On 13/11/2022 10:47, Hyunwoo Kim wrote:
And, even if the mutex_unlock(&unit_mutex); of xillybus_find_inode() is finally moved, xdev may be released before kref_get() is executed if xillyusb_disconnect() ends just before the function returns. (Of course, this is an extremely rare case.) So, in xillyusb_open() we need to move kref_get() above xillybus_find_inode().
First of all, that's impossible. kref_get() is called on a member of a specific @xdev's struct, and it's xillybus_find_inode()'s job to find it. So before the call to xillybus_find_inode(), we don't know which @xdev it is. That's the tricky part of all this.
The solution of this submitted patch was a lock that briefly prevents the kref_put() of all @xdevs. The way it works is that if an @xdev is found by xillybus_find_inode(), it necessarily means that xillyusb_disconnect()'s call to xillybus_cleanup_chrdev() hasn't returned (yet). Therefore, holding @kref_mutex guarantees that the kref_put() call, which is later on, isn't reached for the @xdev that has been found.
If you've found a flaw in this mechanism, please be more specific about it. Regards, Eli
