linux-kernel - [PATCH] ring-buffer: use READ_ONCE() to read cpu_buffer->commit

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Date: Sun, 25 Feb 2024 11:05:06 +0800
From: linke li <lilinke99@...com>
To: 
Cc: lilinke99@...com,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Subject: [PATCH] ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment

In function ring_buffer_iter_empty(), cpu_buffer->commit_page and
curr_commit_page->page->time_stamp is read using READ_ONCE() in 
line 4354, 4355

4354    curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
4355    curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);

while they are read directly in line 4340, 4341

4340    commit_page = cpu_buffer->commit_page;
4341    commit_ts = commit_page->page->time_stamp;

There is patch similar to this. commit c1c0ce31b242 ("r8169: fix the KCSAN reported data-race in rtl_tx() while reading tp->cur_tx")
This patch find two read of same variable while one is protected, another
is not. And READ_ONCE() is added to protect.

Signed-off-by: linke li <lilinke99@...com>
---
 kernel/trace/ring_buffer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 0699027b4f4c..eb3fa629b837 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4337,8 +4337,8 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
 	cpu_buffer = iter->cpu_buffer;
 	reader = cpu_buffer->reader_page;
 	head_page = cpu_buffer->head_page;
-	commit_page = cpu_buffer->commit_page;
-	commit_ts = commit_page->page->time_stamp;
+	commit_page = READ_ONCE(cpu_buffer->commit_page);
+	commit_ts = READ_ONCE(commit_page->page->time_stamp);
 
 	/*
 	 * When the writer goes across pages, it issues a cmpxchg which
-- 
2.39.3 (Apple Git-145)