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ALSA: info: Fix potential deadlock at disconnection

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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>
This commit is contained in:
Takashi Iwai 2023-11-09 15:19:54 +01:00
parent 3e3ab468eb
commit c7a6065195
1 changed files with 13 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

21
sound/core/info.c
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@ -56,7 +56,7 @@ struct snd_info_private_data {
}; };
static int snd_info_version_init(void); static int snd_info_version_init(void);
static void snd_info_disconnect(struct snd_info_entry *entry); static void snd_info_clear_entries(struct snd_info_entry *entry);
/* /*
@ -569,11 +569,16 @@ void snd_info_card_disconnect(struct snd_card *card)
{ {
if (!card) if (!card)
return; return;
mutex_lock(&info_mutex);
proc_remove(card->proc_root_link); proc_remove(card->proc_root_link);
card->proc_root_link = NULL;
if (card->proc_root) if (card->proc_root)
snd_info_disconnect(card->proc_root); proc_remove(card->proc_root->p);
mutex_lock(&info_mutex);
if (card->proc_root)
snd_info_clear_entries(card->proc_root);
card->proc_root_link = NULL;
card->proc_root = NULL;
mutex_unlock(&info_mutex); mutex_unlock(&info_mutex);
} }
@ -745,15 +750,14 @@ struct snd_info_entry *snd_info_create_card_entry(struct snd_card *card,
} }
EXPORT_SYMBOL(snd_info_create_card_entry); EXPORT_SYMBOL(snd_info_create_card_entry);
static void snd_info_disconnect(struct snd_info_entry *entry) static void snd_info_clear_entries(struct snd_info_entry *entry)
{ {
struct snd_info_entry *p; struct snd_info_entry *p;
if (!entry->p) if (!entry->p)
return; return;
list_for_each_entry(p, &entry->children, list) list_for_each_entry(p, &entry->children, list)
snd_info_disconnect(p); snd_info_clear_entries(p);
proc_remove(entry->p);
entry->p = NULL; entry->p = NULL;
} }
@ -770,8 +774,9 @@ void snd_info_free_entry(struct snd_info_entry * entry)
if (!entry) if (!entry)
return; return;
if (entry->p) { if (entry->p) {
proc_remove(entry->p);
mutex_lock(&info_mutex); mutex_lock(&info_mutex);
snd_info_disconnect(entry); snd_info_clear_entries(entry);
mutex_unlock(&info_mutex); mutex_unlock(&info_mutex);
} }