[PATCH] kthread/smpboot: Serialize kthread parking against wakeup

[PATCH] kthread/smpboot: Serialize kthread parking against wakeup

[Gaurav Kohli]

The control cpu thread which initiates hotplug calls kthread_park()
for hotplug thread and sets KTHREAD_SHOULD_PARK. After this control
thread wakes up the hotplug thread. There is a chance that wakeup
code sees the hotplug thread (running on AP core) in INTERRUPTIBLE
state, but sets its state to RUNNING after hotplug thread has entered
kthread_parkme() and changed its state to TASK_PARKED. This can result
in panic later on in kthread_unpark(), as it sees KTHREAD_IS_PARKED
flag set but fails to rebind the kthread, due to it being not in
TASK_PARKED state. Fix this, by serializing wakeup state change,
against state change before parking the kthread.

Below is the possible race:

Control thread				      Hotplug Thread

kthread_park()
set KTHREAD_SHOULD_PARK
					      smpboot_thread_fn
					      set_current_state(TASK_INTERRUPTIBLE);
					      kthread_parkme

wake_up_process()

raw_spin_lock_irqsave(&p->pi_lock, flags);
if (!(p->state & state)) -> this will fail
            goto out;

					      __kthread_parkme
					       __set_current_state(TASK_PARKED);

if (p->on_rq && ttwu_remote(p, wake_flags))
    ttwu_remote()
        p->state = TASK_RUNNING;
						schedule();

So to avoid this race, take pi_lock to serial state changes.

Suggested-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Co-developed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 1650578..514b232 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -121,7 +121,9 @@ static int smpboot_thread_fn(void *data)
 		}
 
 		if (kthread_should_park()) {
+			raw_spin_lock(&current->pi_lock);
 			__set_current_state(TASK_RUNNING);
+			raw_spin_unlock(&current->pi_lock);
 			preempt_enable();
 			if (ht->park && td->status == HP_THREAD_ACTIVE) {
 				BUG_ON(td->cpu != smp_processor_id());
-- 
1.9.1

Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

[Kohli, Gaurav]

Hi ,

We can also fix below race by smpboot code as well:

@@ -109,7 +109,6 @@ static int smpboot_thread_fn(void *data)

         struct smp_hotplug_thread *ht = td->ht;

         while (1) {

-               set_current_state(TASK_INTERRUPTIBLE);

                 preempt_disable();

                 if (kthread_should_stop()) {

                         __set_current_state(TASK_RUNNING);

@@ -157,6 +156,7 @@ static int smpboot_thread_fn(void *data)

                 if (!ht->thread_should_run(td->cpu)) {

                         preempt_enable_no_resched();

+                       set_current_state(TASK_INTERRUPTIBLE);

                         schedule();

                 } else {

                         __set_current_state(TASK_RUNNING);

Please suggest if this approach is better.

Regards

Gaurav

On 4/24/2018 2:58 PM, Gaurav Kohli wrote:

> The control cpu thread which initiates hotplug calls kthread_park()
> for hotplug thread and sets KTHREAD_SHOULD_PARK. After this control
> thread wakes up the hotplug thread. There is a chance that wakeup
> code sees the hotplug thread (running on AP core) in INTERRUPTIBLE
> state, but sets its state to RUNNING after hotplug thread has entered
> kthread_parkme() and changed its state to TASK_PARKED. This can result
> in panic later on in kthread_unpark(), as it sees KTHREAD_IS_PARKED
> flag set but fails to rebind the kthread, due to it being not in
> TASK_PARKED state. Fix this, by serializing wakeup state change,
> against state change before parking the kthread.
>
> Below is the possible race:
>
> Control thread				      Hotplug Thread
>
> kthread_park()
> set KTHREAD_SHOULD_PARK
> 					      smpboot_thread_fn
> 					      set_current_state(TASK_INTERRUPTIBLE);
> 					      kthread_parkme
>
> wake_up_process()
>
> raw_spin_lock_irqsave(&p->pi_lock, flags);
> if (!(p->state & state)) -> this will fail
>              goto out;
>
> 					      __kthread_parkme
> 					       __set_current_state(TASK_PARKED);
>
> if (p->on_rq && ttwu_remote(p, wake_flags))
>      ttwu_remote()
>          p->state = TASK_RUNNING;
> 						schedule();
>
> So to avoid this race, take pi_lock to serial state changes.
>
> Suggested-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
> Co-developed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>
>
> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> index 1650578..514b232 100644
> --- a/kernel/smpboot.c
> +++ b/kernel/smpboot.c
> @@ -121,7 +121,9 @@ static int smpboot_thread_fn(void *data)
>   		}
>   
>   		if (kthread_should_park()) {
> +			raw_spin_lock(&current->pi_lock);
>   			__set_current_state(TASK_RUNNING);
> +			raw_spin_unlock(&current->pi_lock);
>   			preempt_enable();
>   			if (ht->park && td->status == HP_THREAD_ACTIVE) {
>   				BUG_ON(td->cpu != smp_processor_id());
>
-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.

Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

[Peter Zijlstra]

On Tue, Apr 24, 2018 at 08:12:49PM +0530, Kohli, Gaurav wrote:
> @@ -157,6 +156,7 @@ static int smpboot_thread_fn(void *data)
> 
>                 if (!ht->thread_should_run(td->cpu)) {
> 
>                         preempt_enable_no_resched();
> 
> +                       set_current_state(TASK_INTERRUPTIBLE);
> 
>                         schedule();
> 
>                 } else {
> 
>                         __set_current_state(TASK_RUNNING);
> 
> Please suggest if this approach is better.

Bah, my brain isn't working... see below for the 'correct' version of
your second patch.

But this violates the normal pattern; see the comment near
set_current_state(). That pattern ensures the thread either sees the
wakeup condition or the actual wakeup.

I'm thinking that with this patch there is a scenario where we'll miss
both the kthread_should_park() and the actual wakeup and end up not
doing anything.

I do the like the end result, but I suspect it's buggy.


---
 kernel/smpboot.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 5043e7433f4b..5bdf57f2ce68 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -109,10 +109,8 @@ static int smpboot_thread_fn(void *data)
 	struct smp_hotplug_thread *ht = td->ht;
 
 	while (1) {
-		set_current_state(TASK_INTERRUPTIBLE);
 		preempt_disable();
 		if (kthread_should_stop()) {
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			/* cleanup must mirror setup */
 			if (ht->cleanup && td->status != HP_THREAD_NONE)
@@ -122,7 +120,6 @@ static int smpboot_thread_fn(void *data)
 		}
 
 		if (kthread_should_park()) {
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			if (ht->park && td->status == HP_THREAD_ACTIVE) {
 				BUG_ON(td->cpu != smp_processor_id());
@@ -139,7 +136,6 @@ static int smpboot_thread_fn(void *data)
 		/* Check for state change setup */
 		switch (td->status) {
 		case HP_THREAD_NONE:
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			if (ht->setup)
 				ht->setup(td->cpu);
@@ -147,7 +143,6 @@ static int smpboot_thread_fn(void *data)
 			continue;
 
 		case HP_THREAD_PARKED:
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			if (ht->unpark)
 				ht->unpark(td->cpu);
@@ -156,10 +151,10 @@ static int smpboot_thread_fn(void *data)
 		}
 
 		if (!ht->thread_should_run(td->cpu)) {
+			set_current_state(TASK_IDLE);
 			preempt_enable_no_resched();
 			schedule();
 		} else {
-			__set_current_state(TASK_RUNNING);
 			preempt_enable();
 			ht->thread_fn(td->cpu);
 		}

Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

[Peter Zijlstra]

On Tue, Apr 24, 2018 at 02:58:25PM +0530, Gaurav Kohli wrote:
> The control cpu thread which initiates hotplug calls kthread_park()
> for hotplug thread and sets KTHREAD_SHOULD_PARK. After this control
> thread wakes up the hotplug thread. There is a chance that wakeup
> code sees the hotplug thread (running on AP core) in INTERRUPTIBLE
> state, but sets its state to RUNNING after hotplug thread has entered
> kthread_parkme() and changed its state to TASK_PARKED. This can result
> in panic later on in kthread_unpark(), as it sees KTHREAD_IS_PARKED
> flag set but fails to rebind the kthread, due to it being not in
> TASK_PARKED state. Fix this, by serializing wakeup state change,
> against state change before parking the kthread.
> 
> Below is the possible race:
> 
> Control thread				      Hotplug Thread
> 
> kthread_park()
> set KTHREAD_SHOULD_PARK
> 					      smpboot_thread_fn
> 					      set_current_state(TASK_INTERRUPTIBLE);
> 					      kthread_parkme
> 
> wake_up_process()
> 
> raw_spin_lock_irqsave(&p->pi_lock, flags);
> if (!(p->state & state)) -> this will fail
>             goto out;
> 
> 					      __kthread_parkme
> 					       __set_current_state(TASK_PARKED);
> 
> if (p->on_rq && ttwu_remote(p, wake_flags))
>     ttwu_remote()
>         p->state = TASK_RUNNING;
> 						schedule();
> 
> So to avoid this race, take pi_lock to serial state changes.
> 
> Suggested-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
> Co-developed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>
> 
> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> index 1650578..514b232 100644
> --- a/kernel/smpboot.c
> +++ b/kernel/smpboot.c
> @@ -121,7 +121,9 @@ static int smpboot_thread_fn(void *data)
>  		}
>  
>  		if (kthread_should_park()) {
> +			raw_spin_lock(&current->pi_lock);
>  			__set_current_state(TASK_RUNNING);
> +			raw_spin_unlock(&current->pi_lock);
>  			preempt_enable();
>  			if (ht->park && td->status == HP_THREAD_ACTIVE) {
>  				BUG_ON(td->cpu != smp_processor_id());

Note how in your scenario above you didn't actually need the
TASK_RUNNING state; so how is this change going to fix anything?

But yes, I suspect it is right, but it definitely needs a comment
explaining wth we take that lock there.

Like I said earlier, my brain is entirely fried for the day; but I'll
have a try tomorrow.

Re: [PATCH] kthread/smpboot: Serialize kthread parking against wakeup

[Kohli, Gaurav]

On 4/24/2018 11:56 PM, Peter Zijlstra wrote:

> On Tue, Apr 24, 2018 at 02:58:25PM +0530, Gaurav Kohli wrote:
>> The control cpu thread which initiates hotplug calls kthread_park()
>> for hotplug thread and sets KTHREAD_SHOULD_PARK. After this control
>> thread wakes up the hotplug thread. There is a chance that wakeup
>> code sees the hotplug thread (running on AP core) in INTERRUPTIBLE
>> state, but sets its state to RUNNING after hotplug thread has entered
>> kthread_parkme() and changed its state to TASK_PARKED. This can result
>> in panic later on in kthread_unpark(), as it sees KTHREAD_IS_PARKED
>> flag set but fails to rebind the kthread, due to it being not in
>> TASK_PARKED state. Fix this, by serializing wakeup state change,
>> against state change before parking the kthread.
>>
>> Below is the possible race:
>>
>> Control thread				      Hotplug Thread
>>
>> kthread_park()
>> set KTHREAD_SHOULD_PARK
>> 					      smpboot_thread_fn
>> 					      set_current_state(TASK_INTERRUPTIBLE);
>> 					      kthread_parkme
>>
>> wake_up_process()
>>
>> raw_spin_lock_irqsave(&p->pi_lock, flags);
>> if (!(p->state & state)) -> this will fail
>>              goto out;
>>
>> 					      __kthread_parkme
>> 					       __set_current_state(TASK_PARKED);
>>
>> if (p->on_rq && ttwu_remote(p, wake_flags))
>>      ttwu_remote()
>>          p->state = TASK_RUNNING;
>> 						schedule();
>>
>> So to avoid this race, take pi_lock to serial state changes.
>>
>> Suggested-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
>> Co-developed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
>> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
>> Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>
>>
>> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
>> index 1650578..514b232 100644
>> --- a/kernel/smpboot.c
>> +++ b/kernel/smpboot.c
>> @@ -121,7 +121,9 @@ static int smpboot_thread_fn(void *data)
>>   		}
>>   
>>   		if (kthread_should_park()) {
>> +			raw_spin_lock(&current->pi_lock);
>>   			__set_current_state(TASK_RUNNING);
>> +			raw_spin_unlock(&current->pi_lock);
>>   			preempt_enable();
>>   			if (ht->park && td->status == HP_THREAD_ACTIVE) {
>>   				BUG_ON(td->cpu != smp_processor_id());
> Note how in your scenario above you didn't actually need the
> TASK_RUNNING state; so how is this change going to fix anything?

Hi Peter,

As with help of this , if kthread_should_park run first so wake_up call of controller

get exited as task is already set as running, otherwise if controller runs first

then we will block here and set running and then sets TASK_PARKED .

So no chance of cpuhp set as running during  kthread_parkme call.

But as we discussed this can be fix by 2nd patch as well, So once you get time and able to

see , Please let us know or do you want me to try your 2nd patch

for testing first?

>
> But yes, I suspect it is right, but it definitely needs a comment
> explaining wth we take that lock there.
>
> Like I said earlier, my brain is entirely fried for the day; but I'll
> have a try tomorrow.
>
-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.