stop_refcounts(void)
-{
- local_irq_disable();
- return 0;
-}
-static inline void restart_refcounts(void)
-{
- local_irq_enable();
-}
-#endif
-
static unsigned int module_refcount(struct module *mod)
{
unsigned int i, total = 0;
@@ -378,7 +235,8 @@
{
struct module *mod;
char name[MODULE_NAME_LEN];
- int ret, forced = 0;
+ int ret = 0;
+ int forced = 0;

if (!capable(CAP_SYS_MODULE))
return -EPERM;
@@ -431,25 +289,21 @@
goto out;
}
}
- /* Stop the machine so refcounts can't move: irqs disabled. */
- DEBUGP("Stopping refcounts...\n");
- ret = stop_refcounts();
- if (ret != 0)
- goto out;
+
+ down_write(&mod->unload_rwsem);

/* If it's not unused, quit unless we are told to block. */
if ((flags & O_NONBLOCK) && module_refcount(mod) != 0) {
forced = try_force(flags);
- if (!forced)
+ if (!forced) {
+ up_write(&mod->unload_rwsem);
ret = -EWOULDBLOCK;
+ goto out;
+ }
} else {
mod->waiter = current;
mod->state = MODULE_STATE_GOING;
}
- restart_refcounts();
-
- if (ret != 0)
- goto out;

if (forced)
goto destroy;
@@ -471,9 +325,12 @@
destroy:
/* Final destruction now noone is using it. */
mod->exit();
+ up_write(&mod->unload_rwsem);
free_module(mod);
+ mod = NULL;

out:
+
up(&module_mutex);
return ret;
}
@@ -1236,6 +1093,9 @@

/* Start the module */
ret = mod->init();
+#ifdef CONFIG_MODULE_UNLOAD
+ up_read(&mod->unload_rwsem);
+#endif
if (ret < 0) {
/* Init routine failed: abort. Try to protect us from
buggy refcounters. */
-
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