[PATCH 2/2] ALSA: seq: Fix data-race at module auto-loading

[Takashi Iwai]

It's been reported that there is a possible data-race accessing to the
global card_requested[] array at ALSA sequencer core, which is used
for determining whether to call request_module() for the card or not.
This data race itself is almost harmless, as it might end up with one
extra request_module() call for the already loaded module at most.
But it's still better to fix.

This patch addresses the possible data race of card_requested[] and
client_requested[] arrays by replacing them with bitmask.
It's an atomic operation and can work without locks.

Reported-by: Abhishek Shah <abhishek.shah at columbia.edu>
Cc: <stable at vger.kernel.org>
Link: https://lore.kernel.org/r/CAEHB24_ay6YzARpA1zgCsE7=H9CSJJzux618E=Ka4h0YdKn=qA@mail.gmail.com
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/seq/seq_clientmgr.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index 2e9d695d336c..052a690b5e11 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -121,13 +121,13 @@ struct snd_seq_client *snd_seq_client_use_ptr(int clientid)
 	spin_unlock_irqrestore(&clients_lock, flags);
 #ifdef CONFIG_MODULES
 	if (!in_interrupt()) {
-		static char client_requested[SNDRV_SEQ_GLOBAL_CLIENTS];
-		static char card_requested[SNDRV_CARDS];
+		static DECLARE_BITMAP(client_requested, SNDRV_SEQ_GLOBAL_CLIENTS);
+		static DECLARE_BITMAP(card_requested, SNDRV_CARDS);
+
 		if (clientid < SNDRV_SEQ_GLOBAL_CLIENTS) {
 			int idx;
 			
-			if (!client_requested[clientid]) {
-				client_requested[clientid] = 1;
+			if (!test_and_set_bit(clientid, client_requested)) {
 				for (idx = 0; idx < 15; idx++) {
 					if (seq_client_load[idx] < 0)
 						break;
@@ -142,10 +142,8 @@ struct snd_seq_client *snd_seq_client_use_ptr(int clientid)
 			int card = (clientid - SNDRV_SEQ_GLOBAL_CLIENTS) /
 				SNDRV_SEQ_CLIENTS_PER_CARD;
 			if (card < snd_ecards_limit) {
-				if (! card_requested[card]) {
-					card_requested[card] = 1;
+				if (!test_and_set_bit(card_requested, card))
 					snd_request_card(card);
-				}
 				snd_seq_device_load_drivers();
 			}
 		}
-- 
2.35.3