linux-kernel - [for-linus][PATCH 1/3] ring-buffer: Fix wake ups when buffer

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Date: Fri, 29 Dec 2023 16:13:15 -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>,
 Jiri Olsa <jolsa@...nel.org>,
 stable@...r.kernel.org
Subject: [for-linus][PATCH 1/3] ring-buffer: Fix wake ups when buffer_percent is set to 100

From: "Steven Rostedt (Google)" <rostedt@...dmis.org>

The tracefs file "buffer_percent" is to allow user space to set a
water-mark on how much of the tracing ring buffer needs to be filled in
order to wake up a blocked reader.

 0 - is to wait until any data is in the buffer
 1 - is to wait for 1% of the sub buffers to be filled
 50 - would be half of the sub buffers are filled with data
 100 - is not to wake the waiter until the ring buffer is completely full

Unfortunately the test for being full was:

	dirty = ring_buffer_nr_dirty_pages(buffer, cpu);
	return (dirty * 100) > (full * nr_pages);

Where "full" is the value for "buffer_percent".

There is two issues with the above when full == 100.

1. dirty * 100 > 100 * nr_pages will never be true
   That is, the above is basically saying that if the user sets
   buffer_percent to 100, more pages need to be dirty than exist in the
   ring buffer!

2. The page that the writer is on is never considered dirty, as dirty
   pages are only those that are full. When the writer goes to a new
   sub-buffer, it clears the contents of that sub-buffer.

That is, even if the check was ">=" it would still not be equal as the
most pages that can be considered "dirty" is nr_pages - 1.

To fix this, add one to dirty and use ">=" in the compare.

Link: https://lore.kernel.org/linux-trace-kernel/20231226125902.4a057f1d@gandalf.local.home

Cc: stable@...r.kernel.org
Cc: Mark Rutland <mark.rutland@....com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Fixes: 03329f9939781 ("tracing: Add tracefs file buffer_percentage")
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/ring_buffer.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 83eab547f1d1..32c0dd2fd1c3 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -881,9 +881,14 @@ static __always_inline bool full_hit(struct trace_buffer *buffer, int cpu, int f
 	if (!nr_pages || !full)
 		return true;

-	dirty = ring_buffer_nr_dirty_pages(buffer, cpu);
+	/*
+	 * Add one as dirty will never equal nr_pages, as the sub-buffer
+	 * that the writer is on is not counted as dirty.
+	 * This is needed if "buffer_percent" is set to 100.
+	 */
+	dirty = ring_buffer_nr_dirty_pages(buffer, cpu) + 1;

-	return (dirty * 100) > (full * nr_pages);
+	return (dirty * 100) >= (full * nr_pages);
 }

 /*
-- 
2.42.0