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SubjectRe: Patch/resubmit linux-2.5.63-bk4 try_module_get simplification
On Sun, 2 Mar 2003 15:12:37 +0100 (CET)
Roman Zippel <> wrote:

> Hi,
> On Fri, 28 Feb 2003, Adam J. Richter wrote:
> > The following patch shrinks changes the implementation of
> > try_module_get() and friends to eliminate the special stopping of all
> > CPU's when a module is unloaded. Instead, it uses a read/write
> > semaphore in a perhaps slightly non-intuitive way.
> Hmm, I was waiting a bit for Rusty's comment, but there isn't any...

(Sorry, on holiday, skimming mail)

I like it *very* much! There's a small race, but it's easily fixed by
turning the local_dec into a module_put (or variant) (otherwise an
rmmod --wait can sleep forever, waking up and seeing a non-zero
refcount, and going back to sleep).

Three other requests, if I may.

It'd be nice to have a comment for the two smp_mb() eg. /* Must increment
before checking state */ and vice-versa above the one in module.c. Secondly
probably nicer to just rename the modlist_lock to module_lock and use that,
and thirdly merge with the racefix patch below if Linus hasn't already
taken it.

BTW, I'm away from the office for a couple of weeks, and even if I weren't,
my test box doesn't stay up for more than minutes under any recent kernel 8(
Can you stress test this and send to Linus?

> BTW making the module ref functions not inline saves about 5KB with the
> standard config.

Interesting. The "if (!module)" could theortically be dropped, but I
just looked at the sizes of try_module_get as a standalone function
for various config options (x86, gcc 3.2.3):

try_module_get() 78 bytes

try_module_get() 48 bytes

try_module_get() 28 bytes

try_module_get() 21 bytes

These numbers are a bit exaggerated due to function prologue and
epilogues, but on x86 PREEMPT=y case at least I definitely think that
it's worth out-of-lining it (a straight function call costs about 11
bytes). Don't know about other archs.

Thanks for the patch!
there are those who do and those who hang on and you don't see too
many doers quoting their contemporaries. -- Larry McVoy

Name: Fix two module races
Author: Bob Miller, Rusty Russell
Status: Trivial

D: Bob Miller points out that the try_module_get in use_module() can,
D: of course, fail. Secondly, there is a race between setting the module
D: live, and a simultaneous removal of it.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.62-bk6/kernel/module.c working-2.5.62-bk6-modraces/kernel/module.c
--- linux-2.5.62-bk6/kernel/module.c 2003-02-18 11:18:57.000000000 +1100
+++ working-2.5.62-bk6-modraces/kernel/module.c 2003-02-24 13:42:44.000000000 +1100
@@ -173,16 +173,19 @@ static int use_module(struct module *a,
struct module_use *use;
if (b == NULL || already_uses(a, b)) return 1;

+ if (!strong_try_module_get(b))
+ return 0;
DEBUGP("Allocating new usage for %s.\n", a->name);
use = kmalloc(sizeof(*use), GFP_ATOMIC);
if (!use) {
printk("%s: out of memory loading\n", a->name);
+ module_put(b);
return 0;

use->module_which_uses = a;
list_add(&use->list, &b->modules_which_use_me);
- try_module_get(b); /* Can't fail */
return 1;

@@ -1456,10 +1459,12 @@ sys_init_module(void *umod,

/* Now it's a first class citizen! */
+ down(&module_mutex);
mod->state = MODULE_STATE_LIVE;
module_free(mod, mod->module_init);
mod->module_init = NULL;
mod->init_size = 0;
+ up(&module_mutex);

return 0;
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