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 <[email protected]>
Closes: https://lore.kernel.org/r/664457955.21699345385931.JavaMail.epsvc@epcpadp4
Reviewed-by: Jaroslav Kysela <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

sound/core/info.c

@@ -56,7 +56,7 @@ struct snd_info_private_data {
 };
 
 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)
 		return;
-	mutex_lock(&info_mutex);
+
 	proc_remove(card->proc_root_link);
-	card->proc_root_link = NULL;
 	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);
 }
 
@@ -745,15 +750,14 @@ struct snd_info_entry *snd_info_create_card_entry(struct snd_card *card,
 }
 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;
 
 	if (!entry->p)
 		return;
 	list_for_each_entry(p, &entry->children, list)
-		snd_info_disconnect(p);
-	proc_remove(entry->p);
+		snd_info_clear_entries(p);
 	entry->p = NULL;
 }
 
@@ -770,8 +774,9 @@ void snd_info_free_entry(struct snd_info_entry * entry)
 	if (!entry)
 		return;
 	if (entry->p) {
+		proc_remove(entry->p);
 		mutex_lock(&info_mutex);
-		snd_info_disconnect(entry);
+		snd_info_clear_entries(entry);
 		mutex_unlock(&info_mutex);
 	}