From: "Tze-nan Wu" <Tze-nan.Wu@xxxxxxxxxxxx>
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.
Cc: stable@xxxxxxxxxxxxxxx
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>
---
Changes from v4 to v5: https://lore.kernel.org/lkml/20230412112401.25081-1-Tze-nan.Wu@xxxxxxxxxxxx/
- Move the define before the function
---
kernel/trace/ring_buffer.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 76a2d91eecad..253ef85a9ec3 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5345,6 +5345,9 @@ 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
@@ -5361,20 +5364,27 @@ 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);
