[PATCH v6 4/5] unwind deferred: Use SRCU unwind_deferred_task_work()

@@ -23,10 +23,11 @@
  */
 static DEFINE_PER_CPU(u64, unwind_ctx_ctr);
 
-/* Guards adding to and reading the list of callbacks */
+/* Guards adding to or removing from the list of callbacks */
 static DEFINE_MUTEX(callback_mutex);
 static LIST_HEAD(callbacks);
 static unsigned long unwind_mask;
+DEFINE_STATIC_SRCU(unwind_srcu);
 
 /*
  * The context cookie is a unique identifier that is assigned to a user

@@ -137,6 +138,7 @@ static void unwind_deferred_task_work(struct callback_head *head)
 	struct unwind_work *work;
 	struct task_struct *task = current;
 	u64 cookie;
+	int idx;
 
 	if (WARN_ON_ONCE(!info->pending))
 		return;

@@ -155,14 +157,16 @@ static void unwind_deferred_task_work(struct callback_head *head)
 
 	cookie = get_cookie(info);
 
-	guard(mutex)(&callback_mutex);
-	list_for_each_entry(work, &callbacks, list) {
+	idx = srcu_read_lock(&unwind_srcu);
+	list_for_each_entry_srcu(work, &callbacks, list,
+				 srcu_read_lock_held(&unwind_srcu)) {
 		if (task->unwind_mask & (1UL << work->bit)) {
 			work->func(work, &trace, cookie);
 			clear_bit(work->bit, &current->unwind_mask);
 		}
 	}
-	barrier();
+	srcu_read_unlock(&unwind_srcu, idx);
+
 	/* If another task work is pending, reuse the cookie and stack trace */
 	if (!READ_ONCE(info->pending))
 		WRITE_ONCE(info->cookie, 0);

@@ -238,6 +242,7 @@ int unwind_deferred_request(struct unwind_work *work, u64 *cookie)
 {
 	struct unwind_task_info *info = &current->unwind_info;
 	int pending;
+	int bit;
 	int ret;
 
 	*cookie = 0;

@@ -249,12 +254,17 @@ int unwind_deferred_request(struct unwind_work *work, u64 *cookie)
 	if (in_nmi())
 		return unwind_deferred_request_nmi(work, cookie);
 
+	/* Do not allow cancelled works to request again */
+	bit = READ_ONCE(work->bit);
+	if (WARN_ON_ONCE(bit < 0))
+		return -EINVAL;
+
 	guard(irqsave)();
 
 	*cookie = get_cookie(info);
 
 	/* This is already queued */
-	if (current->unwind_mask & (1UL << work->bit))
+	if (current->unwind_mask & (1UL << bit))
 		return 1;
 
 	/* callback already pending? */

@@ -280,19 +290,26 @@ int unwind_deferred_request(struct unwind_work *work, u64 *cookie)
 void unwind_deferred_cancel(struct unwind_work *work)
 {
 	struct task_struct *g, *t;
+	int bit;
 
 	if (!work)
 		return;
 
 	guard(mutex)(&callback_mutex);
-	list_del(&work->list);
+	list_del_rcu(&work->list);
+	bit = work->bit;
+
+	/* Do not allow any more requests and prevent callbacks */
+	work->bit = -1;
+
+	clear_bit(bit, &unwind_mask);
 
-	clear_bit(work->bit, &unwind_mask);
+	synchronize_srcu(&unwind_srcu);
 
 	guard(rcu)();
 	/* Clear this bit from all threads */
 	for_each_process_thread(g, t) {
-		clear_bit(work->bit, &t->unwind_mask);
+		clear_bit(bit, &t->unwind_mask);
 	}
 }
 

@@ -309,7 +326,7 @@ int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func)
 	work->bit = ffz(unwind_mask);
 	unwind_mask |= 1UL << work->bit;
 
-	list_add(&work->list, &callbacks);
+	list_add_rcu(&work->list, &callbacks);
 	work->func = func;
 	return 0;
 }