linux-kernel - [PATCH v2 2/3] tracing: Fix a warning when allocating buffered events fails

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Message-Id: <20231205161736.19663-3-petr.pavlu@suse.com>
Date:   Tue,  5 Dec 2023 17:17:35 +0100
From:   Petr Pavlu <petr.pavlu@...e.com>
To:     rostedt@...dmis.org, mhiramat@...nel.org,
        mathieu.desnoyers@...icios.com
Cc:     zhengyejian1@...wei.com, linux-trace-kernel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Petr Pavlu <petr.pavlu@...e.com>
Subject: [PATCH v2 2/3] tracing: Fix a warning when allocating buffered events fails

Function trace_buffered_event_disable() produces an unexpected warning
when the previous call to trace_buffered_event_enable() fails to
allocate pages for buffered events.

The situation can occur as follows:
* The counter trace_buffered_event_ref is at 0.
* The soft mode gets enabled for some event and
  trace_buffered_event_enable() is called. The function increments
  trace_buffered_event_ref to 1 and starts allocating event pages.
* The allocation fails for some page and trace_buffered_event_disable()
  is called for cleanup.
* Function trace_buffered_event_disable() decrements
  trace_buffered_event_ref back to 0, recognizes that it was the last
  use of buffered events and frees all allocated pages.
* The control goes back to trace_buffered_event_enable() which returns.
  The caller of trace_buffered_event_enable() has no information that
  the function actually failed.
* Some time later, the soft mode is disabled for the same event.
  Function trace_buffered_event_disable() is called. It warns on
  "WARN_ON_ONCE(!trace_buffered_event_ref)" and returns.

Buffered events are just an optimization and can handle failures. Make
trace_buffered_event_enable() exit on the first failure and left any
cleanup later to when trace_buffered_event_disable() is called.

Fixes: 0fc1b09ff1ff ("tracing: Use temp buffer when filtering events")
Signed-off-by: Petr Pavlu <petr.pavlu@...e.com>
---
 kernel/trace/trace.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 6f22a2a31589..dd45020fcdde 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2770,8 +2770,11 @@ void trace_buffered_event_enable(void)
 	for_each_tracing_cpu(cpu) {
 		page = alloc_pages_node(cpu_to_node(cpu),
 					GFP_KERNEL | __GFP_NORETRY, 0);
-		if (!page)
-			goto failed;
+		/* This is just an optimization and can handle failures */
+		if (!page) {
+			pr_err("Failed to allocate event buffer\n");
+			break;
+		}

 		event = page_address(page);
 		memset(event, 0, sizeof(*event));
@@ -2785,10 +2788,6 @@ void trace_buffered_event_enable(void)
 			WARN_ON_ONCE(1);
 		preempt_enable();
 	}
-
-	return;
- failed:
-	trace_buffered_event_disable();
 }

 static void enable_trace_buffered_event(void *data)
-- 
2.35.3