ring-buffer: Ensure proper resetting of atomic variables in ring_buffer_reset_online_cpus

commit 7c339fb4d8577792378136c15fde773cfb863cb8 upstream.

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@mediatek.com
Cc: stable@vger.kernel.org
Cc: <mhiramat@kernel.org>
Cc: npiggin@gmail.com
Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
Reviewed-by: Cheng-Jui Wang <cheng-jui.wang@mediatek.com>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@mediatek.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9d85385..9c42219 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5333,6 +5333,9 @@ 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
@@ -5349,20 +5352,27 @@ 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);