[PATCH v8 11/18] unwind deferred: Use bitmask to determine which callbacks to call

@@ -1649,6 +1649,7 @@ struct task_struct {
 
 #ifdef CONFIG_UNWIND_USER
 	struct unwind_task_info		unwind_info;
+	unsigned long			unwind_mask;
 #endif
 
 	/* CPU-specific state of this task: */

@@ -13,6 +13,7 @@ typedef void (*unwind_callback_t)(struct unwind_work *work, struct unwind_stackt
 struct unwind_work {
 	struct list_head		list;
 	unwind_callback_t		func;
+	int				bit;
 };
 
 #ifdef CONFIG_UNWIND_USER

@@ -16,6 +16,7 @@
 /* Guards adding to and reading the list of callbacks */
 static DEFINE_MUTEX(callback_mutex);
 static LIST_HEAD(callbacks);
+static unsigned long unwind_mask;
 
 /*
  * Read the task context timestamp, if this is the first caller then

@@ -106,6 +107,7 @@ static void unwind_deferred_task_work(struct callback_head *head)
 	struct unwind_stacktrace trace;
 	struct unwind_work *work;
 	u64 timestamp;
+	struct task_struct *task = current;
 
 	if (WARN_ON_ONCE(!info->pending))
 		return;

@@ -133,7 +135,10 @@ static void unwind_deferred_task_work(struct callback_head *head)
 
 	guard(mutex)(&callback_mutex);
 	list_for_each_entry(work, &callbacks, list) {
-		work->func(work, &trace, timestamp);
+		if (task->unwind_mask & (1UL << work->bit)) {
+			work->func(work, &trace, timestamp);
+			clear_bit(work->bit, &current->unwind_mask);
+		}
 	}
 }
 

@@ -159,9 +164,12 @@ static int unwind_deferred_request_nmi(struct unwind_work *work, u64 *timestamp)
 		inited_timestamp = true;
 	}
 
-	if (info->pending)
+	if (current->unwind_mask & (1UL << work->bit))
 		return 1;
 
+	if (info->pending)
+		goto out;
+
 	ret = task_work_add(current, &info->work, TWA_NMI_CURRENT);
 	if (ret) {
 		/*

@@ -175,8 +183,8 @@ static int unwind_deferred_request_nmi(struct unwind_work *work, u64 *timestamp)
 	}
 
 	info->pending = 1;
-
-	return 0;
+out:
+	return test_and_set_bit(work->bit, &current->unwind_mask);
 }
 
 /**

@@ -223,14 +231,18 @@ int unwind_deferred_request(struct unwind_work *work, u64 *timestamp)
 
 	*timestamp = get_timestamp(info);
 
+	/* This is already queued */
+	if (current->unwind_mask & (1UL << work->bit))
+		return 1;
+
 	/* callback already pending? */
 	pending = READ_ONCE(info->pending);
 	if (pending)
-		return 1;
+		goto out;
 
 	/* Claim the work unless an NMI just now swooped in to do so. */
 	if (!try_cmpxchg(&info->pending, &pending, 1))
-		return 1;
+		goto out;
 
 	/* The work has been claimed, now schedule it. */
 	ret = task_work_add(current, &info->work, TWA_RESUME);

@@ -239,16 +251,27 @@ int unwind_deferred_request(struct unwind_work *work, u64 *timestamp)
 		return ret;
 	}
 
-	return 0;
+ out:
+	return test_and_set_bit(work->bit, &current->unwind_mask);
 }
 
 void unwind_deferred_cancel(struct unwind_work *work)
 {
+	struct task_struct *g, *t;
+
 	if (!work)
 		return;
 
 	guard(mutex)(&callback_mutex);
 	list_del(&work->list);
+
+	clear_bit(work->bit, &unwind_mask);
+
+	guard(rcu)();
+	/* Clear this bit from all threads */
+	for_each_process_thread(g, t) {
+		clear_bit(work->bit, &t->unwind_mask);
+	}
 }
 
 int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func)

@@ -256,6 +279,14 @@ int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func)
 	memset(work, 0, sizeof(*work));
 
 	guard(mutex)(&callback_mutex);
+
+	/* See if there's a bit in the mask available */
+	if (unwind_mask == ~0UL)
+		return -EBUSY;
+
+	work->bit = ffz(unwind_mask);
+	unwind_mask |= 1UL << work->bit;
+
 	list_add(&work->list, &callbacks);
 	work->func = func;
 	return 0;

@@ -267,6 +298,7 @@ void unwind_task_init(struct task_struct *task)
 
 	memset(info, 0, sizeof(*info));
 	init_task_work(&info->work, unwind_deferred_task_work);
+	task->unwind_mask = 0;
 }
 
 void unwind_task_free(struct task_struct *task)