Re: [PATCH v8 09/12] sysfs: fix deadlock race with module removal

Ming Lei
2021-10-05T09:24:43Z

On Mon, Sep 27, 2021 at 09:38:02AM -0700, Luis Chamberlain wrote:
> When driver sysfs attributes use a lock also used on module removal we
> can race to deadlock. This happens when for instance a sysfs file on
> a driver is used, then at the same time we have module removal call
> trigger. The module removal call code holds a lock, and then the
> driver's sysfs file entry waits for the same lock. While holding the
> lock the module removal tries to remove the sysfs entries, but these
> cannot be removed yet as one is waiting for a lock. This won't complete
> as the lock is already held. Likewise module removal cannot complete,
> and so we deadlock.
>
> This can now be easily reproducible with our sysfs selftest as follows:
>
> ./tools/testing/selftests/sysfs/sysfs.sh -t 0027
>
> This uses a local driver lock. Test 0028 can also be used, that uses
> the rtnl_lock():
>
> ./tools/testing/selftests/sysfs/sysfs.sh -t 0028
>
> To fix this we extend the struct kernfs_node with a module reference
> and use the try_module_get() after kernfs_get_active() is called. As
> documented in the prior patch, we now know that once kernfs_get_active()
> is called the module is implicitly guarded to exist and cannot be removed.
> This is because the module is the one in charge of removing the same
> sysfs file it created, and removal of sysfs files on module exit will wait
> until they don't have any active references. By using a try_module_get()
> after kernfs_get_active() we yield to let module removal trump calls to
> process a sysfs operation, while also preventing module removal if a sysfs
> operation is in already progress. This prevents the deadlock.
>
> This deadlock was first reported with the zram driver, however the live

Looks not see the lock pattern you mentioned in zram driver, can you
share the related zram code?

> patching folks have acknowledged they have observed this as well with
> live patching, when a live patch is removed. I was then able to
> reproduce easily by creating a dedicated selftest for it.
>
> A sketch of how this can happen follows, consider foo a local mutex
> part of a driver, and used on the driver's module exit routine and
> on one of its sysfs ops:
>
> foo.c:
> static DEFINE_MUTEX(foo);
> static ssize_t foo_store(struct device *dev,
> struct device_attribute *attr,
> const char *buf, size_t count)
> {
> ...
> mutex_lock(&foo);
> ...
> mutex_lock(&foo);
> ...
> }
> static DEVICE_ATTR_RW(foo);
> ...
> void foo_exit(void)
> {
> mutex_lock(&foo);
> ...
> mutex_unlock(&foo);
> }
> module_exit(foo_exit);
>
> And this can lead to this condition:
>
> CPU A CPU B
> foo_store()
> foo_exit()
> mutex_lock(&foo)
> mutex_lock(&foo)
> del_gendisk(some_struct->disk);
> device_del()
> device_remove_groups()

I guess the deadlock exists if foo_exit() is called anywhere. If yes,
look the issue may not be related with removing module directly, right?

Thanks,
Ming