Subject: [PATCH v3 03/25] notifier: Add atomic/blocking_notifier_has_unique_priority()

From: Dmitry Osipenko <[email-masked]>
Date: Mon, 8 Nov 2021 03:45:02 +0300

Add atomic/blocking_notifier_has_unique_priority() helpers which return
true if given handler has unique priority.

Signed-off-by: Dmitry Osipenko <[email-masked]>
---
 include/linux/notifier.h |  5 +++
 kernel/notifier.c        | 69 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+)

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 924c9d7c8e73..2c4036f225e1 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -175,6 +175,11 @@ int raw_notifier_call_chain_robust(struct raw_notifier_head *nh,
 
 bool blocking_notifier_call_chain_is_empty(struct blocking_notifier_head *nh);
 
+bool atomic_notifier_has_unique_priority(struct atomic_notifier_head *nh,
+		struct notifier_block *nb);
+bool blocking_notifier_has_unique_priority(struct blocking_notifier_head *nh,
+		struct notifier_block *nb);
+
 #define NOTIFY_DONE		0x0000		/* Don't care */
 #define NOTIFY_OK		0x0001		/* Suits me */
 #define NOTIFY_STOP_MASK	0x8000		/* Don't call further */
diff --git a/kernel/notifier.c b/kernel/notifier.c
index b20cb7b9b1f0..7a325b742104 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -122,6 +122,19 @@ static int notifier_call_chain_robust(struct notifier_block **nl,
 	return ret;
 }
 
+static int notifier_has_unique_priority(struct notifier_block **nl,
+					struct notifier_block *n)
+{
+	while (*nl && (*nl)->priority >= n->priority) {
+		if ((*nl)->priority == n->priority && *nl != n)
+			return false;
+
+		nl = &((*nl)->next);
+	}
+
+	return true;
+}
+
 /*
  *	Atomic notifier chain routines.  Registration and unregistration
  *	use a spinlock, and call_chain is synchronized by RCU (no locks).
@@ -203,6 +216,30 @@ int atomic_notifier_call_chain(struct atomic_notifier_head *nh,
 EXPORT_SYMBOL_GPL(atomic_notifier_call_chain);
 NOKPROBE_SYMBOL(atomic_notifier_call_chain);
 
+/**
+ *	atomic_notifier_has_unique_priority - Checks whether notifier's priority is unique
+ *	@nh: Pointer to head of the atomic notifier chain
+ *	@n: Entry in notifier chain to check
+ *
+ *	Checks whether there is another notifier in the chain with the same priority.
+ *	Must be called in process context.
+ *
+ *	Returns true if priority is unique, false otherwise.
+ */
+bool atomic_notifier_has_unique_priority(struct atomic_notifier_head *nh,
+		struct notifier_block *n)
+{
+	unsigned long flags;
+	bool ret;
+
+	spin_lock_irqsave(&nh->lock, flags);
+	ret = notifier_has_unique_priority(&nh->head, n);
+	spin_unlock_irqrestore(&nh->lock, flags);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(atomic_notifier_has_unique_priority);
+
 /*
  *	Blocking notifier chain routines.  All access to the chain is
  *	synchronized by an rwsem.
@@ -336,6 +373,38 @@ bool blocking_notifier_call_chain_is_empty(struct blocking_notifier_head *nh)
 }
 EXPORT_SYMBOL_GPL(blocking_notifier_call_chain_is_empty);
 
+/**
+ *	blocking_notifier_has_unique_priority - Checks whether notifier's priority is unique
+ *	@nh: Pointer to head of the blocking notifier chain
+ *	@n: Entry in notifier chain to check
+ *
+ *	Checks whether there is another notifier in the chain with the same priority.
+ *	Must be called in process context.
+ *
+ *	Returns true if priority is unique, false otherwise.
+ */
+bool blocking_notifier_has_unique_priority(struct blocking_notifier_head *nh,
+		struct notifier_block *n)
+{
+	bool ret;
+
+	/*
+	 * This code gets used during boot-up, when task switching is
+	 * not yet working and interrupts must remain disabled. At such
+	 * times we must not call down_read().
+	 */
+	if (system_state != SYSTEM_BOOTING)
+		down_read(&nh->rwsem);
+
+	ret = notifier_has_unique_priority(&nh->head, n);
+
+	if (system_state != SYSTEM_BOOTING)
+		up_read(&nh->rwsem);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(blocking_notifier_has_unique_priority);
+
 /*
  *	Raw notifier chain routines.  There is no protection;
  *	the caller must provide it.  Use at your own risk!
-- 
2.33.1