[RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Yao Cheng]

on vlv, if ipvr is installed, it need be manually unloaded before
i915, otherwise user might run into use-after-free issue.

v2:
added this patch per Daniel's comment

v3:
no change

Signed-off-by: Yao Cheng <yao.cheng@intel.com>
---
 tests/drv_module_reload | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/tests/drv_module_reload b/tests/drv_module_reload
index 5cbff89..82c67bd 100755
--- a/tests/drv_module_reload
+++ b/tests/drv_module_reload
@@ -24,6 +24,14 @@ rmmod snd_hda_intel &> /dev/null
 
 #ignore errors in ips - gen5 only
 rmmod intel_ips &> /dev/null
+
+# vlv only for now:
+# due to platform device model limitation, need unload ipvr manually
+if lsmod | grep ipvr &> /dev/null ; then
+	echo Need manually unload ipvr.ko.
+	rmmod ipvr
+fi
+
 rmmod i915
 #ignore errors in intel-gtt, often built-in
 rmmod intel-gtt &> /dev/null
@@ -31,6 +39,11 @@ rmmod intel-gtt &> /dev/null
 rmmod drm_kms_helper &> /dev/null
 rmmod drm &> /dev/null
 
+if lsmod | grep ipvr &> /dev/null ; then
+	echo WARNING: ipvr.ko still loaded!
+	exit 1
+fi
+
 if lsmod | grep i915 &> /dev/null ; then
 	echo WARNING: i915.ko still loaded!
 	exit 1
@@ -41,6 +54,9 @@ fi
 modprobe i915
 echo 1 > /sys/class/vtconsole/vtcon1/bind
 
+# for vlv, load VED driver
+modprobe ipvr
+
 modprobe snd_hda_intel
 
 # try to run something
-- 
2.1.0

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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Thierry Reding]

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On Sat, Nov 22, 2014 at 03:10:01AM +0800, Yao Cheng wrote:
> on vlv, if ipvr is installed, it need be manually unloaded before
> i915, otherwise user might run into use-after-free issue.

Huh? That doesn't sound right. What exactly is it that's going wrong?
You should never have to do this. If you do you're almost certainly
doing something wrong in the kernel module.

Thierry

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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Daniel Vetter]

On Fri, Nov 21, 2014 at 09:27:04PM +0100, Thierry Reding wrote:
> On Sat, Nov 22, 2014 at 03:10:01AM +0800, Yao Cheng wrote:
> > on vlv, if ipvr is installed, it need be manually unloaded before
> > i915, otherwise user might run into use-after-free issue.
> 
> Huh? That doesn't sound right. What exactly is it that's going wrong?
> You should never have to do this. If you do you're almost certainly
> doing something wrong in the kernel module.

It's the hilarity called platform devices. Removing them is somewhat racy,
so doing that upfront makes the entire thing a bit safer. The use after
free is on the text, since grabbing a module refcount for the platform
device doesn't work (it would pin the module forever).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Thierry Reding]

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On Fri, Nov 21, 2014 at 09:36:33PM +0100, Daniel Vetter wrote:
> On Fri, Nov 21, 2014 at 09:27:04PM +0100, Thierry Reding wrote:
> > On Sat, Nov 22, 2014 at 03:10:01AM +0800, Yao Cheng wrote:
> > > on vlv, if ipvr is installed, it need be manually unloaded before
> > > i915, otherwise user might run into use-after-free issue.
> > 
> > Huh? That doesn't sound right. What exactly is it that's going wrong?
> > You should never have to do this. If you do you're almost certainly
> > doing something wrong in the kernel module.
> 
> It's the hilarity called platform devices. Removing them is somewhat racy,
> so doing that upfront makes the entire thing a bit safer. The use after
> free is on the text, since grabbing a module refcount for the platform
> device doesn't work (it would pin the module forever).

I don't understand what the issue is here. I've used platform devices
quite extensively on ARM and I've never encountered a situation where
they were insufficient (or racy for that matter).

If I understand correctly what this commit tries to achieve, then it
unloads one module before another module that it depends on so that the
dependency can be removed subsequently without causing a crash. That
sounds really brittle to me. How are you going to document this for
users so that they don't accidentally go and unload the i915 module and
crash their system?

Thierry

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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Daniel Vetter]

On Mon, Nov 24, 2014 at 10:55:46AM +0100, Thierry Reding wrote:
> On Fri, Nov 21, 2014 at 09:36:33PM +0100, Daniel Vetter wrote:
> > On Fri, Nov 21, 2014 at 09:27:04PM +0100, Thierry Reding wrote:
> > > On Sat, Nov 22, 2014 at 03:10:01AM +0800, Yao Cheng wrote:
> > > > on vlv, if ipvr is installed, it need be manually unloaded before
> > > > i915, otherwise user might run into use-after-free issue.
> > > 
> > > Huh? That doesn't sound right. What exactly is it that's going wrong?
> > > You should never have to do this. If you do you're almost certainly
> > > doing something wrong in the kernel module.
> > 
> > It's the hilarity called platform devices. Removing them is somewhat racy,
> > so doing that upfront makes the entire thing a bit safer. The use after
> > free is on the text, since grabbing a module refcount for the platform
> > device doesn't work (it would pin the module forever).
> 
> I don't understand what the issue is here. I've used platform devices
> quite extensively on ARM and I've never encountered a situation where
> they were insufficient (or racy for that matter).
> 
> If I understand correctly what this commit tries to achieve, then it
> unloads one module before another module that it depends on so that the
> dependency can be removed subsequently without causing a crash. That
> sounds really brittle to me. How are you going to document this for
> users so that they don't accidentally go and unload the i915 module and
> crash their system?

Module unloading taints your kernel and isn't an end-user supported
feature. That simple ;-)

Also afaik the problem is that you actually can't unload i915 until you've
unloaded the subordinate driver, since i915 registering the platform
driver prevents unload. Or at least that was my understanding, I didn't
test this myself. I just asked whether the unload script still works and
apparently it breaks.

I guess what's different with ARM is that DT creates all the platform
devices, and not modules themselves?
-Daniel
-- 
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Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Cheng, Yao]

> -----Original Message-----
> From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch] On Behalf Of Daniel
> Vetter
> Sent: Monday, November 24, 2014 21:15
> To: Thierry Reding
> Cc: Daniel Vetter; Cheng, Yao; intel-gfx@lists.freedesktop.org; dri-
> devel@lists.freedesktop.org; daniel.vetter@ffwll.ch; Kelley, Sean V; Chehab,
> John; emil.l.velikov@gmail.com; Jiang, Fei
> Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support
> 
> On Mon, Nov 24, 2014 at 10:55:46AM +0100, Thierry Reding wrote:
> > On Fri, Nov 21, 2014 at 09:36:33PM +0100, Daniel Vetter wrote:
> > > On Fri, Nov 21, 2014 at 09:27:04PM +0100, Thierry Reding wrote:
> > > > On Sat, Nov 22, 2014 at 03:10:01AM +0800, Yao Cheng wrote:
> > > > > on vlv, if ipvr is installed, it need be manually unloaded
> > > > > before i915, otherwise user might run into use-after-free issue.
> > > >
> > > > Huh? That doesn't sound right. What exactly is it that's going wrong?
> > > > You should never have to do this. If you do you're almost
> > > > certainly doing something wrong in the kernel module.
> > >
> > > It's the hilarity called platform devices. Removing them is somewhat
> > > racy, so doing that upfront makes the entire thing a bit safer. The
> > > use after free is on the text, since grabbing a module refcount for
> > > the platform device doesn't work (it would pin the module forever).
> >
> > I don't understand what the issue is here. I've used platform devices
> > quite extensively on ARM and I've never encountered a situation where
> > they were insufficient (or racy for that matter).
> >
> > If I understand correctly what this commit tries to achieve, then it
> > unloads one module before another module that it depends on so that
> > the dependency can be removed subsequently without causing a crash.
> > That sounds really brittle to me. How are you going to document this
> > for users so that they don't accidentally go and unload the i915
> > module and crash their system?
> 
> Module unloading taints your kernel and isn't an end-user supported feature.
> That simple ;-)
> 
> Also afaik the problem is that you actually can't unload i915 until you've
> unloaded the subordinate driver, since i915 registering the platform driver
> prevents unload. Or at least that was my understanding, I didn't test this
> myself. I just asked whether the unload script still works and apparently it
> breaks.
> 
> I guess what's different with ARM is that DT creates all the platform devices,
> and not modules themselves?
> -Daniel

Thierry/Daniel, the actual symptom is, after "rmmod i915", though drm_drv_release() is also called on the child device "ipvr", I still see the module exist in the system (check it by "lsmod"). This causes issue when I modprobe i915 and ipvr again later. 
I don't understand why this happens but I believe what Daniel said: "grabbing a module refcount for the platform device doesn't work (it would pin the module forever)".

> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Thierry Reding]

[-- Attachment #1.1: Type: text/plain, Size: 2557 bytes --]

On Mon, Nov 24, 2014 at 02:14:48PM +0100, Daniel Vetter wrote:
> On Mon, Nov 24, 2014 at 10:55:46AM +0100, Thierry Reding wrote:
> > On Fri, Nov 21, 2014 at 09:36:33PM +0100, Daniel Vetter wrote:
> > > On Fri, Nov 21, 2014 at 09:27:04PM +0100, Thierry Reding wrote:
> > > > On Sat, Nov 22, 2014 at 03:10:01AM +0800, Yao Cheng wrote:
> > > > > on vlv, if ipvr is installed, it need be manually unloaded before
> > > > > i915, otherwise user might run into use-after-free issue.
> > > > 
> > > > Huh? That doesn't sound right. What exactly is it that's going wrong?
> > > > You should never have to do this. If you do you're almost certainly
> > > > doing something wrong in the kernel module.
> > > 
> > > It's the hilarity called platform devices. Removing them is somewhat racy,
> > > so doing that upfront makes the entire thing a bit safer. The use after
> > > free is on the text, since grabbing a module refcount for the platform
> > > device doesn't work (it would pin the module forever).
> > 
> > I don't understand what the issue is here. I've used platform devices
> > quite extensively on ARM and I've never encountered a situation where
> > they were insufficient (or racy for that matter).
> > 
> > If I understand correctly what this commit tries to achieve, then it
> > unloads one module before another module that it depends on so that the
> > dependency can be removed subsequently without causing a crash. That
> > sounds really brittle to me. How are you going to document this for
> > users so that they don't accidentally go and unload the i915 module and
> > crash their system?
> 
> Module unloading taints your kernel and isn't an end-user supported
> feature. That simple ;-)
> 
> Also afaik the problem is that you actually can't unload i915 until you've
> unloaded the subordinate driver, since i915 registering the platform
> driver prevents unload.

That doesn't sound at all like use-after-free, so if that's really the
only problem then the commit description should be more accurate.

> Or at least that was my understanding, I didn't test this myself. I
> just asked whether the unload script still works and apparently it
> breaks.
> 
> I guess what's different with ARM is that DT creates all the platform
> devices, and not modules themselves?

No, I don't think that has anything to do with it. I'm pretty sure I've
seen this work reliably with something like MFD where one module can
create a number of platform devices, and remove them again, just as
well.

Thierry

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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Thierry Reding]

[-- Attachment #1.1: Type: text/plain, Size: 4060 bytes --]

On Mon, Dec 01, 2014 at 03:06:08AM +0000, Cheng, Yao wrote:
> > -----Original Message-----
> > From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch] On Behalf Of Daniel
> > Vetter
> > Sent: Monday, November 24, 2014 21:15
> > To: Thierry Reding
> > Cc: Daniel Vetter; Cheng, Yao; intel-gfx@lists.freedesktop.org; dri-
> > devel@lists.freedesktop.org; daniel.vetter@ffwll.ch; Kelley, Sean V; Chehab,
> > John; emil.l.velikov@gmail.com; Jiang, Fei
> > Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support
> > 
> > On Mon, Nov 24, 2014 at 10:55:46AM +0100, Thierry Reding wrote:
> > > On Fri, Nov 21, 2014 at 09:36:33PM +0100, Daniel Vetter wrote:
> > > > On Fri, Nov 21, 2014 at 09:27:04PM +0100, Thierry Reding wrote:
> > > > > On Sat, Nov 22, 2014 at 03:10:01AM +0800, Yao Cheng wrote:
> > > > > > on vlv, if ipvr is installed, it need be manually unloaded
> > > > > > before i915, otherwise user might run into use-after-free issue.
> > > > >
> > > > > Huh? That doesn't sound right. What exactly is it that's going wrong?
> > > > > You should never have to do this. If you do you're almost
> > > > > certainly doing something wrong in the kernel module.
> > > >
> > > > It's the hilarity called platform devices. Removing them is somewhat
> > > > racy, so doing that upfront makes the entire thing a bit safer. The
> > > > use after free is on the text, since grabbing a module refcount for
> > > > the platform device doesn't work (it would pin the module forever).
> > >
> > > I don't understand what the issue is here. I've used platform devices
> > > quite extensively on ARM and I've never encountered a situation where
> > > they were insufficient (or racy for that matter).
> > >
> > > If I understand correctly what this commit tries to achieve, then it
> > > unloads one module before another module that it depends on so that
> > > the dependency can be removed subsequently without causing a crash.
> > > That sounds really brittle to me. How are you going to document this
> > > for users so that they don't accidentally go and unload the i915
> > > module and crash their system?
> > 
> > Module unloading taints your kernel and isn't an end-user supported feature.
> > That simple ;-)
> > 
> > Also afaik the problem is that you actually can't unload i915 until you've
> > unloaded the subordinate driver, since i915 registering the platform driver
> > prevents unload. Or at least that was my understanding, I didn't test this
> > myself. I just asked whether the unload script still works and apparently it
> > breaks.
> > 
> > I guess what's different with ARM is that DT creates all the platform devices,
> > and not modules themselves?
> > -Daniel
> 
> Thierry/Daniel, the actual symptom is, after "rmmod i915", though
> drm_drv_release() is also called on the child device "ipvr", I still
> see the module exist in the system (check it by "lsmod").

Which module? ipvr or i915?

> This causes issue when I modprobe i915 and ipvr again later.

What issue are you seeing? If your driver can't deal with a situation
where it's probed again after being removed then you have a bug.

> I don't understand why this happens but I believe what Daniel said:
> "grabbing a module refcount for the platform device doesn't work (it
> would pin the module forever)"

What I'd expect to happen is this:

	# modprobe i915
	i915 registers a platform devices
	# modprobe ipvr
	driver core probes ipvr device
	# modprobe -r i915
	i915 removes the platform device (ipvr's ->remove() is called)

I guess if you don't do anything else, then indeed the ipvr module will
stay around, but the above should work idempotently, that is you should
be able to repeat it an unlimited number of times and nothing should
break.

In fact you should be able to run the following in any permutation
without causing a crash:

	# modprobe i915
	# modprobe ipvr
	# modprobe -r ipvr
	# modprobe -r i915

If any permutation results in a crash you have a bug.

Thierry

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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Cheng, Yao]

> -----Original Message-----
> From: Thierry Reding [mailto:thierry.reding@gmail.com]
> Sent: Wednesday, December 17, 2014 16:13
> To: Cheng, Yao
> Cc: Daniel Vetter; intel-gfx@lists.freedesktop.org; dri-
> devel@lists.freedesktop.org; daniel.vetter@ffwll.ch; Kelley, Sean V; Chehab,
> John; emil.l.velikov@gmail.com; Jiang, Fei
> Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

Thanks Thiery for the suggestion, pls see my inline comments

> 
> > Thierry/Daniel, the actual symptom is, after "rmmod i915", though
> > drm_drv_release() is also called on the child device "ipvr", I still
> > see the module exist in the system (check it by "lsmod").
> 
> Which module? ipvr or i915?

The ipvr module still exist by checking "lsmod" after rmmod i915

> 
> > This causes issue when I modprobe i915 and ipvr again later.
> 
> What issue are you seeing? If your driver can't deal with a situation where it's
> probed again after being removed then you have a bug.
>

I double checked the symptom and found it was a deadlock on drm_global_mutex.
When i915_driver_load() registers the platform device while ipvr module is in the system, ipvr's probe() function tries to lock drm_global_mutex which was already held by i915.
I think either of the following 2 actions need to be moved to a bottom half e.g. a work queue:
	platform_device_add () call in i915_ved.c (called during i915_driver_load())
	drm_dev_register() call during ipvr's probe()
Which one makes more sense? pls kindly advise (I personally prefer the former one.).
 
> > I don't understand why this happens but I believe what Daniel said:
> > "grabbing a module refcount for the platform device doesn't work (it
> > would pin the module forever)"
> 
> What I'd expect to happen is this:
> 
> 	# modprobe i915
> 	i915 registers a platform devices
> 	# modprobe ipvr
> 	driver core probes ipvr device
> 	# modprobe -r i915
> 	i915 removes the platform device (ipvr's ->remove() is called)
> 
> I guess if you don't do anything else, then indeed the ipvr module will stay
> around, but the above should work idempotently, that is you should be able
> to repeat it an unlimited number of times and nothing should break.
> 
> In fact you should be able to run the following in any permutation without
> causing a crash:
> 
> 	# modprobe i915
> 	# modprobe ipvr
> 	# modprobe -r ipvr
> 	# modprobe -r i915
> 
> If any permutation results in a crash you have a bug.

I assume all the permutations will work after fixing the deadlock.

> 
> Thierry
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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Thierry Reding]

[-- Attachment #1.1: Type: text/plain, Size: 1882 bytes --]

On Thu, Dec 18, 2014 at 05:44:37AM +0000, Cheng, Yao wrote:
> > -----Original Message-----
> > From: Thierry Reding [mailto:thierry.reding@gmail.com]
> > Sent: Wednesday, December 17, 2014 16:13
> > To: Cheng, Yao
> > Cc: Daniel Vetter; intel-gfx@lists.freedesktop.org; dri-
> > devel@lists.freedesktop.org; daniel.vetter@ffwll.ch; Kelley, Sean V; Chehab,
> > John; emil.l.velikov@gmail.com; Jiang, Fei
> > Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support
> 
> Thanks Thiery for the suggestion, pls see my inline comments
> 
> > 
> > > Thierry/Daniel, the actual symptom is, after "rmmod i915", though
> > > drm_drv_release() is also called on the child device "ipvr", I still
> > > see the module exist in the system (check it by "lsmod").
> > 
> > Which module? ipvr or i915?
> 
> The ipvr module still exist by checking "lsmod" after rmmod i915
> 
> > 
> > > This causes issue when I modprobe i915 and ipvr again later.
> > 
> > What issue are you seeing? If your driver can't deal with a situation where it's
> > probed again after being removed then you have a bug.
> >
> 
> I double checked the symptom and found it was a deadlock on drm_global_mutex.
> When i915_driver_load() registers the platform device while ipvr module is in the system, ipvr's probe() function tries to lock drm_global_mutex which was already held by i915.
> I think either of the following 2 actions need to be moved to a bottom half e.g. a work queue:
> 	platform_device_add () call in i915_ved.c (called during i915_driver_load())
> 	drm_dev_register() call during ipvr's probe()
> Which one makes more sense? pls kindly advise (I personally prefer the former one.).

Yes, that's somewhat ugly, but I don't see a way around that. I'd also
think that moving platform_device_add() to a workqueue would be the best
option here.

Thierry

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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Daniel Vetter]

On Thu, Dec 18, 2014 at 11:04 AM, Thierry Reding
<thierry.reding@gmail.com> wrote:
>> I double checked the symptom and found it was a deadlock on drm_global_mutex.
>> When i915_driver_load() registers the platform device while ipvr module is in the system, ipvr's probe() function tries to lock drm_global_mutex which was already held by i915.
>> I think either of the following 2 actions need to be moved to a bottom half e.g. a work queue:
>>       platform_device_add () call in i915_ved.c (called during i915_driver_load())
>>       drm_dev_register() call during ipvr's probe()
>> Which one makes more sense? pls kindly advise (I personally prefer the former one.).
>
> Yes, that's somewhat ugly, but I don't see a way around that. I'd also
> think that moving platform_device_add() to a workqueue would be the best
> option here.

Or we simply kill drm_global_mutex for platform drivers that don't use
the ->probe hook. It should work when they have a correct order betwen
drm_dev_alloc and _register and all the code in between. So just ditch
the ->load callback in teh ipvr driver and rework the load sequence as
suggested somewhere else and this is fixed already. No need for bottom
halfs I think.
-Daniel
-- 
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Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Cheng, Yao]

> -----Original Message-----
> From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch]
> Sent: Thursday, December 18, 2014 19:21
> To: Thierry Reding
> Cc: Cheng, Yao; intel-gfx@lists.freedesktop.org; dri-
> devel@lists.freedesktop.org; Kelley, Sean V; Chehab, John;
> emil.l.velikov@gmail.com; Jiang, Fei
> Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support
> 
> On Thu, Dec 18, 2014 at 11:04 AM, Thierry Reding <thierry.reding@gmail.com>
> wrote:
> >> I double checked the symptom and found it was a deadlock on
> drm_global_mutex.
> >> When i915_driver_load() registers the platform device while ipvr module
> is in the system, ipvr's probe() function tries to lock drm_global_mutex which
> was already held by i915.
> >> I think either of the following 2 actions need to be moved to a bottom half
> e.g. a work queue:
> >>       platform_device_add () call in i915_ved.c (called during
> i915_driver_load())
> >>       drm_dev_register() call during ipvr's probe() Which one makes
> >> more sense? pls kindly advise (I personally prefer the former one.).
> >
> > Yes, that's somewhat ugly, but I don't see a way around that. I'd also
> > think that moving platform_device_add() to a workqueue would be the
> > best option here.
> 
> Or we simply kill drm_global_mutex for platform drivers that don't use the -
> >probe hook. It should work when they have a correct order betwen
> drm_dev_alloc and _register and all the code in between. So just ditch the -
> >load callback in teh ipvr driver and rework the load sequence as suggested
> somewhere else and this is fixed already. No need for bottom halfs I think.

Daniel, sorry I didn't quite understand "platform drivers that don't use the probe hook". For initialization, the ipvr platform driver's probe() is called in following 2 possible paths:
1. ipvr installed before i915. In this case, ipvr's probe() is called inside i915_driver_load() and falls into the drm_global_mutex dead lock.
2. i915 installed before ipvr. In this case, ipvr's probe() is called without drm_global_mutex held by i915 and no dead lock issue.
If we kill drm_global_mutex, will path 2 run into issue? And in your suggestion, how to rework the load sequence? Do you mean calling ipvr's load() callback directly during platform driver probe()?

> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
_______________________________________________
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Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Daniel Vetter]

On Sun, Dec 21, 2014 at 02:40:24PM +0000, Cheng, Yao wrote:
> > -----Original Message-----
> > From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch]
> > Sent: Thursday, December 18, 2014 19:21
> > To: Thierry Reding
> > Cc: Cheng, Yao; intel-gfx@lists.freedesktop.org; dri-
> > devel@lists.freedesktop.org; Kelley, Sean V; Chehab, John;
> > emil.l.velikov@gmail.com; Jiang, Fei
> > Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support
> > 
> > On Thu, Dec 18, 2014 at 11:04 AM, Thierry Reding <thierry.reding@gmail.com>
> > wrote:
> > >> I double checked the symptom and found it was a deadlock on
> > drm_global_mutex.
> > >> When i915_driver_load() registers the platform device while ipvr module
> > is in the system, ipvr's probe() function tries to lock drm_global_mutex which
> > was already held by i915.
> > >> I think either of the following 2 actions need to be moved to a bottom half
> > e.g. a work queue:
> > >>       platform_device_add () call in i915_ved.c (called during
> > i915_driver_load())
> > >>       drm_dev_register() call during ipvr's probe() Which one makes
> > >> more sense? pls kindly advise (I personally prefer the former one.).
> > >
> > > Yes, that's somewhat ugly, but I don't see a way around that. I'd also
> > > think that moving platform_device_add() to a workqueue would be the
> > > best option here.
> > 
> > Or we simply kill drm_global_mutex for platform drivers that don't use the -
> > >probe hook. It should work when they have a correct order betwen
> > drm_dev_alloc and _register and all the code in between. So just ditch the -
> > >load callback in teh ipvr driver and rework the load sequence as suggested
> > somewhere else and this is fixed already. No need for bottom halfs I think.
> 
> Daniel, sorry I didn't quite understand "platform drivers that don't use
> the probe hook". For initialization, the ipvr platform driver's probe()
> is called in following 2 possible paths:
> 1. ipvr installed before i915. In this case, ipvr's probe() is called
> inside i915_driver_load() and falls into the drm_global_mutex dead lock.
> 2. i915 installed before ipvr. In this case, ipvr's probe() is called
> without drm_global_mutex held by i915 and no dead lock issue.
> If we kill drm_global_mutex, will path 2 run into issue? And in your
> suggestion, how to rework the load sequence? Do you mean calling ipvr's
> load() callback directly during platform driver probe()?

Hm right it's not that simple really. What we need in more detail is:
- Move the mutex_lock(&drm_global_mutex) out of drm_dev_register into
  all the callers. If a driver has a ->load() callback it most likely is
  racy with the usual load ordering issues.

- Rework ipvr to no longer have a ->load callback. Insteaed use the
  following sequence (in the platform ->probe callback):

  drm_dev_alloc();
  ipvr_load();
  drm_dev_register();

  With that ordering we don't need the additional guarantees that
  drm_global_mutex provides and we can avoid to take that lock around
  drm_dev_registrer() call in the ipvr code.

This should resolve the deadlock I hope.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
_______________________________________________
dri-devel mailing list
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http://lists.freedesktop.org/mailman/listinfo/dri-devel

Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Cheng, Yao]

> -----Original Message-----
> From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch] On Behalf Of Daniel
> Vetter
> Sent: Monday, January 5, 2015 16:40
> To: Cheng, Yao
> Cc: Daniel Vetter; Thierry Reding; intel-gfx@lists.freedesktop.org; dri-
> devel@lists.freedesktop.org; Kelley, Sean V; Chehab, John;
> emil.l.velikov@gmail.com; Jiang, Fei; Beckett, Robert; Barbalho, Rafael
> Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support
> 
> On Sun, Dec 21, 2014 at 02:40:24PM +0000, Cheng, Yao wrote:
> > > -----Original Message-----
> > > From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch]
> > > Sent: Thursday, December 18, 2014 19:21
> > > To: Thierry Reding
> > > Cc: Cheng, Yao; intel-gfx@lists.freedesktop.org; dri-
> > > devel@lists.freedesktop.org; Kelley, Sean V; Chehab, John;
> > > emil.l.velikov@gmail.com; Jiang, Fei
> > > Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr
> > > support
> > >
> > > On Thu, Dec 18, 2014 at 11:04 AM, Thierry Reding
> > > <thierry.reding@gmail.com>
> > > wrote:
> > > >> I double checked the symptom and found it was a deadlock on
> > > drm_global_mutex.
> > > >> When i915_driver_load() registers the platform device while ipvr
> > > >> module
> > > is in the system, ipvr's probe() function tries to lock
> > > drm_global_mutex which was already held by i915.
> > > >> I think either of the following 2 actions need to be moved to a
> > > >> bottom half
> > > e.g. a work queue:
> > > >>       platform_device_add () call in i915_ved.c (called during
> > > i915_driver_load())
> > > >>       drm_dev_register() call during ipvr's probe() Which one
> > > >> makes more sense? pls kindly advise (I personally prefer the former
> one.).
> > > >
> > > > Yes, that's somewhat ugly, but I don't see a way around that. I'd
> > > > also think that moving platform_device_add() to a workqueue would
> > > > be the best option here.
> > >
> > > Or we simply kill drm_global_mutex for platform drivers that don't
> > > use the -
> > > >probe hook. It should work when they have a correct order betwen
> > > drm_dev_alloc and _register and all the code in between. So just
> > > ditch the -
> > > >load callback in teh ipvr driver and rework the load sequence as
> > > >suggested
> > > somewhere else and this is fixed already. No need for bottom halfs I
> think.
> >
> > Daniel, sorry I didn't quite understand "platform drivers that don't
> > use the probe hook". For initialization, the ipvr platform driver's
> > probe() is called in following 2 possible paths:
> > 1. ipvr installed before i915. In this case, ipvr's probe() is called
> > inside i915_driver_load() and falls into the drm_global_mutex dead lock.
> > 2. i915 installed before ipvr. In this case, ipvr's probe() is called
> > without drm_global_mutex held by i915 and no dead lock issue.
> > If we kill drm_global_mutex, will path 2 run into issue? And in your
> > suggestion, how to rework the load sequence? Do you mean calling
> > ipvr's
> > load() callback directly during platform driver probe()?
> 
> Hm right it's not that simple really. What we need in more detail is:
> - Move the mutex_lock(&drm_global_mutex) out of drm_dev_register into
>   all the callers. If a driver has a ->load() callback it most likely is
>   racy with the usual load ordering issues.
> 
> - Rework ipvr to no longer have a ->load callback. Insteaed use the
>   following sequence (in the platform ->probe callback):
> 
>   drm_dev_alloc();
>   ipvr_load();
>   drm_dev_register();
> 
>   With that ordering we don't need the additional guarantees that
>   drm_global_mutex provides and we can avoid to take that lock around
>   drm_dev_registrer() call in the ipvr code.

Thanks for the detailed explanation, Daniel!
That sounds to be a small refactor on drm core, and need change many drm drivers: nouveau, tegra, udl.
Should it be a standalone RFC patch?

> 
> This should resolve the deadlock I hope.
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
http://lists.freedesktop.org/mailman/listinfo/intel-gfx

Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support

[Daniel Vetter]

On Tue, Jan 06, 2015 at 02:14:27PM +0000, Cheng, Yao wrote:
> > -----Original Message-----
> > From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch] On Behalf Of Daniel
> > Vetter
> > Sent: Monday, January 5, 2015 16:40
> > To: Cheng, Yao
> > Cc: Daniel Vetter; Thierry Reding; intel-gfx@lists.freedesktop.org; dri-
> > devel@lists.freedesktop.org; Kelley, Sean V; Chehab, John;
> > emil.l.velikov@gmail.com; Jiang, Fei; Beckett, Robert; Barbalho, Rafael
> > Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr support
> > 
> > On Sun, Dec 21, 2014 at 02:40:24PM +0000, Cheng, Yao wrote:
> > > > -----Original Message-----
> > > > From: Daniel Vetter [mailto:daniel.vetter@ffwll.ch]
> > > > Sent: Thursday, December 18, 2014 19:21
> > > > To: Thierry Reding
> > > > Cc: Cheng, Yao; intel-gfx@lists.freedesktop.org; dri-
> > > > devel@lists.freedesktop.org; Kelley, Sean V; Chehab, John;
> > > > emil.l.velikov@gmail.com; Jiang, Fei
> > > > Subject: Re: [RFC PATCH v3 4/4] tests/drv_module_reload: add ipvr
> > > > support
> > > >
> > > > On Thu, Dec 18, 2014 at 11:04 AM, Thierry Reding
> > > > <thierry.reding@gmail.com>
> > > > wrote:
> > > > >> I double checked the symptom and found it was a deadlock on
> > > > drm_global_mutex.
> > > > >> When i915_driver_load() registers the platform device while ipvr
> > > > >> module
> > > > is in the system, ipvr's probe() function tries to lock
> > > > drm_global_mutex which was already held by i915.
> > > > >> I think either of the following 2 actions need to be moved to a
> > > > >> bottom half
> > > > e.g. a work queue:
> > > > >>       platform_device_add () call in i915_ved.c (called during
> > > > i915_driver_load())
> > > > >>       drm_dev_register() call during ipvr's probe() Which one
> > > > >> makes more sense? pls kindly advise (I personally prefer the former
> > one.).
> > > > >
> > > > > Yes, that's somewhat ugly, but I don't see a way around that. I'd
> > > > > also think that moving platform_device_add() to a workqueue would
> > > > > be the best option here.
> > > >
> > > > Or we simply kill drm_global_mutex for platform drivers that don't
> > > > use the -
> > > > >probe hook. It should work when they have a correct order betwen
> > > > drm_dev_alloc and _register and all the code in between. So just
> > > > ditch the -
> > > > >load callback in teh ipvr driver and rework the load sequence as
> > > > >suggested
> > > > somewhere else and this is fixed already. No need for bottom halfs I
> > think.
> > >
> > > Daniel, sorry I didn't quite understand "platform drivers that don't
> > > use the probe hook". For initialization, the ipvr platform driver's
> > > probe() is called in following 2 possible paths:
> > > 1. ipvr installed before i915. In this case, ipvr's probe() is called
> > > inside i915_driver_load() and falls into the drm_global_mutex dead lock.
> > > 2. i915 installed before ipvr. In this case, ipvr's probe() is called
> > > without drm_global_mutex held by i915 and no dead lock issue.
> > > If we kill drm_global_mutex, will path 2 run into issue? And in your
> > > suggestion, how to rework the load sequence? Do you mean calling
> > > ipvr's
> > > load() callback directly during platform driver probe()?
> > 
> > Hm right it's not that simple really. What we need in more detail is:
> > - Move the mutex_lock(&drm_global_mutex) out of drm_dev_register into
> >   all the callers. If a driver has a ->load() callback it most likely is
> >   racy with the usual load ordering issues.
> > 
> > - Rework ipvr to no longer have a ->load callback. Insteaed use the
> >   following sequence (in the platform ->probe callback):
> > 
> >   drm_dev_alloc();
> >   ipvr_load();
> >   drm_dev_register();
> > 
> >   With that ordering we don't need the additional guarantees that
> >   drm_global_mutex provides and we can avoid to take that lock around
> >   drm_dev_registrer() call in the ipvr code.
> 
> Thanks for the detailed explanation, Daniel!
> That sounds to be a small refactor on drm core, and need change many drm drivers: nouveau, tegra, udl.
> Should it be a standalone RFC patch?

I think the locking shuffling should be doable in just one patch, but
definitely needs to be split out.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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