[alsa-devel] [RFC PATCH 1/2] ASoC: core: don't increase component module refcount unconditionally

[Pierre-Louis Bossart]

The ASoC core has for the longest time increased the module reference
counts, even before the transition to the component model. This is
probably fine on most platforms, but it introduces a deadlock case on
Intel devices with the Skylake and SOF drivers which cannot be removed
due to their reference counts being modified by the core.

In these 2 cases, the PCI or ACPI driver .probe creates a platform
device to let the machine driver .probe register the audio
card. Conversely the PCI or ACPI driver .remove will unregister the
platform device which results in the card being removed by the machine
driver .remove.

With ascii art, this can be represented as

modprobe
snd_soc_skl/
soc-pci-dev/sof-acpci-dev  ----------> pci/acpi probe
       ^                                    |
       |                     ---------------|
       |                    |               |
       |                    V               V
    increase            register        register machine
    refcount            component       platform_device
       ^                                    |
       |                                    |
       |                                    V
    component <----   register card  <---- probe
    probe

The issue is that by playing with the component's module reference
counts during the card registration, it's no longer possible to remove
the module which controls the component. This can be shown, e.g. with
the following error:

root at plb-XPS-13-9350:~# lsmod | grep snd_soc_skl
snd_soc_skl           110592  1

root at plb-XPS-13-9350:~# rmmod snd_soc_skl
rmmod: ERROR: Module snd_soc_skl is in use

Increasing the reference count during the component probe is not
useful. If the PCI/ACPI module is removed, the card will be removed
anyway.

To avoid breaking existing platforms and allowing Intel platforms to
safely deal with module load/unload cases, this patch introduces a
flag which needs to be set during the component initialization. This
is a strictly opt-in capability that should only be used when the
handling of the component module does not require a reference count
increase to prevent removal during use.

Note that this solution is not directly applicable to the legacy
Atom/SST driver, which uses a different device hierarchy. There are
however additional refcount issues which prevent the ACPI driver from
being removed. This is a different issue which would need a different
patch.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
---
 include/sound/soc.h  | 3 +++
 sound/soc/soc-core.c | 6 ++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/sound/soc.h b/include/sound/soc.h
index 95689680336b..eb7db605955b 100644
--- a/include/sound/soc.h
+++ b/include/sound/soc.h
@@ -802,6 +802,9 @@ struct snd_soc_component_driver {
 	int probe_order;
 	int remove_order;
 
+	/* signal if the module handling the component cannot be removed */
+	unsigned int ignore_module_refcount:1;
+
 	/* bits */
 	unsigned int idle_bias_on:1;
 	unsigned int suspend_bias_off:1;
diff --git a/sound/soc/soc-core.c b/sound/soc/soc-core.c
index 9dad2b1498c1..80ab81f1df3d 100644
--- a/sound/soc/soc-core.c
+++ b/sound/soc/soc-core.c
@@ -947,7 +947,8 @@ static void soc_cleanup_component(struct snd_soc_component *component)
 	snd_soc_dapm_free(snd_soc_component_get_dapm(component));
 	soc_cleanup_component_debugfs(component);
 	component->card = NULL;
-	module_put(component->dev->driver->owner);
+	if (!component->driver->ignore_module_refcount)
+		module_put(component->dev->driver->owner);
 }
 
 static void soc_remove_component(struct snd_soc_component *component)
@@ -1362,7 +1363,8 @@ static int soc_probe_component(struct snd_soc_card *card,
 		return 0;
 	}
 
-	if (!try_module_get(component->dev->driver->owner))
+	if (!component->driver->ignore_module_refcount &&
+	    !try_module_get(component->dev->driver->owner))
 		return -ENODEV;
 
 	component->card = card;
-- 
2.17.1