linux-kernel - [for-linus][PATCH 5/8] tracing: Fix a warning when allocating buffered events fails

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Message-ID: <20231207023819.931186714@goodmis.org>
Date:   Wed, 06 Dec 2023 21:37:57 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Petr Pavlu <petr.pavlu@...e.com>
Subject: [for-linus][PATCH 5/8] tracing: Fix a warning when allocating buffered events fails

From: Petr Pavlu <petr.pavlu@...e.com>

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.

Link: https://lore.kernel.org/all/20231127151248.7232-2-petr.pavlu@suse.com/
Link: https://lkml.kernel.org/r/20231205161736.19663-3-petr.pavlu@suse.com

Fixes: 0fc1b09ff1ff ("tracing: Use temp buffer when filtering events")
Signed-off-by: Petr Pavlu <petr.pavlu@...e.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 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 6aeffa4a6994..ef72354f61ce 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2728,8 +2728,11 @@ 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));
@@ -2743,10 +2746,6 @@ 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)
-- 
2.42.0