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SubjectRe: [PATCH 2/2] module: fix bne2 "gave up waiting for init of module libcrc32c"


On Tue, 1 Jun 2010, Brandon Philips wrote:
>
> When I tested a Kernel with Rusty's modules branch pulled onto
> 2.6.35-rc1 I got duplicate sysfs path errors:

Hmm. Yeah, the module_mutex used to be held across the whole "find -> add"
state, but isn't any more.

> To fix this we need to make sure that we only have one copy of a module
> going through load_module at a time. Patch suggestion follows which
> boots without errors. I am sure there is a better way to do what it does
> ;)

I think Rusty may have made the lock a bit _too_ finegrained there, and
didn't add it to some places that needed it. It looks, for example, like
PATCH 1/2 actually drops the lock in places where it's needed
("find_module()" is documented to need it, but now load_module() didn't
hold it at all when it did the find_module()).

Rather than adding a new "module_loading" list, I think we should be able
to just use the existing "modules" list, and just fix up the locking a
bit.

In fact, maybe we could just move the "look up existing module" a bit
later - optimistically assuming that the module doesn't exist, and then
just undoing the work if it turns out that we were wrong, just before
adding ourselves to the list.

A patch something like the appended (TOTALLY UNTESTED!)

Linus
---
kernel/module.c | 13 ++++++++-----
1 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index a1f46a5..21f7ffa 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2198,11 +2198,6 @@ static noinline struct module *load_module(void __user *umod,
goto free_mod;
}

- if (find_module(mod->name)) {
- err = -EEXIST;
- goto free_mod;
- }
-
mod->state = MODULE_STATE_COMING;

/* Allow arches to frob section contents and sizes. */
@@ -2486,6 +2481,13 @@ static noinline struct module *load_module(void __user *umod,
* The mutex protects against concurrent writers.
*/
mutex_lock(&module_mutex);
+
+ if (find_module(mod->name)) {
+ err = -EEXIST;
+ /* This will also unlock the mutex */
+ goto already_exists;
+ }
+
list_add_rcu(&mod->list, &modules);
mutex_unlock(&module_mutex);

@@ -2511,6 +2513,7 @@ static noinline struct module *load_module(void __user *umod,
mutex_lock(&module_mutex);
/* Unlink carefully: kallsyms could be walking list. */
list_del_rcu(&mod->list);
+ already_exists:
mutex_unlock(&module_mutex);
synchronize_sched();
module_arch_cleanup(mod);

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