[PATCH v6 1/2] driver core: Introduce device_link_wait_removal()

[PATCH v6 1/2] driver core: Introduce device_link_wait_removal()

[Herve Codina]

The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
introduces a workqueue to release the consumer and supplier devices used
in the devlink.
In the job queued, devices are release and in turn, when all the
references to these devices are dropped, the release function of the
device itself is called.

Nothing is present to provide some synchronisation with this workqueue
in order to ensure that all ongoing releasing operations are done and
so, some other operations can be started safely.

For instance, in the following sequence:
  1) of_platform_depopulate()
  2) of_overlay_remove()

During the step 1, devices are released and related devlinks are removed
(jobs pushed in the workqueue).
During the step 2, OF nodes are destroyed but, without any
synchronisation with devlink removal jobs, of_overlay_remove() can raise
warnings related to missing of_node_put():
  ERROR: memory leak, expected refcount 1 instead of 2

Indeed, the missing of_node_put() call is going to be done, too late,
from the workqueue job execution.

Introduce device_link_wait_removal() to offer a way to synchronize
operations waiting for the end of devlink removals (i.e. end of
workqueue jobs).
Also, as a flushing operation is done on the workqueue, the workqueue
used is moved from a system-wide workqueue to a local one.

Cc: stable@vger.kernel.org
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Saravana Kannan <saravanak@google.com>
---
 drivers/base/core.c    | 26 +++++++++++++++++++++++---
 include/linux/device.h |  1 +
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 7e3af0ad770a..f2242aadffb0 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -44,6 +44,7 @@ static bool fw_devlink_is_permissive(void);
 static void __fw_devlink_link_to_consumers(struct device *dev);
 static bool fw_devlink_drv_reg_done;
 static bool fw_devlink_best_effort;
+static struct workqueue_struct *device_link_wq;
 
 /**
  * __fwnode_link_add - Create a link between two fwnode_handles.
@@ -533,12 +534,26 @@ static void devlink_dev_release(struct device *dev)
 	/*
 	 * It may take a while to complete this work because of the SRCU
 	 * synchronization in device_link_release_fn() and if the consumer or
-	 * supplier devices get deleted when it runs, so put it into the "long"
-	 * workqueue.
+	 * supplier devices get deleted when it runs, so put it into the
+	 * dedicated workqueue.
 	 */
-	queue_work(system_long_wq, &link->rm_work);
+	queue_work(device_link_wq, &link->rm_work);
 }
 
+/**
+ * device_link_wait_removal - Wait for ongoing devlink removal jobs to terminate
+ */
+void device_link_wait_removal(void)
+{
+	/*
+	 * devlink removal jobs are queued in the dedicated work queue.
+	 * To be sure that all removal jobs are terminated, ensure that any
+	 * scheduled work has run to completion.
+	 */
+	flush_workqueue(device_link_wq);
+}
+EXPORT_SYMBOL_GPL(device_link_wait_removal);
+
 static struct class devlink_class = {
 	.name = "devlink",
 	.dev_groups = devlink_groups,
@@ -4165,9 +4180,14 @@ int __init devices_init(void)
 	sysfs_dev_char_kobj = kobject_create_and_add("char", dev_kobj);
 	if (!sysfs_dev_char_kobj)
 		goto char_kobj_err;
+	device_link_wq = alloc_workqueue("device_link_wq", 0, 0);
+	if (!device_link_wq)
+		goto wq_err;
 
 	return 0;
 
+ wq_err:
+	kobject_put(sysfs_dev_char_kobj);
  char_kobj_err:
 	kobject_put(sysfs_dev_block_kobj);
  block_kobj_err:
diff --git a/include/linux/device.h b/include/linux/device.h
index 1795121dee9a..d7d8305a72e8 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -1249,6 +1249,7 @@ void device_link_del(struct device_link *link);
 void device_link_remove(void *consumer, struct device *supplier);
 void device_links_supplier_sync_state_pause(void);
 void device_links_supplier_sync_state_resume(void);
+void device_link_wait_removal(void);
 
 /* Create alias, so I can be autoloaded. */
 #define MODULE_ALIAS_CHARDEV(major,minor) \
-- 
2.44.0

Re: [PATCH v6 1/2] driver core: Introduce device_link_wait_removal()

[Rob Herring]

On Mon, Mar 25, 2024 at 04:21:25PM +0100, Herve Codina wrote:
> The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> introduces a workqueue to release the consumer and supplier devices used
> in the devlink.
> In the job queued, devices are release and in turn, when all the
> references to these devices are dropped, the release function of the
> device itself is called.
> 
> Nothing is present to provide some synchronisation with this workqueue
> in order to ensure that all ongoing releasing operations are done and
> so, some other operations can be started safely.
> 
> For instance, in the following sequence:
>   1) of_platform_depopulate()
>   2) of_overlay_remove()
> 
> During the step 1, devices are released and related devlinks are removed
> (jobs pushed in the workqueue).
> During the step 2, OF nodes are destroyed but, without any
> synchronisation with devlink removal jobs, of_overlay_remove() can raise
> warnings related to missing of_node_put():
>   ERROR: memory leak, expected refcount 1 instead of 2
> 
> Indeed, the missing of_node_put() call is going to be done, too late,
> from the workqueue job execution.
> 
> Introduce device_link_wait_removal() to offer a way to synchronize
> operations waiting for the end of devlink removals (i.e. end of
> workqueue jobs).
> Also, as a flushing operation is done on the workqueue, the workqueue
> used is moved from a system-wide workqueue to a local one.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> Reviewed-by: Nuno Sa <nuno.sa@analog.com>
> Reviewed-by: Saravana Kannan <saravanak@google.com>
> ---
>  drivers/base/core.c    | 26 +++++++++++++++++++++++---
>  include/linux/device.h |  1 +
>  2 files changed, 24 insertions(+), 3 deletions(-)

Greg, can you ack and I'll take this series.

Rob

Re: [PATCH v6 1/2] driver core: Introduce device_link_wait_removal()

[Greg Kroah-Hartman]

On Mon, Mar 25, 2024 at 11:44:01AM -0500, Rob Herring wrote:
> On Mon, Mar 25, 2024 at 04:21:25PM +0100, Herve Codina wrote:
> > The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> > introduces a workqueue to release the consumer and supplier devices used
> > in the devlink.
> > In the job queued, devices are release and in turn, when all the
> > references to these devices are dropped, the release function of the
> > device itself is called.
> > 
> > Nothing is present to provide some synchronisation with this workqueue
> > in order to ensure that all ongoing releasing operations are done and
> > so, some other operations can be started safely.
> > 
> > For instance, in the following sequence:
> >   1) of_platform_depopulate()
> >   2) of_overlay_remove()
> > 
> > During the step 1, devices are released and related devlinks are removed
> > (jobs pushed in the workqueue).
> > During the step 2, OF nodes are destroyed but, without any
> > synchronisation with devlink removal jobs, of_overlay_remove() can raise
> > warnings related to missing of_node_put():
> >   ERROR: memory leak, expected refcount 1 instead of 2
> > 
> > Indeed, the missing of_node_put() call is going to be done, too late,
> > from the workqueue job execution.
> > 
> > Introduce device_link_wait_removal() to offer a way to synchronize
> > operations waiting for the end of devlink removals (i.e. end of
> > workqueue jobs).
> > Also, as a flushing operation is done on the workqueue, the workqueue
> > used is moved from a system-wide workqueue to a local one.
> > 
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> > Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> > Reviewed-by: Nuno Sa <nuno.sa@analog.com>
> > Reviewed-by: Saravana Kannan <saravanak@google.com>
> > ---
> >  drivers/base/core.c    | 26 +++++++++++++++++++++++---
> >  include/linux/device.h |  1 +
> >  2 files changed, 24 insertions(+), 3 deletions(-)
> 
> Greg, can you ack and I'll take this series.

Looks semi-sane:

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Re: [PATCH v6 1/2] driver core: Introduce device_link_wait_removal()

[Greg Kroah-Hartman]

On Mon, Mar 25, 2024 at 04:21:25PM +0100, Herve Codina wrote:
> The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> introduces a workqueue to release the consumer and supplier devices used
> in the devlink.
> In the job queued, devices are release and in turn, when all the
> references to these devices are dropped, the release function of the
> device itself is called.
> 
> Nothing is present to provide some synchronisation with this workqueue
> in order to ensure that all ongoing releasing operations are done and
> so, some other operations can be started safely.
> 
> For instance, in the following sequence:
>   1) of_platform_depopulate()
>   2) of_overlay_remove()
> 
> During the step 1, devices are released and related devlinks are removed
> (jobs pushed in the workqueue).
> During the step 2, OF nodes are destroyed but, without any
> synchronisation with devlink removal jobs, of_overlay_remove() can raise
> warnings related to missing of_node_put():
>   ERROR: memory leak, expected refcount 1 instead of 2
> 
> Indeed, the missing of_node_put() call is going to be done, too late,
> from the workqueue job execution.
> 
> Introduce device_link_wait_removal() to offer a way to synchronize
> operations waiting for the end of devlink removals (i.e. end of
> workqueue jobs).
> Also, as a flushing operation is done on the workqueue, the workqueue
> used is moved from a system-wide workqueue to a local one.
> 
> Cc: stable@vger.kernel.org

Why is this for stable?  You are just adding a new api, no one is using
it.

Or if they are, you didn't send me that patch...

greg k-h

Re: [PATCH v6 1/2] driver core: Introduce device_link_wait_removal()

[Herve Codina]

On Mon, 25 Mar 2024 19:37:10 +0100
Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> On Mon, Mar 25, 2024 at 04:21:25PM +0100, Herve Codina wrote:
> > The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> > introduces a workqueue to release the consumer and supplier devices used
> > in the devlink.
> > In the job queued, devices are release and in turn, when all the
> > references to these devices are dropped, the release function of the
> > device itself is called.
> > 
> > Nothing is present to provide some synchronisation with this workqueue
> > in order to ensure that all ongoing releasing operations are done and
> > so, some other operations can be started safely.
> > 
> > For instance, in the following sequence:
> >   1) of_platform_depopulate()
> >   2) of_overlay_remove()
> > 
> > During the step 1, devices are released and related devlinks are removed
> > (jobs pushed in the workqueue).
> > During the step 2, OF nodes are destroyed but, without any
> > synchronisation with devlink removal jobs, of_overlay_remove() can raise
> > warnings related to missing of_node_put():
> >   ERROR: memory leak, expected refcount 1 instead of 2
> > 
> > Indeed, the missing of_node_put() call is going to be done, too late,
> > from the workqueue job execution.
> > 
> > Introduce device_link_wait_removal() to offer a way to synchronize
> > operations waiting for the end of devlink removals (i.e. end of
> > workqueue jobs).
> > Also, as a flushing operation is done on the workqueue, the workqueue
> > used is moved from a system-wide workqueue to a local one.
> > 
> > Cc: stable@vger.kernel.org  
> 
> Why is this for stable?  You are just adding a new api, no one is using
> it.
> 
> Or if they are, you didn't send me that patch...

The patch 2 in this current series uses the new api.

Best regards,
Hervé

Re: [PATCH v6 1/2] driver core: Introduce device_link_wait_removal()

[Greg Kroah-Hartman]

On Mon, Mar 25, 2024 at 04:21:25PM +0100, Herve Codina wrote:
> The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> introduces a workqueue to release the consumer and supplier devices used
> in the devlink.
> In the job queued, devices are release and in turn, when all the
> references to these devices are dropped, the release function of the
> device itself is called.
> 
> Nothing is present to provide some synchronisation with this workqueue
> in order to ensure that all ongoing releasing operations are done and
> so, some other operations can be started safely.
> 
> For instance, in the following sequence:
>   1) of_platform_depopulate()
>   2) of_overlay_remove()

So this is only an issue for overlays?  Why has no one noticed this in
the years since 80dd33cf72d1 was added?  Why is this an issue now
suddenly?

thanks,

greg k-h

Re: [PATCH v6 1/2] driver core: Introduce device_link_wait_removal()

[Nuno Sá]

On Mon, 2024-03-25 at 19:38 +0100, Greg Kroah-Hartman wrote:
> On Mon, Mar 25, 2024 at 04:21:25PM +0100, Herve Codina wrote:
> > The commit 80dd33cf72d1 ("drivers: base: Fix device link removal")
> > introduces a workqueue to release the consumer and supplier devices used
> > in the devlink.
> > In the job queued, devices are release and in turn, when all the
> > references to these devices are dropped, the release function of the
> > device itself is called.
> > 
> > Nothing is present to provide some synchronisation with this workqueue
> > in order to ensure that all ongoing releasing operations are done and
> > so, some other operations can be started safely.
> > 
> > For instance, in the following sequence:
> >   1) of_platform_depopulate()
> >   2) of_overlay_remove()
> 
> So this is only an issue for overlays?  Why has no one noticed this in
> the years since 80dd33cf72d1 was added?  Why is this an issue now
> suddenly?
> 

Not sure either... Note this is only an issue if device links are in place. So the
overlay needs to have nodes creating those links. You need to have regulators, pwm,
eth phy (at least these ones I'm aware they create links) to trigger this. We would
have to dig through git to understand when would this be noticeable. But note this is
very straight to trigger.

May also very well be that most people don't really "play" with overlay removal...
For example, I have been dealing with overlays on rpi's for the last 5 years and only
noticed this last year when we had an usecase that involved overlay removal.

- Nuno Sá

[PATCH v6 2/2] of: dynamic: Synchronize of_changeset_destroy() with the devlink removals

[Herve Codina]

In the following sequence:
  1) of_platform_depopulate()
  2) of_overlay_remove()

During the step 1, devices are destroyed and devlinks are removed.
During the step 2, OF nodes are destroyed but
__of_changeset_entry_destroy() can raise warnings related to missing
of_node_put():
  ERROR: memory leak, expected refcount 1 instead of 2 ...

Indeed, during the devlink removals performed at step 1, the removal
itself releasing the device (and the attached of_node) is done by a job
queued in a workqueue and so, it is done asynchronously with respect to
function calls.
When the warning is present, of_node_put() will be called but wrongly
too late from the workqueue job.

In order to be sure that any ongoing devlink removals are done before
the of_node destruction, synchronize the of_changeset_destroy() with the
devlink removals.

Fixes: 80dd33cf72d1 ("drivers: base: Fix device link removal")
Cc: stable@vger.kernel.org
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Saravana Kannan <saravanak@google.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
---
 drivers/of/dynamic.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/of/dynamic.c b/drivers/of/dynamic.c
index 3bf27052832f..4d57a4e34105 100644
--- a/drivers/of/dynamic.c
+++ b/drivers/of/dynamic.c
@@ -9,6 +9,7 @@
 
 #define pr_fmt(fmt)	"OF: " fmt
 
+#include <linux/device.h>
 #include <linux/of.h>
 #include <linux/spinlock.h>
 #include <linux/slab.h>
@@ -667,6 +668,17 @@ void of_changeset_destroy(struct of_changeset *ocs)
 {
 	struct of_changeset_entry *ce, *cen;
 
+	/*
+	 * When a device is deleted, the device links to/from it are also queued
+	 * for deletion. Until these device links are freed, the devices
+	 * themselves aren't freed. If the device being deleted is due to an
+	 * overlay change, this device might be holding a reference to a device
+	 * node that will be freed. So, wait until all already pending device
+	 * links are deleted before freeing a device node. This ensures we don't
+	 * free any device node that has a non-zero reference count.
+	 */
+	device_link_wait_removal();
+
 	list_for_each_entry_safe_reverse(ce, cen, &ocs->entries, node)
 		__of_changeset_entry_destroy(ce);
 }
-- 
2.44.0