On Wed, Jun 29, 2016 at 06:02:58PM +0200, Takashi Iwai wrote:
> Now I reread the code again, and I guess it's really the racy
> accesses. Basically you can't call alsa-lib functions concurrently.
> For example, calling snd_pcm_avail_update() from multiple threads
> concurrently may lead to such an error. Internally it copies /
> converts the content (e.g. for softvol plugin), and this would
> conflict when called in parallel.
>
> You can try a hackish patch below to see whether it emits any
> messages. It has no mutex, so the code itself is racy, but it should
> be enough just as a check.
>
> Meanwhile, the crash itself might be avoided by disabling
> "pcm->mmap_shadow = 1" line in pcm_softvol.c. Then it'll be copied to
> the external buffer.
Thank you Takashi, I will give it a try!
Baptiste
> ---
> diff --git a/src/pcm/pcm_plugin.c b/src/pcm/pcm_plugin.c
> index 8527783c3569..571defad6e12 100644
> --- a/src/pcm/pcm_plugin.c
> +++ b/src/pcm/pcm_plugin.c
> @@ -483,17 +483,25 @@ snd_pcm_plugin_mmap_commit(snd_pcm_t *pcm,
>
> static snd_pcm_sframes_t snd_pcm_plugin_avail_update(snd_pcm_t *pcm)
> {
> + static snd_pcm_t *check = NULL;
> snd_pcm_plugin_t *plugin = pcm->private_data;
> snd_pcm_t *slave = plugin->gen.slave;
> snd_pcm_sframes_t slave_size;
> int err;
>
> + if (!check)
> + check = pcm;
> + else if (pcm == check)
> + fprintf(stderr, "XXX RACY CALL\n");
> +
> slave_size = snd_pcm_avail_update(slave);
> if (pcm->stream == SND_PCM_STREAM_CAPTURE &&
> pcm->access != SND_PCM_ACCESS_RW_INTERLEAVED &&
> pcm->access != SND_PCM_ACCESS_RW_NONINTERLEAVED)
> goto _capture;
> *pcm->hw.ptr = *slave->hw.ptr;
> + if (pcm == check)
> + check = NULL;
> return slave_size;
> _capture:
> {
> @@ -545,11 +553,15 @@ static snd_pcm_sframes_t snd_pcm_plugin_avail_update(snd_pcm_t *pcm)
> slave_size -= slave_frames;
> xfer += frames;
> }
> + if (pcm == check)
> + check = NULL;
> return (snd_pcm_sframes_t)xfer;
>
> error_atomic:
> snd_atomic_write_end(&plugin->watom);
> error:
> + if (pcm == check)
> + check = NULL;
> return xfer > 0 ? (snd_pcm_sframes_t)xfer : err;
> }
> }
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