Problem with component helpers and probe deferral in 4.5-rc1

Message ID 1453831153.2850.107.camel@linaro.org
State New
Headers show

Commit Message

Jon Medhurst (Tixy) Jan. 26, 2016, 5:59 p.m.
I believe I've found a problem with the component helpers and/or how
drivers use them. I discovered this whilst trying to get ARM's HDLCD
driver [1] working on 4.5-rc1, however I believe that code is following
a pattern used by drivers already in 4.5 and the problem isn't specific
to it. This is what I have observed...

The master device's probe function uses component_match_add to create a
match list then it passes this to component_master_add_with_match.

That creates a struct master and then calls try_to_bring_up_master
which:
- Calls find_components to attach all components to the master.
- Calls master->ops->bind()

If this bind call fails (with -EPROBE_DEFER due to missing clock in my
case) then this error is returned to component_master_add_with_match
which proceeds to delete the master struct. However, find_components has
already attached components to that deleted master, so I think we also
need something to detach components as well. I've added a patch at the
end of this email which does that directly, but I wonder if instead it's
the responsibility of the driver for the master to call
component_master_del on error?

Finally, with my scenario which has probe deferring, some time later the
original master device will be probed again, repeating all the above.
Except that now find_components doesn't find the components because they
are already attached to a different master (the old master struct which
was deleted) this results in a permanent error. Which is what lead me to
investigate.

[1] https://lkml.org/lkml/2015/12/22/451

The patch to detach components when master is deleted...

-------------------------------------------------------------------------

From: Jon Medhurst <tixy@linaro.org>

Subject: [PATCH] component: Detach components when deleting master struct

component_master_add_with_match calls find_components which, if any
components already exist, it attaches to the master struct. However, if
we later encounter an error the master struct is deleted, leaving
components with a dangling pointer to it.

If the error was a temporary one, e.g. for probe deferral, then when
the master device is re-probed, it will fail to find the required
components as they appear to already be attached to a master.

Fix this by nulling components pointers to the master struct when it is
deleted. This code is factored out into a separate function so it can be
shared with component_master_del.

Signed-off-by: Jon Medhurst <tixy@linaro.org>

---
 drivers/base/component.c | 41 ++++++++++++++++++++++-------------------
 1 file changed, 22 insertions(+), 19 deletions(-)

-- 
2.1.4



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Comments

Jon Medhurst (Tixy) Jan. 27, 2016, 9:18 a.m. | #1
On Tue, 2016-01-26 at 22:35 +0000, Russell King - ARM Linux wrote:
> On Tue, Jan 26, 2016 at 05:59:13PM +0000, Jon Medhurst (Tixy) wrote:

> > I believe I've found a problem with the component helpers and/or how

> > drivers use them. I discovered this whilst trying to get ARM's HDLCD

> > driver [1] working on 4.5-rc1, however I believe that code is following

> > a pattern used by drivers already in 4.5 and the problem isn't specific

> > to it. This is what I have observed...

> 

> Hmm, it all looks plausible, and I'm again left wondering how the code

> passed testing over the last year (I've been running this code for

> ages both on iMX6 and Dove, where deferred probing does happen.)


It depends on the order of things. To go wrong, components must already
be there before the master is added, then the master needs to defer
probing from it's bind callback. So, components deferring won't trigger
the issue, neither will master deferring before any components have been
added.

> Your patch looks like the right thing to do, so I'll add it to the

> component tree shortly - it should end up in linux-next in a few days

> time.


Thanks. I wasn't sure if that patch was correct as I hadn't looked at
any of this code before this week.

-- 
Tixy


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Jon Medhurst (Tixy) Jan. 27, 2016, 9:50 a.m. | #2
On Wed, 2016-01-27 at 09:18 +0000, Jon Medhurst (Tixy) wrote:
> On Tue, 2016-01-26 at 22:35 +0000, Russell King - ARM Linux wrote:

> > On Tue, Jan 26, 2016 at 05:59:13PM +0000, Jon Medhurst (Tixy) wrote:

> > > I believe I've found a problem with the component helpers and/or how

> > > drivers use them. I discovered this whilst trying to get ARM's HDLCD

> > > driver [1] working on 4.5-rc1, however I believe that code is following

> > > a pattern used by drivers already in 4.5 and the problem isn't specific

> > > to it. This is what I have observed...

> > 

> > Hmm, it all looks plausible, and I'm again left wondering how the code

> > passed testing over the last year (I've been running this code for

> > ages both on iMX6 and Dove, where deferred probing does happen.)

> 

> It depends on the order of things. To go wrong, components must already

> be there before the master is added, then the master needs to defer

> probing from it's bind callback. So, components deferring won't trigger

> the issue,


Actually, looks like drm drivers for armada and imx will end up calling
component_bind_all from their bind callpath, so if components return
-EPROBE_DEFER from their bind function, then the master will bail out
with that, triggering my failure scenario. Hmmm...

-- 
Tixy


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Patch hide | download patch | download mbox

diff --git a/drivers/base/component.c b/drivers/base/component.c
index 89f5cf68..a3a1394 100644
--- a/drivers/base/component.c
+++ b/drivers/base/component.c
@@ -283,6 +283,24 @@  void component_match_add_release(struct device *master,
 }
 EXPORT_SYMBOL(component_match_add_release);
 
+static void free_master(struct master *master)
+{
+	struct component_match *match = master->match;
+	int i;
+
+	list_del(&master->node);
+
+	if (match) {
+		for (i = 0; i < match->num; i++) {
+			struct component *c = match->compare[i].component;
+			if (c)
+				c->master = NULL;
+		}
+	}
+
+	kfree(master);
+}
+
 int component_master_add_with_match(struct device *dev,
 	const struct component_master_ops *ops,
 	struct component_match *match)
@@ -309,11 +327,9 @@  int component_master_add_with_match(struct device *dev,
 
 	ret = try_to_bring_up_master(master, NULL);
 
-	if (ret < 0) {
-		/* Delete off the list if we weren't successful */
-		list_del(&master->node);
-		kfree(master);
-	}
+	if (ret < 0)
+		free_master(master);
+
 	mutex_unlock(&component_mutex);
 
 	return ret < 0 ? ret : 0;
@@ -324,25 +340,12 @@  void component_master_del(struct device *dev,
 	const struct component_master_ops *ops)
 {
 	struct master *master;
-	int i;
 
 	mutex_lock(&component_mutex);
 	master = __master_find(dev, ops);
 	if (master) {
-		struct component_match *match = master->match;
-
 		take_down_master(master);
-
-		list_del(&master->node);
-
-		if (match) {
-			for (i = 0; i < match->num; i++) {
-				struct component *c = match->compare[i].component;
-				if (c)
-					c->master = NULL;
-			}
-		}
-		kfree(master);
+		free_master(master);
 	}
 	mutex_unlock(&component_mutex);
 }