When booting a debug PREEMPT_RT kernel on an arm64 system with grace
processor, a "inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage"
lockdep warning message was reported to the console.

During early boot, interrupts are getting enabled before the scheduler
is enabled. In this window (before SYSTEM_SCHEDULING is set) interrupts
can fire and attempt to fill the pool from within the hardirq. This can
lead to a deadlock the interrupt occurred while in the memory allocator.

Add a new can_fill_pool() helper and reorder the exception rule and
forbid this scenario by excluding allocations from hardirq.

Fixes: 06e0ae988f6e ("debugobjects: Allow to refill the pool before SYSTEM_SCHEDULING")
Co-developed-by: Waiman Long <longman@redhat.com>
Co-developed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 lib/debugobjects.c | 46 +++++++++++++++++++++++++++++++++++++---------
 1 file changed, 37 insertions(+), 9 deletions(-)

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index b18a682fe3da..6fb00e08a4e2 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -720,6 +720,41 @@ static inline bool debug_objects_is_pi_blocked_on(void)
 #endif
 }
 
+static inline bool can_fill_pool(void)
+{
+	/*
+	 * On !RT enabled kernels there are no restrictions and spinlock_t and
+	 * raw_spinlock_t are the same types.
+	 */
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		return true;
+
+	/*
+	 * On RT enabled kernels, the task must not be blocked on a lock as
+	 * that could corrupt the PI state when blocking on a lock in the
+	 * allocation path.
+	 */
+	if (debug_objects_is_pi_blocked_on())
+		return false;
+
+	/*
+	 * On RT enabled kernels the pool refill should happen in preemptible
+	 * context.
+	 */
+	if (preemptible())
+		return true;
+
+	/*
+	 * Though during system boot before scheduling is set up, preemption is
+	 * disabled and the pool can get exhausted. Before scheduling is active
+	 * a task cannot be blocked on a sleeping lock, but it might hold a lock
+	 * and if interrupted then hard interrupt context might run into a lock
+	 * inversion. So exclude hard interrupt context from allocations before
+	 * scheduling is active.
+	 */
+	return system_state < SYSTEM_SCHEDULING && !in_hardirq();
+}
+
 static void debug_objects_fill_pool(void)
 {
 	if (!static_branch_likely(&obj_cache_enabled))
@@ -734,18 +769,11 @@ static void debug_objects_fill_pool(void)
 	if (likely(!pool_should_refill(&pool_global)))
 		return;
 
-	/*
-	 * On RT enabled kernels the pool refill must happen in preemptible
-	 * context and not enqueued on an rt_mutex -- for !RT kernels we rely
-	 * on the fact that spinlock_t and raw_spinlock_t are basically the
-	 * same type and this lock-type inversion works just fine.
-	 */
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || system_state < SYSTEM_SCHEDULING ||
-	    (preemptible() && !debug_objects_is_pi_blocked_on())) {
+	if (can_fill_pool()) {
 		/*
 		 * Annotate away the spinlock_t inside raw_spinlock_t warning
 		 * by temporarily raising the wait-type to LD_WAIT_CONFIG, matching
-		 * the preemptible() condition above.
+		 * the preemptible() condition in can_fill_pool().
 		 */
 		static DEFINE_WAIT_OVERRIDE_MAP(fill_pool_map, LD_WAIT_CONFIG);
 		lock_map_acquire_try(&fill_pool_map);
-- 
2.54.0

* Waiman Long <longman@redhat.com> wrote:

> When booting a debug PREEMPT_RT kernel on an arm64 system with grace
> processor, a "inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage"
> lockdep warning message was reported to the console.
> 
> During early boot, interrupts are getting enabled before the scheduler
> is enabled. In this window (before SYSTEM_SCHEDULING is set) interrupts
> can fire and attempt to fill the pool from within the hardirq. This can
> lead to a deadlock the interrupt occurred while in the memory allocator.
> 
> Add a new can_fill_pool() helper and reorder the exception rule and
> forbid this scenario by excluding allocations from hardirq.
> 
> Fixes: 06e0ae988f6e ("debugobjects: Allow to refill the pool before SYSTEM_SCHEDULING")
> Co-developed-by: Waiman Long <longman@redhat.com>
> Co-developed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  lib/debugobjects.c | 46 +++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 37 insertions(+), 9 deletions(-)
> 
> diff --git a/lib/debugobjects.c b/lib/debugobjects.c
> index b18a682fe3da..6fb00e08a4e2 100644
> --- a/lib/debugobjects.c
> +++ b/lib/debugobjects.c
> @@ -720,6 +720,41 @@ static inline bool debug_objects_is_pi_blocked_on(void)
>  #endif
>  }
>  
> +static inline bool can_fill_pool(void)
> +{
> +	/*
> +	 * On !RT enabled kernels there are no restrictions and spinlock_t and
> +	 * raw_spinlock_t are the same types.
> +	 */
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		return true;

> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || system_state < SYSTEM_SCHEDULING ||
> -	    (preemptible() && !debug_objects_is_pi_blocked_on())) {
> +	if (can_fill_pool()) {

Would have been nice to have this as two patches: first one
factors out the check into can_fill_pool() without changing the
logic, while the second one applies the fix to can_fill_pool().

Thanks,

	Ingo

On 6/7/26 1:15 AM, Ingo Molnar wrote:
> * Waiman Long <longman@redhat.com> wrote:
>
>> When booting a debug PREEMPT_RT kernel on an arm64 system with grace
>> processor, a "inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage"
>> lockdep warning message was reported to the console.
>>
>> During early boot, interrupts are getting enabled before the scheduler
>> is enabled. In this window (before SYSTEM_SCHEDULING is set) interrupts
>> can fire and attempt to fill the pool from within the hardirq. This can
>> lead to a deadlock the interrupt occurred while in the memory allocator.
>>
>> Add a new can_fill_pool() helper and reorder the exception rule and
>> forbid this scenario by excluding allocations from hardirq.
>>
>> Fixes: 06e0ae988f6e ("debugobjects: Allow to refill the pool before SYSTEM_SCHEDULING")
>> Co-developed-by: Waiman Long <longman@redhat.com>
>> Co-developed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
>> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   lib/debugobjects.c | 46 +++++++++++++++++++++++++++++++++++++---------
>>   1 file changed, 37 insertions(+), 9 deletions(-)
>>
>> diff --git a/lib/debugobjects.c b/lib/debugobjects.c
>> index b18a682fe3da..6fb00e08a4e2 100644
>> --- a/lib/debugobjects.c
>> +++ b/lib/debugobjects.c
>> @@ -720,6 +720,41 @@ static inline bool debug_objects_is_pi_blocked_on(void)
>>   #endif
>>   }
>>   
>> +static inline bool can_fill_pool(void)
>> +{
>> +	/*
>> +	 * On !RT enabled kernels there are no restrictions and spinlock_t and
>> +	 * raw_spinlock_t are the same types.
>> +	 */
>> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
>> +		return true;
>> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || system_state < SYSTEM_SCHEDULING ||
>> -	    (preemptible() && !debug_objects_is_pi_blocked_on())) {
>> +	if (can_fill_pool()) {
> Would have been nice to have this as two patches: first one
> factors out the check into can_fill_pool() without changing the
> logic, while the second one applies the fix to can_fill_pool().

OK, I will send a v6 patch as suggested. However, Thomas had merged the 
v5 patch into tip. I am fine whether it will be one patch or 2 as long 
as the problem is going to be fixed.

Cheers,
Longman

The following commit has been merged into the core/urgent branch of tip:

Commit-ID:     0d046ae106255cba5eb83b23f78ee93f3620247d
Gitweb:        https://git.kernel.org/tip/0d046ae106255cba5eb83b23f78ee93f3620247d
Author:        Waiman Long <longman@redhat.com>
AuthorDate:    Fri, 05 Jun 2026 13:30:38 -04:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Sat, 06 Jun 2026 14:36:25 +02:00

debugobjects: Don't call fill_pool() in early boot hardirq context

When booting a debug PREEMPT_RT kernel on an ARM64 system, a "inconsistent
{HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage" lockdep warning message was
reported to the console.

During early boot, interrupts are enabled before the scheduler is
enabled. In this window (before SYSTEM_SCHEDULING is set) interrupts can
fire and in the hard interrupt context handler attempt to fill the pool

This can lead to a deadlock when the interrupt occurred when the interrupt
hits a region which holds a lock that is required to be taken in the
allocation path.

Add a new can_fill_pool() helper and reorder the exception rule and forbid
this scenario by excluding allocations from hard interrupt context.

Fixes: 06e0ae988f6e ("debugobjects: Allow to refill the pool before SYSTEM_SCHEDULING")
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260605173038.495075-1-longman@redhat.com
---
 lib/debugobjects.c | 46 ++++++++++++++++++++++++++++++++++++---------
 1 file changed, 37 insertions(+), 9 deletions(-)

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 772ddab..1fa156c 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -720,6 +720,41 @@ static inline bool debug_objects_is_pi_blocked_on(void)
 #endif
 }
 
+static inline bool can_fill_pool(void)
+{
+	/*
+	 * On !RT enabled kernels there are no restrictions and spinlock_t and
+	 * raw_spinlock_t are the same types.
+	 */
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		return true;
+
+	/*
+	 * On RT enabled kernels, the task must not be blocked on a lock as
+	 * that could corrupt the PI state when blocking on a lock in the
+	 * allocation path.
+	 */
+	if (debug_objects_is_pi_blocked_on())
+		return false;
+
+	/*
+	 * On RT enabled kernels the pool refill should happen in preemptible
+	 * context.
+	 */
+	if (preemptible())
+		return true;
+
+	/*
+	 * Though during system boot before scheduling is set up, preemption is
+	 * disabled and the pool can get exhausted. Before scheduling is active
+	 * a task cannot be blocked on a sleeping lock, but it might hold a lock
+	 * and if interrupted then hard interrupt context might run into a lock
+	 * inversion. So exclude hard interrupt context from allocations before
+	 * scheduling is active.
+	 */
+	return system_state < SYSTEM_SCHEDULING && !in_hardirq();
+}
+
 static void debug_objects_fill_pool(void)
 {
 	if (!static_branch_likely(&obj_cache_enabled))
@@ -734,18 +769,11 @@ static void debug_objects_fill_pool(void)
 	if (likely(!pool_should_refill(&pool_global)))
 		return;
 
-	/*
-	 * On RT enabled kernels the pool refill must happen in preemptible
-	 * context and not enqueued on an rt_mutex -- for !RT kernels we rely
-	 * on the fact that spinlock_t and raw_spinlock_t are basically the
-	 * same type and this lock-type inversion works just fine.
-	 */
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || system_state < SYSTEM_SCHEDULING ||
-	    (preemptible() && !debug_objects_is_pi_blocked_on())) {
+	if (can_fill_pool()) {
 		/*
 		 * Annotate away the spinlock_t inside raw_spinlock_t warning
 		 * by temporarily raising the wait-type to LD_WAIT_CONFIG, matching
-		 * the preemptible() condition above.
+		 * the preemptible() condition in can_fill_pool().
 		 */
 		static DEFINE_WAIT_OVERRIDE_MAP(fill_pool_map, LD_WAIT_CONFIG);
 		lock_map_acquire_try(&fill_pool_map);