linux-kernel - Re: [PATCH 09/10] ALSA: pcm: Add snd_pcm_ops for snd_pcm

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Date:   Tue, 26 Mar 2019 09:35:36 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     <twischer@...adit-jv.com>
Cc:     <patch@...a-project.org>, <broonie@...nel.org>, <perex@...ex.cz>,
        <alsa-devel@...a-project.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/10] ALSA: pcm: Add snd_pcm_ops for snd_pcm_link()

On Tue, 26 Mar 2019 08:49:33 +0100,
<twischer@...adit-jv.com> wrote:
> 
> From: Timo Wischer <twischer@...adit-jv.com>
> 
> snd_pcm_link() can be called by the user as long as the device is not
> yet started. Therefore currently a driver which wants to iterate over
> the linked substreams has to do this at the start trigger. But the start
> trigger should not block for a long time. Therefore there is no callback
> which can be used to iterate over the linked substreams without delaying
> the start trigger.
> This patch introduces a new callback function which will be called after
> the linked substream list was updated by snd_pcm_link(). This callback
> function is allowed to block for a longer time without interfering the
> synchronized start up of linked substreams.
> 
> Signed-off-by: Timo Wischer <twischer@...adit-jv.com>

Well, the idea appears interesting, but I'm afraid that the
implementation is still racy.  The place you're calling the new
callback isn't protected, hence the stream can be triggered while
calling it.  That is, even during operating your loopback link_changed
callback, another thread is able to start the stream.


thanks,

Takashi