ALSA: info: Fix potential deadlock at disconnection

ALSA: info: Fix potential deadlock at disconnection

As reported recently, ALSA core info helper may cause a deadlock at
the forced device disconnection during the procfs operation.

The proc_remove() (that is called from the snd_card_disconnect()
helper) has a synchronization of the pending procfs accesses via
wait_for_completion().  Meanwhile, ALSA procfs helper takes the global
mutex_lock(&info_mutex) at both the proc_open callback and
snd_card_info_disconnect() helper.  Since the proc_open can't finish
due to the mutex lock, wait_for_completion() never returns, either,
hence it deadlocks.

	TASK#1				TASK#2
	proc_reg_open()
	  takes use_pde()
	snd_info_text_entry_open()
					snd_card_disconnect()
					snd_info_card_disconnect()
					  takes mutex_lock(&info_mutex)
					proc_remove()
					wait_for_completion(unused_pde)
					  ... waiting task#1 closes
	mutex_lock(&info_mutex)
		=> DEADLOCK

This patch is a workaround for avoiding the deadlock scenario above.

The basic strategy is to move proc_remove() call outside the mutex
lock.  proc_remove() can work gracefully without extra locking, and it
can delete the tree recursively alone.  So, we call proc_remove() at
snd_info_card_disconnection() at first, then delete the rest resources
recursively within the info_mutex lock.

After the change, the function snd_info_disconnect() doesn't do
disconnection by itself any longer, but it merely clears the procfs
pointer.  So rename the function to snd_info_clear_entries() for
avoiding confusion.

The similar change is applied to snd_info_free_entry(), too.  Since
the proc_remove() is called only conditionally with the non-NULL
entry->p, it's skipped after the snd_info_clear_entries() call.

Reported-by: Shinhyung Kang <s47.kang@samsung.com>
Closes: https://lore.kernel.org/r/664457955.21699345385931.JavaMail.epsvc@epcpadp4
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20231109141954.4283-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>