Hi,
On Fri, May 2, 2025 at 1:53 PM Steven Rostedt <rost...@goodmis.org> wrote:
>
> From: Steven Rostedt <rost...@goodmis.org>
>
> The per CPU "disabled" value was the original way to disable tracing when
> the tracing subsystem was first created. Today, the ring buffer
> infrastructure has its own way to disable tracing. In fact, things have
> changed so much since 2008 that many things ignore the disable flag.
>
> The kdb_ftdump() function iterates over all the current tracing CPUs and
> increments the "disabled" counter before doing the dump, and decrements it
> afterward.
>
> As the disabled flag can be ignored, doing this today is not reliable.
> Instead, simply call tracer_tracing_off() and then tracer_tracing_on() to
> disable and then enabled the entire ring buffer in one go!
>
> Cc: Jason Wessel <jason.wes...@windriver.com>
> Cc: Daniel Thompson <dani...@kernel.org>
> Cc: Douglas Anderson <diand...@chromium.org>
> Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org>
> ---
> kernel/trace/trace_kdb.c | 8 ++------
> 1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
> index 1e72d20b3c2f..b5cf3fdde8cb 100644
> --- a/kernel/trace/trace_kdb.c
> +++ b/kernel/trace/trace_kdb.c
> @@ -120,9 +120,7 @@ static int kdb_ftdump(int argc, const char **argv)
> trace_init_global_iter(&iter);
> iter.buffer_iter = buffer_iter;
>
> - for_each_tracing_cpu(cpu) {
> - atomic_inc(&per_cpu_ptr(iter.array_buffer->data,
> cpu)->disabled);
> - }
> + tracer_tracing_off(iter.tr);
>
> /* A negative skip_entries means skip all but the last entries */
> if (skip_entries < 0) {
> @@ -135,9 +133,7 @@ static int kdb_ftdump(int argc, const char **argv)
>
> ftrace_dump_buf(skip_entries, cpu_file);
>
> - for_each_tracing_cpu(cpu) {
> - atomic_dec(&per_cpu_ptr(iter.array_buffer->data,
> cpu)->disabled);
> - }
> + tracer_tracing_on(iter.tr);
This new change seems less safe than the old one. Previously you'd
always increment by one at the start of the function and decrement by
one at the end. Now at the start of the function you'll set
"buffer_disabled" to 1 and at the end you'll set it to 0. If
"buffer_disabled" was already 1 at the start of the function your new
sequence will end up having the side effect of changing it to 0.
-Doug
