On 5/9/07, Andrew Morton <[email protected]> wrote:
On Wed, 09 May 2007 16:52:41 -0700
Jeremy Fitzhardinge <[email protected]> wrote:
> Seems to be getting a 0 refcount. I don't see anything in the recent
> changes which might cause this, but this is relatively new behaviour.
> It was working for me in the 2.6.21-pre time period, but I haven't tried
> this since 2.6.21 was released.
>
> The BUG is actually triggered by the __module_get(THIS_MODULE) in
> loop_set_fd.
A few people have been playing with module refcounting lately. Did you
work out a reproduce-it recipe?
Ah, it's a mis-understanding on what kobj_probe_t function is suppose
to return on success. When we open loop device that has not been
initialized, we probe it via:
do_open
get_gendisk
kobj_lookup
loop_probe
Notice that in kobj_lookup(), when p->probe() returns non-zero value
(I presume it is an -ERRNO), it breaks out of the loop and propagate
the return value, otherwise, loops back to the beginning of the for
loop and retry, and in there get_disk() will be called via p->lock()
to get a ref against the module.
kobj_look_up(...) {
retry:
mutex_lock(domain->lock);
for (p = domain->probes[MAJOR(dev) % 255]; p; p = p->next) {
...
if (kobj)
return kobj;
goto retry;
}
So loop_probe() mistakenly returned wrong status and leads to future
oops on inconsistent module ref count. The following patch fixes the
issue.
Signed-off-by: Ken Chen <[email protected]>
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 18cdd8c..40f7bc2 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1460,6 +1460,7 @@ static void loop_del_one(struct loop_device *lo)
kfree(lo);
}
+/* return NULL for success, or return non-zero value if there are error */
static struct kobject *loop_probe(dev_t dev, int *part, void *data)
{
unsigned int number = dev & MINORMASK;
@@ -1474,8 +1475,8 @@ static struct kobject *loop_probe
*part = 0;
if (IS_ERR(lo))
return (void *)lo;
- else
- return &lo->lo_disk->kobj;
+
+ return NULL;
}
static int __init loop_init(void)
-
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