[for-next][PATCH 07/10] tracing/osnoise: Fix possible recursive locking for cpus_read_lock()

[for-next][PATCH 07/10] tracing/osnoise: Fix possible recursive locking for cpus_read_lock()

[Steven Rostedt]

From: Ran Xiaokai <ran.xiaokai@zte.com.cn>

Lockdep reports this deadlock log:

osnoise: could not start sampling thread
============================================
WARNING: possible recursive locking detected
--------------------------------------------
       CPU0
       ----
  lock(cpu_hotplug_lock);
  lock(cpu_hotplug_lock);

 Call Trace:
  <TASK>
  print_deadlock_bug+0x282/0x3c0
  __lock_acquire+0x1610/0x29a0
  lock_acquire+0xcb/0x2d0
  cpus_read_lock+0x49/0x120
  stop_per_cpu_kthreads+0x7/0x60
  start_kthread+0x103/0x120
  osnoise_hotplug_workfn+0x5e/0x90
  process_one_work+0x44f/0xb30
  worker_thread+0x33e/0x5e0
  kthread+0x206/0x3b0
  ret_from_fork+0x31/0x50
  ret_from_fork_asm+0x11/0x20
  </TASK>

This is the deadlock scenario:
osnoise_hotplug_workfn()
  guard(cpus_read_lock)();      // first lock call
  start_kthread(cpu)
    if (IS_ERR(kthread)) {
      stop_per_cpu_kthreads(); {
        cpus_read_lock();      // second lock call. Cause the AA deadlock
      }
    }

It is not necessary to call stop_per_cpu_kthreads() which stops osnoise
kthread for every other CPUs in the system if a failure occurs during
hotplug of a certain CPU.
For start_per_cpu_kthreads(), if the start_kthread() call fails,
this function calls stop_per_cpu_kthreads() to handle the error.
Therefore, similarly, there is no need to call stop_per_cpu_kthreads()
again within start_kthread().
So just remove stop_per_cpu_kthreads() from start_kthread to solve this issue.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/20250321095249.2739397-1-ranxiaokai627@163.com
Fixes: c8895e271f79 ("trace/osnoise: Support hotplug operations")
Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace_osnoise.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index f3a2722ee4c0..c83a51218ee5 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -2032,7 +2032,6 @@ static int start_kthread(unsigned int cpu)
 
 	if (IS_ERR(kthread)) {
 		pr_err(BANNER "could not start sampling thread\n");
-		stop_per_cpu_kthreads();
 		return -ENOMEM;
 	}
 
-- 
2.47.2

[for-next][PATCH 09/10] tracing: Ensure module defining synth event cannot be unloaded while tracing

[Steven Rostedt]

From: Douglas Raillard <douglas.raillard@arm.com>

Currently, using synth_event_delete() will fail if the event is being
used (tracing in progress), but that is normally done in the module exit
function. At that stage, failing is problematic as returning a non-zero
status means the module will become locked (impossible to unload or
reload again).

Instead, ensure the module exit function does not get called in the
first place by increasing the module refcnt when the event is enabled.

Cc: stable@vger.kernel.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fixes: 35ca5207c2d11 ("tracing: Add synthetic event command generation functions")
Link: https://lore.kernel.org/20250318180906.226841-1-douglas.raillard@arm.com
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace_events_synth.c | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index 07ff8be8267e..463b0073629a 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/kernel/trace/trace_events_synth.c
@@ -852,6 +852,34 @@ static struct trace_event_fields synth_event_fields_array[] = {
 	{}
 };
 
+static int synth_event_reg(struct trace_event_call *call,
+		    enum trace_reg type, void *data)
+{
+	struct synth_event *event = container_of(call, struct synth_event, call);
+
+	switch (type) {
+	case TRACE_REG_REGISTER:
+	case TRACE_REG_PERF_REGISTER:
+		if (!try_module_get(event->mod))
+			return -EBUSY;
+		break;
+	default:
+		break;
+	}
+
+	int ret = trace_event_reg(call, type, data);
+
+	switch (type) {
+	case TRACE_REG_UNREGISTER:
+	case TRACE_REG_PERF_UNREGISTER:
+		module_put(event->mod);
+		break;
+	default:
+		break;
+	}
+	return ret;
+}
+
 static int register_synth_event(struct synth_event *event)
 {
 	struct trace_event_call *call = &event->call;
@@ -881,7 +909,7 @@ static int register_synth_event(struct synth_event *event)
 		goto out;
 	}
 	call->flags = TRACE_EVENT_FL_TRACEPOINT;
-	call->class->reg = trace_event_reg;
+	call->class->reg = synth_event_reg;
 	call->class->probe = trace_event_raw_event_synth;
 	call->data = event;
 	call->tp = event->tp;
-- 
2.47.2