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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>
c7a6065195