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

[Gabriel Ryan]

We have tested this patch and confirm it eliminates the race we initially
reported.

Best,

Gabe

On Tue, Aug 23, 2022 at 3:27 AM Takashi Iwai <tiwai at suse.de> wrote:

> 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://urldefense.proofpoint.com/v2/url?u=https-3A__lore.kernel.org_r_CAEHB24-5Fay6YzARpA1zgCsE7-3DH9CSJJzux618E-3DKa4h0YdKn-3DqA-40mail.gmail.com&d=DwIDAg&c=009klHSCxuh5AI1vNQzSO0KGjl4nbi2Q0M1QLJX9BeE&r=EyAJYRJu01oaAhhVVY3o8zKgZvacDAXd_PNRtaqACCo&m=oPAvNa2pfJuUIAiwbiE1IJgoQhWb8AB7IBdJqWslhhuZ-LwBrrgAnFUthdapQska&s=K26ukWoQce9A8OzoQEGfRXynlOouHQ79dnAnD7xriNw&e=
>
> 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
>
>