CVE-2024-56532: Improper Locking

In the Linux kernel, the following vulnerability has been resolved:

ALSA: us122l: Use snd_card_free_when_closed() at disconnection

The USB disconnect callback is supposed to be short and not too-long waiting. OTOH, the current code uses snd_card_free() at disconnection, but this waits for the close of all used fds, hence it can take long. It eventually blocks the upper layer USB ioctls, which may trigger a soft lockup.

An easy workaround is to replace snd_card_free() with snd_card_free_when_closed(). This variant returns immediately while the release of resources is done asynchronously by the card device release at the last close.

The loop of us122l->mmap_count check is dropped as well. The check is useless for the asynchronous operation with *_when_closed().