[PATCH v3] ring-buffer: Prevent inconsistent operation on cpu_buffer->resize_disabled

[Tze-nan Wu]

Write to buffer_size_kb can permanently fail, due to cpu_online_mask may
changed between two for_each_online_buffer_cpu loops.
The number of increasing and decreasing on cpu_buffer->resize_disable
may be inconsistent, leading that the resize_disabled in some CPUs
becoming none zero after ring_buffer_reset_online_cpus return.

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.

Prevent the two "loops" in this function from iterating through different
online cpus by copying cpu_online_mask at the entry of the function.

Changes from v1 to v3:
  Declare the cpumask variable statically rather than dynamically.

Changes from v2 to v3:
  Considering holding cpu_hotplug_lock too long because of the
  synchronize_rcu(), maybe it's better to prevent the issue by copying
  cpu_online_mask at the entry of the function as V1 does, instead of
  using cpus_read_lock().

Link: https://lore.kernel.org/lkml/20230408052226.25268-1-Tze-nan.Wu@xxxxxxxxxxxx/
Link: https://lore.kernel.org/oe-kbuild-all/202304082051.Dp50upfS-lkp@xxxxxxxxx/
Link: https://lore.kernel.org/oe-kbuild-all/202304081615.eiaqpbV8-lkp@xxxxxxxxx/

Cc: stable@xxxxxxxxxxxxxxx
Cc: npiggin@xxxxxxxxx
Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Reviewed-by: Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
---
 kernel/trace/ring_buffer.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 76a2d91eecad..dc758930dacb 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -288,9 +288,6 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data);
 #define for_each_buffer_cpu(buffer, cpu)		\
 	for_each_cpu(cpu, buffer->cpumask)
 
-#define for_each_online_buffer_cpu(buffer, cpu)		\
-	for_each_cpu_and(cpu, buffer->cpumask, cpu_online_mask)
-
 #define TS_SHIFT	27
 #define TS_MASK		((1ULL << TS_SHIFT) - 1)
 #define TS_DELTA_TEST	(~TS_MASK)
@@ -5353,12 +5350,19 @@ EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
 void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
+	cpumask_t reset_online_cpumask;
 	int cpu;
 
+	/*
+	 * Record cpu_online_mask here to make sure we iterate through the same
+	 * online CPUs in the following two loops.
+	 */
+	cpumask_copy(&reset_online_cpumask, cpu_online_mask);
+
 	/* prevent another thread from changing buffer sizes */
 	mutex_lock(&buffer->mutex);
 
-	for_each_online_buffer_cpu(buffer, cpu) {
+	for_each_cpu_and(cpu, buffer->cpumask, &reset_online_cpumask) {
 		cpu_buffer = buffer->buffers[cpu];
 
 		atomic_inc(&cpu_buffer->resize_disabled);
@@ -5368,7 +5372,7 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
 	/* Make sure all commits have finished */
 	synchronize_rcu();
 
-	for_each_online_buffer_cpu(buffer, cpu) {
+	for_each_cpu_and(cpu, buffer->cpumask, &reset_online_cpumask) {
 		cpu_buffer = buffer->buffers[cpu];
 
 		reset_disabled_cpu_buffer(cpu_buffer);
-- 
2.18.0