FAILED: patch "[PATCH] ring-buffer: Ensure proper resetting of atomic variables in" failed to apply to 5.15-stable tree

[<gregkh@xxxxxxxxxxxxxxxxxxx>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 7c339fb4d8577792378136c15fde773cfb863cb8
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@xxxxxxxxxxxxxxx>' --in-reply-to '2023050648-brethren-harmony-6fc1@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 7c339fb4d8577792378136c15fde773cfb863cb8 Mon Sep 17 00:00:00 2001
From: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
Date: Wed, 26 Apr 2023 14:20:23 +0800
Subject: [PATCH] ring-buffer: Ensure proper resetting of atomic variables in
 ring_buffer_reset_online_cpus

In ring_buffer_reset_online_cpus, the buffer_size_kb write operation
may permanently fail if the cpu_online_mask changes between two
for_each_online_buffer_cpu loops. The number of increases and decreases
on both cpu_buffer->resize_disabled and cpu_buffer->record_disabled may be
inconsistent, causing some CPUs to have non-zero values for these atomic
variables after the function returns.

This issue can be reproduced by "echo 0 > trace" while hotplugging cpu.
After reproducing success, we can find out buffer_size_kb will not be
functional anymore.

To prevent leaving 'resize_disabled' and 'record_disabled' non-zero after
ring_buffer_reset_online_cpus returns, we ensure that each atomic variable
has been set up before atomic_sub() to it.

Link: https://lore.kernel.org/linux-trace-kernel/20230426062027.17451-1-Tze-nan.Wu@xxxxxxxxxxxx

Cc: stable@xxxxxxxxxxxxxxx
Cc: <mhiramat@xxxxxxxxxx>
Cc: npiggin@xxxxxxxxx
Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
Reviewed-by: Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 58be5b409f72..9a0cb94c3972 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5326,6 +5326,9 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
 
+/* Flag to ensure proper resetting of atomic variables */
+#define RESET_BIT	(1 << 30)
+
 /**
  * ring_buffer_reset_online_cpus - reset a ring buffer per CPU buffer
  * @buffer: The ring buffer to reset a per cpu buffer of
@@ -5342,20 +5345,27 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
 	for_each_online_buffer_cpu(buffer, cpu) {
 		cpu_buffer = buffer->buffers[cpu];
 
-		atomic_inc(&cpu_buffer->resize_disabled);
+		atomic_add(RESET_BIT, &cpu_buffer->resize_disabled);
 		atomic_inc(&cpu_buffer->record_disabled);
 	}
 
 	/* Make sure all commits have finished */
 	synchronize_rcu();
 
-	for_each_online_buffer_cpu(buffer, cpu) {
+	for_each_buffer_cpu(buffer, cpu) {
 		cpu_buffer = buffer->buffers[cpu];
 
+		/*
+		 * If a CPU came online during the synchronize_rcu(), then
+		 * ignore it.
+		 */
+		if (!(atomic_read(&cpu_buffer->resize_disabled) & RESET_BIT))
+			continue;
+
 		reset_disabled_cpu_buffer(cpu_buffer);
 
 		atomic_dec(&cpu_buffer->record_disabled);
-		atomic_dec(&cpu_buffer->resize_disabled);
+		atomic_sub(RESET_BIT, &cpu_buffer->resize_disabled);
 	}
 
 	mutex_unlock(&buffer->mutex);