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Another idea for simplifying locking in kernel/module.c

PATCH Adam J. Richter 2 participants 2 messages Jan 7, 2003 – Jan 10, 2003 Module support

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Adam J. Richter

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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
[email protected]     \ /                  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."

include/linux/module.h +6 -2
@@ -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;
 }
kernel/module.c +19 -159

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#1 lore raw

In message <[email protected]> you write:

Quoted 4 lines

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.

And now you can't modularize netfilter modules.

Rusty.
--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.