Konrad Rzeszutek Wilk
2013-Dec-03 15:59 UTC
[PATCH] cpuidle: Check for dev before deregistering it.
If not, we could end up in the unfortunate situation where
we dereference a NULL pointer b/c we have cpuidle disabled.
This is the case when booting under Xen (which uses the
ACPI P/C states but disables the CPU idle driver) - and can
be easily reproduced when booting with cpuidle.off=1.
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff8156db4a>] cpuidle_unregister_device+0x2a/0x90
.. snip..
Call Trace:
[<ffffffff813b15b4>] acpi_processor_power_exit+0x3c/0x5c
[<ffffffff813af0a9>] acpi_processor_stop+0x61/0xb6
[<ffffffff814215bf>] __device_release_driver+0fffff81421653>]
device_release_driver+0x23/0x30
[<ffffffff81420ed8>] bus_remove_device+0x108/0x180
[<ffffffff8141d9d9>] device_del+0x129/0x1c0
[<ffffffff813cb4b0>] ? unregister_xenbus_watch+0x1f0/0x1f0
[<ffffffff8141da8e>] device_unregister+0x1e/0x60
[<ffffffff814243e9>] unregister_cpu+0x39/0x60
[<ffffffff81019e03>] arch_unregister_cpu+0x23/0x30
[<ffffffff813c3c51>] handle_vcpu_hotplug_event+0xc1/0xe0
[<ffffffff813cb4f5>] xenwatch_thread+0x45/0x120
[<ffffffff810af010>] ? abort_exclusive_wait+0xb0/0xb0
[<ffffffff8108ec42>] kthread+0xd2/0xf0
[<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180
[<ffffffff816ce17c>] ret_from_fork+0x7c/0xb0
[<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180
This problem also appears in 3.12 and could be a candidate for backport.
CC: "Rafael J. Wysocki" <rjw@rjwysocki.net>
CC: Daniel Lezcano <daniel.lezcano@linaro.org>
CC: linux-pm@vger.kernel.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
drivers/cpuidle/cpuidle.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 2a991e4..a55e68f 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -400,7 +400,7 @@ EXPORT_SYMBOL_GPL(cpuidle_register_device);
*/
void cpuidle_unregister_device(struct cpuidle_device *dev)
{
- if (dev->registered == 0)
+ if (!dev || dev->registered == 0)
return;
cpuidle_pause_and_lock();
--
1.8.3.1
Rafael J. Wysocki
2013-Dec-03 21:33 UTC
Re: [PATCH] cpuidle: Check for dev before deregistering it.
On Tuesday, December 03, 2013 10:59:58 AM Konrad Rzeszutek Wilk wrote:> If not, we could end up in the unfortunate situation where > we dereference a NULL pointer b/c we have cpuidle disabled. > > This is the case when booting under Xen (which uses the > ACPI P/C states but disables the CPU idle driver) - and can > be easily reproduced when booting with cpuidle.off=1. > > BUG: unable to handle kernel NULL pointer dereference at (null) > IP: [<ffffffff8156db4a>] cpuidle_unregister_device+0x2a/0x90 > .. snip.. > Call Trace: > [<ffffffff813b15b4>] acpi_processor_power_exit+0x3c/0x5c > [<ffffffff813af0a9>] acpi_processor_stop+0x61/0xb6 > [<ffffffff814215bf>] __device_release_driver+0fffff81421653>] device_release_driver+0x23/0x30 > [<ffffffff81420ed8>] bus_remove_device+0x108/0x180 > [<ffffffff8141d9d9>] device_del+0x129/0x1c0 > [<ffffffff813cb4b0>] ? unregister_xenbus_watch+0x1f0/0x1f0 > [<ffffffff8141da8e>] device_unregister+0x1e/0x60 > [<ffffffff814243e9>] unregister_cpu+0x39/0x60 > [<ffffffff81019e03>] arch_unregister_cpu+0x23/0x30 > [<ffffffff813c3c51>] handle_vcpu_hotplug_event+0xc1/0xe0 > [<ffffffff813cb4f5>] xenwatch_thread+0x45/0x120 > [<ffffffff810af010>] ? abort_exclusive_wait+0xb0/0xb0 > [<ffffffff8108ec42>] kthread+0xd2/0xf0 > [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 > [<ffffffff816ce17c>] ret_from_fork+0x7c/0xb0 > [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 > > This problem also appears in 3.12 and could be a candidate for backport. > > CC: "Rafael J. Wysocki" <rjw@rjwysocki.net> > CC: Daniel Lezcano <daniel.lezcano@linaro.org> > CC: linux-pm@vger.kernel.org > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>Applied, thanks!> --- > drivers/cpuidle/cpuidle.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c > index 2a991e4..a55e68f 100644 > --- a/drivers/cpuidle/cpuidle.c > +++ b/drivers/cpuidle/cpuidle.c > @@ -400,7 +400,7 @@ EXPORT_SYMBOL_GPL(cpuidle_register_device); > */ > void cpuidle_unregister_device(struct cpuidle_device *dev) > { > - if (dev->registered == 0) > + if (!dev || dev->registered == 0) > return; > > cpuidle_pause_and_lock(); >-- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center.
Daniel Lezcano
2013-Dec-04 09:10 UTC
Re: [PATCH] cpuidle: Check for dev before deregistering it.
On 12/03/2013 10:33 PM, Rafael J. Wysocki wrote:> On Tuesday, December 03, 2013 10:59:58 AM Konrad Rzeszutek Wilk wrote: >> If not, we could end up in the unfortunate situation where >> we dereference a NULL pointer b/c we have cpuidle disabled. >> >> This is the case when booting under Xen (which uses the >> ACPI P/C states but disables the CPU idle driver) - and can >> be easily reproduced when booting with cpuidle.off=1. >> >> BUG: unable to handle kernel NULL pointer dereference at (null) >> IP: [<ffffffff8156db4a>] cpuidle_unregister_device+0x2a/0x90 >> .. snip.. >> Call Trace: >> [<ffffffff813b15b4>] acpi_processor_power_exit+0x3c/0x5c >> [<ffffffff813af0a9>] acpi_processor_stop+0x61/0xb6 >> [<ffffffff814215bf>] __device_release_driver+0fffff81421653>] device_release_driver+0x23/0x30 >> [<ffffffff81420ed8>] bus_remove_device+0x108/0x180 >> [<ffffffff8141d9d9>] device_del+0x129/0x1c0 >> [<ffffffff813cb4b0>] ? unregister_xenbus_watch+0x1f0/0x1f0 >> [<ffffffff8141da8e>] device_unregister+0x1e/0x60 >> [<ffffffff814243e9>] unregister_cpu+0x39/0x60 >> [<ffffffff81019e03>] arch_unregister_cpu+0x23/0x30 >> [<ffffffff813c3c51>] handle_vcpu_hotplug_event+0xc1/0xe0 >> [<ffffffff813cb4f5>] xenwatch_thread+0x45/0x120 >> [<ffffffff810af010>] ? abort_exclusive_wait+0xb0/0xb0 >> [<ffffffff8108ec42>] kthread+0xd2/0xf0 >> [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 >> [<ffffffff816ce17c>] ret_from_fork+0x7c/0xb0 >> [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 >> >> This problem also appears in 3.12 and could be a candidate for backport. >> >> CC: "Rafael J. Wysocki" <rjw@rjwysocki.net> >> CC: Daniel Lezcano <daniel.lezcano@linaro.org> >> CC: linux-pm@vger.kernel.org >> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> > > Applied, thanks! > >> --- >> drivers/cpuidle/cpuidle.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c >> index 2a991e4..a55e68f 100644 >> --- a/drivers/cpuidle/cpuidle.c >> +++ b/drivers/cpuidle/cpuidle.c >> @@ -400,7 +400,7 @@ EXPORT_SYMBOL_GPL(cpuidle_register_device); >> */ >> void cpuidle_unregister_device(struct cpuidle_device *dev) >> { >> - if (dev->registered == 0) >> + if (!dev || dev->registered == 0) >> return; >> >> cpuidle_pause_and_lock();Oops, wait. Are we sure the problem is coming from cpuidle ? The cpuidle_unregister_device is called with a NULL pointer, that shouldn't happen. Konrad, you say that could be easily reproduced. How do you produce it ? Unplugging a cpu ? -- <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
Konrad Rzeszutek Wilk
2013-Dec-04 16:09 UTC
Re: [PATCH] cpuidle: Check for dev before deregistering it.
On Wed, Dec 04, 2013 at 10:10:28AM +0100, Daniel Lezcano wrote:> On 12/03/2013 10:33 PM, Rafael J. Wysocki wrote: > >On Tuesday, December 03, 2013 10:59:58 AM Konrad Rzeszutek Wilk wrote: > >>If not, we could end up in the unfortunate situation where > >>we dereference a NULL pointer b/c we have cpuidle disabled. > >> > >>This is the case when booting under Xen (which uses the > >>ACPI P/C states but disables the CPU idle driver) - and can > >>be easily reproduced when booting with cpuidle.off=1. > >> > >>BUG: unable to handle kernel NULL pointer dereference at (null) > >>IP: [<ffffffff8156db4a>] cpuidle_unregister_device+0x2a/0x90 > >>.. snip.. > >>Call Trace: > >> [<ffffffff813b15b4>] acpi_processor_power_exit+0x3c/0x5c > >> [<ffffffff813af0a9>] acpi_processor_stop+0x61/0xb6 > >> [<ffffffff814215bf>] __device_release_driver+0fffff81421653>] device_release_driver+0x23/0x30 > >> [<ffffffff81420ed8>] bus_remove_device+0x108/0x180 > >> [<ffffffff8141d9d9>] device_del+0x129/0x1c0 > >> [<ffffffff813cb4b0>] ? unregister_xenbus_watch+0x1f0/0x1f0 > >> [<ffffffff8141da8e>] device_unregister+0x1e/0x60 > >> [<ffffffff814243e9>] unregister_cpu+0x39/0x60 > >> [<ffffffff81019e03>] arch_unregister_cpu+0x23/0x30 > >> [<ffffffff813c3c51>] handle_vcpu_hotplug_event+0xc1/0xe0 > >> [<ffffffff813cb4f5>] xenwatch_thread+0x45/0x120 > >> [<ffffffff810af010>] ? abort_exclusive_wait+0xb0/0xb0 > >> [<ffffffff8108ec42>] kthread+0xd2/0xf0 > >> [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 > >> [<ffffffff816ce17c>] ret_from_fork+0x7c/0xb0 > >> [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 > >> > >>This problem also appears in 3.12 and could be a candidate for backport. > >> > >>CC: "Rafael J. Wysocki" <rjw@rjwysocki.net> > >>CC: Daniel Lezcano <daniel.lezcano@linaro.org> > >>CC: linux-pm@vger.kernel.org > >>Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> > > > >Applied, thanks! > > > >>--- > >> drivers/cpuidle/cpuidle.c | 2 +- > >> 1 file changed, 1 insertion(+), 1 deletion(-) > >> > >>diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c > >>index 2a991e4..a55e68f 100644 > >>--- a/drivers/cpuidle/cpuidle.c > >>+++ b/drivers/cpuidle/cpuidle.c > >>@@ -400,7 +400,7 @@ EXPORT_SYMBOL_GPL(cpuidle_register_device); > >> */ > >> void cpuidle_unregister_device(struct cpuidle_device *dev) > >> { > >>- if (dev->registered == 0) > >>+ if (!dev || dev->registered == 0) > >> return; > >> > >> cpuidle_pause_and_lock(); > > Oops, wait. Are we sure the problem is coming from cpuidle ?It is acpi_processor_power_exit assuming that the cpuidle is initialized. It could be fixed there too, but there are multiple entries in cpuidle where it does the : "if (!dev) return .." so I figured this should be done as well here.> > The cpuidle_unregister_device is called with a NULL pointer, that > shouldn''t happen.It does :-)> > Konrad, you say that could be easily reproduced. How do you produce > it ? Unplugging a cpu ?Yes.
Daniel Lezcano
2013-Dec-04 21:24 UTC
Re: [PATCH] cpuidle: Check for dev before deregistering it.
On 12/04/2013 05:09 PM, Konrad Rzeszutek Wilk wrote:> On Wed, Dec 04, 2013 at 10:10:28AM +0100, Daniel Lezcano wrote: >> On 12/03/2013 10:33 PM, Rafael J. Wysocki wrote: >>> On Tuesday, December 03, 2013 10:59:58 AM Konrad Rzeszutek Wilk wrote: >>>> If not, we could end up in the unfortunate situation where >>>> we dereference a NULL pointer b/c we have cpuidle disabled. >>>> >>>> This is the case when booting under Xen (which uses the >>>> ACPI P/C states but disables the CPU idle driver) - and can >>>> be easily reproduced when booting with cpuidle.off=1. >>>> >>>> BUG: unable to handle kernel NULL pointer dereference at (null) >>>> IP: [<ffffffff8156db4a>] cpuidle_unregister_device+0x2a/0x90 >>>> .. snip.. >>>> Call Trace: >>>> [<ffffffff813b15b4>] acpi_processor_power_exit+0x3c/0x5c >>>> [<ffffffff813af0a9>] acpi_processor_stop+0x61/0xb6 >>>> [<ffffffff814215bf>] __device_release_driver+0fffff81421653>] device_release_driver+0x23/0x30 >>>> [<ffffffff81420ed8>] bus_remove_device+0x108/0x180 >>>> [<ffffffff8141d9d9>] device_del+0x129/0x1c0 >>>> [<ffffffff813cb4b0>] ? unregister_xenbus_watch+0x1f0/0x1f0 >>>> [<ffffffff8141da8e>] device_unregister+0x1e/0x60 >>>> [<ffffffff814243e9>] unregister_cpu+0x39/0x60 >>>> [<ffffffff81019e03>] arch_unregister_cpu+0x23/0x30 >>>> [<ffffffff813c3c51>] handle_vcpu_hotplug_event+0xc1/0xe0 >>>> [<ffffffff813cb4f5>] xenwatch_thread+0x45/0x120 >>>> [<ffffffff810af010>] ? abort_exclusive_wait+0xb0/0xb0 >>>> [<ffffffff8108ec42>] kthread+0xd2/0xf0 >>>> [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 >>>> [<ffffffff816ce17c>] ret_from_fork+0x7c/0xb0 >>>> [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 >>>> >>>> This problem also appears in 3.12 and could be a candidate for backport. >>>> >>>> CC: "Rafael J. Wysocki" <rjw@rjwysocki.net> >>>> CC: Daniel Lezcano <daniel.lezcano@linaro.org> >>>> CC: linux-pm@vger.kernel.org >>>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> >>> >>> Applied, thanks! >>> >>>> --- >>>> drivers/cpuidle/cpuidle.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c >>>> index 2a991e4..a55e68f 100644 >>>> --- a/drivers/cpuidle/cpuidle.c >>>> +++ b/drivers/cpuidle/cpuidle.c >>>> @@ -400,7 +400,7 @@ EXPORT_SYMBOL_GPL(cpuidle_register_device); >>>> */ >>>> void cpuidle_unregister_device(struct cpuidle_device *dev) >>>> { >>>> - if (dev->registered == 0) >>>> + if (!dev || dev->registered == 0) >>>> return; >>>> >>>> cpuidle_pause_and_lock(); >> >> Oops, wait. Are we sure the problem is coming from cpuidle ? > > It is acpi_processor_power_exit assuming that the cpuidle is > initialized. It could be fixed there too, but there are multiple > entries in cpuidle where it does the : "if (!dev) return .." > so I figured this should be done as well here.I understand. From my POV the bug is coming from the acpi processor idle driver. The function acpi_processor_power_init registers the cpuidle driver and the cpuidle device when acpi_processor_registered is zero. Then it increments acpi_processor_registered preventing the next call to this function to register the driver but it will register the device. As cpuidle is disabled, the cpuidle_register_driver fails, thus the device is not registered and acpi_processor_registered is not incremented. So all calls to acpi_processor_power_init prevents the driver and the device to be registered. No problem with that. But the function acpi_processor_power_exit does not take care of the value of acpi_processor_registered and just unregister the device. Then it decrements acpi_processor_registered which is zero to -1. Trying to be immune from a NULL pointer in cpuidle_unregister_device hides bogus code from the caller. So IMO, this check shouldn't be there and the acpi_processor_power_exit function should be fixed instead.>> The cpuidle_unregister_device is called with a NULL pointer, that >> shouldn't happen. > > It does :-) >> >> Konrad, you say that could be easily reproduced. How do you produce >> it ? Unplugging a cpu ? > > Yes. >-- <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
Rafael J. Wysocki
2013-Dec-04 22:46 UTC
Re: [PATCH] cpuidle: Check for dev before deregistering it.
On Wednesday, December 04, 2013 10:24:25 PM Daniel Lezcano wrote:> On 12/04/2013 05:09 PM, Konrad Rzeszutek Wilk wrote: > > On Wed, Dec 04, 2013 at 10:10:28AM +0100, Daniel Lezcano wrote: > >> On 12/03/2013 10:33 PM, Rafael J. Wysocki wrote: > >>> On Tuesday, December 03, 2013 10:59:58 AM Konrad Rzeszutek Wilk wrote: > >>>> If not, we could end up in the unfortunate situation where > >>>> we dereference a NULL pointer b/c we have cpuidle disabled. > >>>> > >>>> This is the case when booting under Xen (which uses the > >>>> ACPI P/C states but disables the CPU idle driver) - and can > >>>> be easily reproduced when booting with cpuidle.off=1. > >>>> > >>>> BUG: unable to handle kernel NULL pointer dereference at (null) > >>>> IP: [<ffffffff8156db4a>] cpuidle_unregister_device+0x2a/0x90 > >>>> .. snip.. > >>>> Call Trace: > >>>> [<ffffffff813b15b4>] acpi_processor_power_exit+0x3c/0x5c > >>>> [<ffffffff813af0a9>] acpi_processor_stop+0x61/0xb6 > >>>> [<ffffffff814215bf>] __device_release_driver+0fffff81421653>] device_release_driver+0x23/0x30 > >>>> [<ffffffff81420ed8>] bus_remove_device+0x108/0x180 > >>>> [<ffffffff8141d9d9>] device_del+0x129/0x1c0 > >>>> [<ffffffff813cb4b0>] ? unregister_xenbus_watch+0x1f0/0x1f0 > >>>> [<ffffffff8141da8e>] device_unregister+0x1e/0x60 > >>>> [<ffffffff814243e9>] unregister_cpu+0x39/0x60 > >>>> [<ffffffff81019e03>] arch_unregister_cpu+0x23/0x30 > >>>> [<ffffffff813c3c51>] handle_vcpu_hotplug_event+0xc1/0xe0 > >>>> [<ffffffff813cb4f5>] xenwatch_thread+0x45/0x120 > >>>> [<ffffffff810af010>] ? abort_exclusive_wait+0xb0/0xb0 > >>>> [<ffffffff8108ec42>] kthread+0xd2/0xf0 > >>>> [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 > >>>> [<ffffffff816ce17c>] ret_from_fork+0x7c/0xb0 > >>>> [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180 > >>>> > >>>> This problem also appears in 3.12 and could be a candidate for backport. > >>>> > >>>> CC: "Rafael J. Wysocki" <rjw@rjwysocki.net> > >>>> CC: Daniel Lezcano <daniel.lezcano@linaro.org> > >>>> CC: linux-pm@vger.kernel.org > >>>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> > >>> > >>> Applied, thanks! > >>> > >>>> --- > >>>> drivers/cpuidle/cpuidle.c | 2 +- > >>>> 1 file changed, 1 insertion(+), 1 deletion(-) > >>>> > >>>> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c > >>>> index 2a991e4..a55e68f 100644 > >>>> --- a/drivers/cpuidle/cpuidle.c > >>>> +++ b/drivers/cpuidle/cpuidle.c > >>>> @@ -400,7 +400,7 @@ EXPORT_SYMBOL_GPL(cpuidle_register_device); > >>>> */ > >>>> void cpuidle_unregister_device(struct cpuidle_device *dev) > >>>> { > >>>> - if (dev->registered == 0) > >>>> + if (!dev || dev->registered == 0) > >>>> return; > >>>> > >>>> cpuidle_pause_and_lock(); > >> > >> Oops, wait. Are we sure the problem is coming from cpuidle ? > > > > It is acpi_processor_power_exit assuming that the cpuidle is > > initialized. It could be fixed there too, but there are multiple > > entries in cpuidle where it does the : "if (!dev) return .." > > so I figured this should be done as well here. > > I understand. > > From my POV the bug is coming from the acpi processor idle driver. > > The function acpi_processor_power_init registers the cpuidle driver and > the cpuidle device when acpi_processor_registered is zero. Then it > increments acpi_processor_registered preventing the next call to this > function to register the driver but it will register the device. > > As cpuidle is disabled, the cpuidle_register_driver fails, thus the > device is not registered and acpi_processor_registered is not > incremented. So all calls to acpi_processor_power_init prevents the > driver and the device to be registered. No problem with that. > > But the function acpi_processor_power_exit does not take care of the > value of acpi_processor_registered and just unregister the device. Then > it decrements acpi_processor_registered which is zero to -1. > > Trying to be immune from a NULL pointer in cpuidle_unregister_device > hides bogus code from the caller. So IMO, this check shouldn''t be there > and the acpi_processor_power_exit function should be fixed instead.I''m not sure I understand the "hides bogus code from the caller" phrase. It prevents the kernel from crashing, which may be due to a driver bug, but still. We can add a WARN_ON() around the check to indicate suspiciousness, but otherwise I don''t have problems with preventing kernel crashes from happening. Thanks, Rafael