[PATCH v6 2/2] debugobjects: Don't call fill_pool() in early boot hardirq context

[Waiman Long <longman@redhat.com>]

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.

Reorder the exception rules and forbid this scenario by excluding
allocations from hardirq.

Fixes: 06e0ae988f6e ("debugobjects: Allow to refill the pool before SYSTEM_SCHEDULING")
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 lib/debugobjects.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 1d826689611f..6fb00e08a4e2 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -730,18 +730,29 @@ static inline bool can_fill_pool(void)
 		return true;
 
 	/*
-	 * On RT enabled kernels the pool refill must happen in preemptible
-	 * context and 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.
+	 * 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 (preemptible() && !debug_objects_is_pi_blocked_on())
+	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;
 
 	/*
-	 * Allow refilling to happen early in the boot with disabled interrupts
-	 * as long as the scheduler is not operational.
+	 * 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;
+	return system_state < SYSTEM_SCHEDULING && !in_hardirq();
 }
 
 static void debug_objects_fill_pool(void)
-- 
2.54.0