Re: [PATCH v4 1/3] ring-buffer: Flush and stop persistent ring buffer on panic

[Google]

On Tue, 24 Feb 2026 15:52:42 +0900
"Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> On real hardware, panic and machine reboot may not flush hardware cache
> to memory. This means the persistent ring buffer, which relies on a
> coherent state of memory, may not have its events written to the buffer
> and they may be lost. Moreover, there may be inconsistency with the
> counters which are used for validation of the integrity of the
> persistent ring buffer which may cause all data to be discarded.
> 
> To avoid this issue, stop recording of the ring buffer on panic and
> flush the cache of the ring buffer's memory.
> 
> Fixes: e645535a954a ("tracing: Add option to use memmapped memory for trace boot instance")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
>  Changes in v3:
>    - update patch description.
> ---
>  kernel/trace/ring_buffer.c |   21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index f16f053ef77d..3eb124c93d72 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -6,6 +6,7 @@
>   */
>  #include <linux/sched/isolation.h>
>  #include <linux/trace_recursion.h>
> +#include <linux/panic_notifier.h>
>  #include <linux/trace_events.h>
>  #include <linux/ring_buffer.h>
>  #include <linux/trace_clock.h>
> @@ -589,6 +590,7 @@ struct trace_buffer {
>  
>  	unsigned long			range_addr_start;
>  	unsigned long			range_addr_end;
> +	struct notifier_block		flush_nb;
>  
>  	struct ring_buffer_meta		*meta;
>  
> @@ -2471,6 +2473,16 @@ static void rb_free_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
>  	kfree(cpu_buffer);
>  }
>  
> +static int rb_flush_buffer_cb(struct notifier_block *nb, unsigned long event, void *data)
> +{
> +	struct trace_buffer *buffer = container_of(nb, struct trace_buffer, flush_nb);
> +
> +	ring_buffer_record_disable(buffer);

I found this was a wrong API. I have to use ring_buffer_record_off().

> +	flush_kernel_vmap_range((void *)buffer->range_addr_start,
> +				buffer->range_addr_end - buffer->range_addr_start);

This does nothing on arm64.



Thanks,

> +	return NOTIFY_DONE;
> +}
> +
>  static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
>  					 int order, unsigned long start,
>  					 unsigned long end,
> @@ -2590,6 +2602,12 @@ static struct trace_buffer *alloc_buffer(unsigned long size, unsigned flags,
>  
>  	mutex_init(&buffer->mutex);
>  
> +	/* Persistent ring buffer needs to flush cache before reboot. */
> +	if (start & end) {
> +		buffer->flush_nb.notifier_call = rb_flush_buffer_cb;
> +		atomic_notifier_chain_register(&panic_notifier_list, &buffer->flush_nb);
> +	}
> +
>  	return_ptr(buffer);
>  
>   fail_free_buffers:
> @@ -2677,6 +2695,9 @@ ring_buffer_free(struct trace_buffer *buffer)
>  {
>  	int cpu;
>  
> +	if (buffer->range_addr_start && buffer->range_addr_end)
> +		atomic_notifier_chain_unregister(&panic_notifier_list, &buffer->flush_nb);
> +
>  	cpuhp_state_remove_instance(CPUHP_TRACE_RB_PREPARE, &buffer->node);
>  
>  	irq_work_sync(&buffer->irq_work.work);
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>