On 2023/2/9 06:36, Steven Rostedt wrote:

On Wed, 8 Feb 2023 17:08:14 +0800
Zheng Yejian [off-list ref] wrote:

quoted

Concurrently closing "trace" file and writing into ring buffer [1] can
cause WARNINGs [2]. It has been reported in
Link: https://lore.kernel.org/all/20230203035608.2336906-1-zhengyejian1@huawei.com/ (local)

It seems a data race between ring_buffer writing and integrity check.
That is, RB_FLAG of head_page is been updating, while at same time RB_FLAG
was cleared when doing integrity check:
   rb_check_pages()            rb_handle_head_page():
   --------                    --------
   rb_head_page_deactivate()
                               rb_head_page_set_normal()
   rb_head_page_activate()

Good catch!

Thanks!

quoted

Integrity check at end of iter read was added since commit 659f451ff213
("ring-buffer: Add integrity check at end of iter read"). As it's commit
message said:
   > As reading via an iterator requires disabling the ring buffer, it
   > is a perfect place to have it.
However, since commit 1039221cc278 ("ring-buffer: Do not disable recording
when there is an iterator"), ring buffer was not disabled at that place,
so that integrity check should be removed.

1:
``` read_trace.sh
   while true;
   do
     # the "trace" file is closed after read
     head -1 /sys/kernel/tracing/trace > /dev/null
   done

``` repro.sh
   sysctl -w kernel.panic_on_warn=1
   # function tracer will writing enough data into ring_buffer
   echo function > /sys/kernel/tracing/current_tracer
   ./read_trace.sh &
   ./read_trace.sh &
   ./read_trace.sh &
   ./read_trace.sh &
   ./read_trace.sh &
   ./read_trace.sh &
   ./read_trace.sh &
   ./read_trace.sh &

quoted

Fixes: 1039221cc278 ("ring-buffer: Do not disable recording when there is an iterator")
Signed-off-by: Zheng Yejian <redacted>
---
  kernel/trace/ring_buffer.c | 11 -----------
  1 file changed, 11 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index c366a0a9ddba..34e955bd1e59 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c

@@ -5203,17 +5203,6 @@ void
  ring_buffer_read_finish(struct ring_buffer_iter *iter)
  {
  	struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
-	unsigned long flags;
-
-	/*
-	 * Ring buffer is disabled from recording, here's a good place
-	 * to check the integrity of the ring buffer.
-	 * Must prevent readers from trying to read, as the check
-	 * clears the HEAD page and readers require it.
-	 */
-	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
-	rb_check_pages(cpu_buffer);
-	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);

I would rather find a way to make this still work than just removing it.

Yes, we can try to find the way.

Perhaps there's no reason to clear the flags, and change rb_check_pages()
to mask them out before testing. Something like:

static int rb_check_pages(struct ring_buffer_per_cpu *cpu_buffer)
{
	struct list_head *head = cpu_buffer->pages;
	struct buffer_page *bpage, *tmp;

	if (RB_WARN_ON(cpu_buffer, rb_list_head(rb_list_head(head->next)->prev) != head))
		return -1;
	if (RB_WARN_ON(cpu_buffer, rb_list_head(rb_list_head(head->prev)->next) != head))
		return -1;

	if (rb_check_list(cpu_buffer, head))

rb_check_list() expect to check a page with RB_FLAG being cleared,
but in this solution, rb_head_page_deactivate() is not called before,
so we may not call it directly? The same problem with below check for
"bpage->list".

		return -1;

	list_for_each_entry_safe(bpage, tmp, head, list) {

I'd like to know if there is a case that "head" happens to be a
"reader_page", and the ring buffer is not exactly being traversed?

		if (RB_WARN_ON(cpu_buffer,
		     rb_list_head(rb_list_head(bpage->list.next)->prev) != &bpage->list))
			return -1;
		if (RB_WARN_ON(cpu_buffer,
		     rb_list_head(rb_list_head(bpage->list.prev)->next) != &bpage->list))
			return -1;
		if (rb_check_list(cpu_buffer, &bpage->list))
			return -1;
	}

	return 0;
}

I haven't tested the above.

?

-- Steve

quoted

  
  	atomic_dec(&cpu_buffer->resize_disabled);
  	kfree(iter->event);