Re: [PATCH 0/2] Synchronize DT overlay removal with devlink removals

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hello Saravana, Rob, Hervé,

[+Miquèl, who contributed to the discussion with Hervé and me]

On Wed, 6 Dec 2023 19:09:06 -0800
Saravana Kannan <saravanak@google.com> wrote:

> On Wed, Dec 6, 2023 at 9:15 AM Rob Herring <robh@kernel.org> wrote:
> >
> > On Thu, Nov 30, 2023 at 06:41:07PM +0100, Herve Codina wrote:  
> > > Hi,  
> >
> > +Saravana for comment  
> 
> I'll respond to this within a week -- very swamped at the moment. The
> main thing I want to make sure is that we don't cause an indirect
> deadlock with this wait(). I'll go back and look at why we added the
> work queue and then check for device/devlink locking issues.

While working on a project unrelated to Hervé's work, I also ended up
in getting sporadic but frequent "ERROR: memory leak, expected refcount
1 instead of..." messages, which persisted even after adding this patch
series on my tree.

My use case is the insertion and removal of a simple overlay describing
a regulator-fixed and an I2C GPIO expander using it. The messages appear
regardless of whether the insertion and removal is done from kernel code
or via the configfs interface (out-of-tree patches from [0]).

I reconstructed the sequence of operations, all of which stem from
of_overlay_remove():

int of_overlay_remove(int *ovcs_id)
{
    ...

    device_link_wait_removal(); // proposed by this patch series

    mutex_lock(&of_mutex);

    ...

    ret = __of_changeset_revert_notify(&ovcs->cset);
    // this ends up calling (excerpt from a long stack trace):
    // -> of_i2c_notify
    // -> device_remove
    // -> devm_regulator_release
    // -> device_link_remove
    // -> devlink_dev_release, which queues work for
    //      device_link_release_fn, which in turn calls:
    //      -> device_put
    //      -> device_release
    //      -> {platform,regulator,...}_dev*_release
    //      -> of_node_put() [**]

    ...

    free_overlay_changeset(ovcs);
    // calls:
    // -> of_changeset_destroy
    // -> __of_changeset_entry_destroy
    // -> pr_err("ERROR: memory leak, expected refcount 1 instead of %d...
    // The error appears or not, based on when the workqueue runs

err_unlock:
    mutex_unlock(&of_mutex);

    ...
}

So this adds up to the question of whether devlink removal should actually
be run asynchronously or not.

A simple short-term solution is to move the call to
device_link_wait_removal() later, just before free_overlay_changeset():


diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
index 1a8a6620748c..eccf08cf2160 100644
--- a/drivers/of/overlay.c
+++ b/drivers/of/overlay.c
@@ -1375,12 +1375,6 @@ int of_overlay_remove(int *ovcs_id)
 		goto out;
 	}
 
-	/*
-	 * Wait for any ongoing device link removals before removing some of
-	 * nodes
-	 */
-	device_link_wait_removal();
-
 	mutex_lock(&of_mutex);
 
 	ovcs = idr_find(&ovcs_idr, *ovcs_id);
@@ -1427,6 +1421,14 @@ int of_overlay_remove(int *ovcs_id)
 		if (!ret)
 			ret = ret_tmp;
 
+	/*
+	 * Wait for any ongoing device link removals before removing some of
+	 * nodes
+	 */
+	mutex_unlock(&of_mutex);
+	device_link_wait_removal();
+	mutex_lock(&of_mutex);
+
 	free_overlay_changeset(ovcs);
 
 err_unlock:


This obviously raises the question of whether unlocking and re-locking
the mutex is potentially dangerous. I have no answer to this right away,
but I tested this change with CONFIG_PROVE_LOCKING=y and no issue showed
up after several overlay load/unload sequences so I am not aware of any
actual issues with this change.

[0] https://git.kernel.org/pub/scm/linux/kernel/git/geert/renesas-drivers.git/log/?h=topic/overlays

Luca
-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com