[PATCH v2 05/11] arm64: entry: Add entry and exit functions for debug exceptions

[Ada Couprie Diaz]

Duplicate the `debug_exception_entry()` and `debug_exception_exit()`
functions from mm/fault.c, as they are needed to split
the debug exceptions entry paths from the current unified one.
The original will be removed in a cleanup patch.

Signed-off-by: Ada Couprie Diaz <ada.coupriediaz at arm.com>
---
 arch/arm64/kernel/entry-common.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index b260ddc4d3e9..92d78b329e67 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -418,6 +418,28 @@ static inline void fp_user_discard(void)
 	}
 }
 
+/*
+ * In debug exception context, we explicitly disable preemption despite
+ * having interrupts disabled.
+ * This serves two purposes: it makes it much less likely that we would
+ * accidentally schedule in exception context and it will force a warning
+ * if we somehow manage to schedule by accident.
+ */
+static void debug_exception_enter(struct pt_regs *regs)
+{
+	preempt_disable();
+
+	/* This code is a bit fragile.  Test it. */
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
+}
+NOKPROBE_SYMBOL(debug_exception_enter);
+
+static void debug_exception_exit(struct pt_regs *regs)
+{
+	preempt_enable_no_resched();
+}
+NOKPROBE_SYMBOL(debug_exception_exit);
+
 UNHANDLED(el1t, 64, sync)
 UNHANDLED(el1t, 64, irq)
 UNHANDLED(el1t, 64, fiq)
-- 
2.43.0