Hi there,
I have a quick debugfs question:
How can I protect against a module being unloaded while debugfs files it
provides are open?
I can do something like the following:
static int my_debugfs_file_open(struct inode *, struct file *)
{
if (!try_module_get())
return -EIO;
/* ... */
}
static int my_debugfs_file_release(struct inode *, struct file *)
{
/* ... */
module_put();
return 0;
}
static const struct file_operations my_debugfs_fops = {
/* ... */
.open = my_debugfs_file_open,
.release = my_debugfs_file_release,
/* ... */
};
static struct device_driver my_driver {
/* ... */
.owner = THIS_MODULE;
/* ... */
};
static int my_module_init(void)
{
driver_register(&my_driver);
debugfs_create_file(..., &my_debugfs_fops);
}
static void my_module_exit(void)
{
debugfs_remove_recusrive(...);
driver_unregister(&my_driver);
}
... but it doesn't quite work. debugfs_remove_recursive() prevents any
new file handles being opened, but files already open remain open -- so
it's still possible for the module text to get unloaded between the
module refcount being decreased inside module_put() and the time
my_debugfs_file_release() returns.
The scenario to consider is when a request to unload the module races
with closure of the last debugfs file.
The only obvious way I can see to solve this without changing the debugfs
code is to make the module impossible to unload by calling __module_get()
during initialisation, before any debugfs file is created.
A similar dependency problem exists when a pointer to some device
instance data is passed to debugfs_create_file(). For pluggable
devices, the device might go away at any time. I hoped this could
be solved by calling get_device() ... put_device() in the debugfs file
open and release methods, but I found that a device instance can
get removed, and the module unloaded, even though the struct device
refcount is not zero.
Am I missing something?
Cheers
---Dave
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