linux-kernel - [PATCH] ftrace: Comment that ftrace_func_mapper is freed with free_ftrace

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Message-ID: <20250416165420.5c717420@gandalf.local.home>
Date: Wed, 16 Apr 2025 16:54:20 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Mark Rutland <mark.rutland@....com>
Subject: [PATCH] ftrace: Comment that ftrace_func_mapper is freed with
 free_ftrace_hash()

From: Steven Rostedt <rostedt@...dmis.org>

The structure ftrace_func_mapper only contains a single field and that is
a ftrace_hash. It is used to abstract it out from a normal hash to control
users of how it gets modified.

The freeing of a ftrace_func_mapper structure is:

  free_ftrace_hash(&mapper->hash);

Without context, this looks like a bug. It should be commented that it is
not a bug and it is freed this way.

Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/ftrace.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index a8a02868b435..19b902b8de2b 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5160,8 +5160,12 @@ struct ftrace_func_map {
 	void				*data;
 };
 
+/*
+ * Note, ftrace_func_mapper is freed by free_ftrace_hash(&mapper->hash).
+ * The hash field must be the first field.
+ */
 struct ftrace_func_mapper {
-	struct ftrace_hash		hash;
+	struct ftrace_hash		hash;	/* Must be first! */
 };
 
 /**
@@ -5296,6 +5300,7 @@ void free_ftrace_func_mapper(struct ftrace_func_mapper *mapper,
 			}
 		}
 	}
+	/* This also frees the mapper itself */
 	free_ftrace_hash(&mapper->hash);
 }
 
-- 
2.47.2