linux-kernel - [PATCH 2/2] tracing: Add autoremove feature to the backup instance

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Message-ID:  <176779716708.4193242.18165634129732047971.stgit@mhiramat.tok.corp.google.com>
Date: Wed,  7 Jan 2026 23:46:07 +0900
From: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Subject: [PATCH 2/2] tracing: Add autoremove feature to the backup instance

From: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Since the backup instance is readonly, after reading all data
via pipe, no data is left on the instance. Thus it can be
removed safely after closing all files.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
---
 kernel/trace/trace.c |   64 +++++++++++++++++++++++++++++++++++++++++++++++++-
 kernel/trace/trace.h |    6 +++++
 2 files changed, 69 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 725930f5980e..dfd4385603e6 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -590,6 +590,55 @@ void trace_set_ring_buffer_expanded(struct trace_array *tr)
 	tr->ring_buffer_expanded = true;
 }
 
+static int __remove_instance(struct trace_array *tr);
+
+static void trace_array_autoremove(struct work_struct *work)
+{
+	struct trace_array *tr = container_of(work, struct trace_array, autoremove_work);
+
+	guard(mutex)(&event_mutex);
+	guard(mutex)(&trace_types_lock);
+
+	/*
+	 * This can be fail if someone gets @tr before starting this
+	 * function, but in that case, this will be kicked again when
+	 * putting it. So we don't care the result.
+	 */
+	__remove_instance(tr);
+}
+
+static struct workqueue_struct *autoremove_wq;
+
+static void trace_array_init_autoremove(struct trace_array *tr)
+{
+	INIT_WORK(&tr->autoremove_work, trace_array_autoremove);
+}
+
+static void trace_array_kick_autoremove(struct trace_array *tr)
+{
+	if (!work_pending(&tr->autoremove_work) && autoremove_wq)
+		queue_work(autoremove_wq, &tr->autoremove_work);
+}
+
+static void trace_array_cancel_autoremove(struct trace_array *tr)
+{
+	if (work_pending(&tr->autoremove_work))
+		cancel_work(&tr->autoremove_work);
+}
+
+__init static int trace_array_init_autoremove_wq(void)
+{
+	autoremove_wq = alloc_workqueue("tr_autoremove_wq",
+					WQ_UNBOUND | WQ_HIGHPRI, 0);
+	if (!autoremove_wq) {
+		pr_err("Unable to allocate tr_autoremove_wq\n");
+		return -ENOMEM;
+	}
+	return 0;
+}
+
+late_initcall_sync(trace_array_init_autoremove_wq);
+
 LIST_HEAD(ftrace_trace_arrays);
 
 int trace_array_get(struct trace_array *this_tr)
@@ -598,7 +647,7 @@ int trace_array_get(struct trace_array *this_tr)
 
 	guard(mutex)(&trace_types_lock);
 	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
-		if (tr == this_tr) {
+		if (tr == this_tr && !tr->free_on_close) {
 			tr->ref++;
 			return 0;
 		}
@@ -611,6 +660,12 @@ static void __trace_array_put(struct trace_array *this_tr)
 {
 	WARN_ON(!this_tr->ref);
 	this_tr->ref--;
+	/*
+	 * When free_on_close is set, prepare removing the array
+	 * when the last reference is released.
+	 */
+	if (this_tr->ref == 1 && this_tr->free_on_close)
+		trace_array_kick_autoremove(this_tr);
 }
 
 /**
@@ -6212,6 +6267,10 @@ static void update_last_data(struct trace_array *tr)
 	/* Only if the buffer has previous boot data clear and update it. */
 	tr->flags &= ~TRACE_ARRAY_FL_LAST_BOOT;
 
+	/* If this is a backup instance, mark it for autoremove. */
+	if (tr->flags & TRACE_ARRAY_FL_VMALLOC)
+		tr->free_on_close = true;
+
 	/* Reset the module list and reload them */
 	if (tr->scratch) {
 		struct trace_scratch *tscratch = tr->scratch;
@@ -10392,6 +10451,8 @@ trace_array_create_systems(const char *name, const char *systems,
 	if (ftrace_allocate_ftrace_ops(tr) < 0)
 		goto out_free_tr;
 
+	trace_array_init_autoremove(tr);
+
 	ftrace_init_trace_array(tr);
 
 	init_trace_flags_index(tr);
@@ -10540,6 +10601,7 @@ static int __remove_instance(struct trace_array *tr)
 	if (update_marker_trace(tr, 0))
 		synchronize_rcu();
 
+	trace_array_cancel_autoremove(tr);
 	tracing_set_nop(tr);
 	clear_ftrace_function_probes(tr);
 	event_trace_del_tracer(tr);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index bc0eeb2d1d07..a8088a106d67 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -446,6 +446,12 @@ struct trace_array {
 	 * we do not waste memory on systems that are not using tracing.
 	 */
 	bool ring_buffer_expanded;
+	/*
+	 * If the ring buffer is a read only backup instance, it will be
+	 * removed after dumping all data via pipe, because no readable data.
+	 */
+	bool free_on_close;
+	struct work_struct	autoremove_work;
 };
 
 enum {