This is a note to let you know that I've just added the patch titled
ring-buffer: Fix race while reader and writer are on the same page
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ring-buffer-fix-race-while-reader-and-writer-are-on-the-same-page.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 6455b6163d8c680366663cdb8c679514d55fc30c Mon Sep 17 00:00:00 2001
From: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
Date: Sat, 25 Mar 2023 10:12:47 +0800
Subject: ring-buffer: Fix race while reader and writer are on the same page
From: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
commit 6455b6163d8c680366663cdb8c679514d55fc30c upstream.
When user reads file 'trace_pipe', kernel keeps printing following logs
that warn at "cpu_buffer->reader_page->read > rb_page_size(reader)" in
rb_get_reader_page(). It just looks like there's an infinite loop in
tracing_read_pipe(). This problem occurs several times on arm64 platform
when testing v5.10 and below.
Call trace:
rb_get_reader_page+0x248/0x1300
rb_buffer_peek+0x34/0x160
ring_buffer_peek+0xbc/0x224
peek_next_entry+0x98/0xbc
__find_next_entry+0xc4/0x1c0
trace_find_next_entry_inc+0x30/0x94
tracing_read_pipe+0x198/0x304
vfs_read+0xb4/0x1e0
ksys_read+0x74/0x100
__arm64_sys_read+0x24/0x30
el0_svc_common.constprop.0+0x7c/0x1bc
do_el0_svc+0x2c/0x94
el0_svc+0x20/0x30
el0_sync_handler+0xb0/0xb4
el0_sync+0x160/0x180
Then I dump the vmcore and look into the problematic per_cpu ring_buffer,
I found that tail_page/commit_page/reader_page are on the same page while
reader_page->read is obviously abnormal:
tail_page == commit_page == reader_page == {
.write = 0x100d20,
.read = 0x8f9f4805, // Far greater than 0xd20, obviously abnormal!!!
.entries = 0x10004c,
.real_end = 0x0,
.page = {
.time_stamp = 0x857257416af0,
.commit = 0xd20, // This page hasn't been full filled.
// .data[0...0xd20] seems normal.
}
}
The root cause is most likely the race that reader and writer are on the
same page while reader saw an event that not fully committed by writer.
To fix this, add memory barriers to make sure the reader can see the
content of what is committed. Since commit a0fcaaed0c46 ("ring-buffer: Fix
race between reset page and reading page") has added the read barrier in
rb_get_reader_page(), here we just need to add the write barrier.
Link: https://lore.kernel.org/linux-trace-kernel/20230325021247.2923907-1-zhengyejian1@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 77ae365eca89 ("ring-buffer: make lockless")
Suggested-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/trace/ring_buffer.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2532,6 +2532,10 @@ rb_set_commit_to_write(struct ring_buffe
if (RB_WARN_ON(cpu_buffer,
rb_is_reader_page(cpu_buffer->tail_page)))
return;
+ /*
+ * No need for a memory barrier here, as the update
+ * of the tail_page did it for this page.
+ */
local_set(&cpu_buffer->commit_page->page->commit,
rb_page_write(cpu_buffer->commit_page));
rb_inc_page(cpu_buffer, &cpu_buffer->commit_page);
@@ -2545,6 +2549,8 @@ rb_set_commit_to_write(struct ring_buffe
while (rb_commit_index(cpu_buffer) !=
rb_page_write(cpu_buffer->commit_page)) {
+ /* Make sure the readers see the content of what is committed. */
+ smp_wmb();
local_set(&cpu_buffer->commit_page->page->commit,
rb_page_write(cpu_buffer->commit_page));
RB_WARN_ON(cpu_buffer,
@@ -3920,7 +3926,12 @@ rb_get_reader_page(struct ring_buffer_pe
/*
* Make sure we see any padding after the write update
- * (see rb_reset_tail())
+ * (see rb_reset_tail()).
+ *
+ * In addition, a writer may be writing on the reader page
+ * if the page has not been fully filled, so the read barrier
+ * is also needed to make sure we see the content of what is
+ * committed by the writer (see rb_set_commit_to_write()).
*/
smp_rmb();
Patches currently in stable-queue which might be from zhengyejian1@xxxxxxxxxx are
queue-5.4/ring-buffer-fix-race-while-reader-and-writer-are-on-the-same-page.patch
