From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The kernel command line ftrace_boot_snapshot by itself is supposed to
trigger a snapshot at the end of boot up of the main top level trace
buffer. A ftrace_boot_snapshot=foo will do the same for an instance called
foo that was created by trace_instance=foo,...

The logic was broken where if ftrace_boot_snapshot was by itself, it would
trigger a snapshot for all instances that had tracing enabled, regardless
if it asked for a snapshot or not.

When a snapshot is requested for a buffer, the buffer's
tr->allocated_snapshot is set to true. Use that to know if a trace buffer
wants a snapshot at boot up or not.

Since the top level buffer is part of the ftrace_trace_arrays list,
there's no reason to treat it differently than the other buffers. Just
iterate the list if ftrace_boot_snapshot was specified.

Link: https://lkml.kernel.org/r/20230405022341.895334039@goodmis.org

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ross Zwisler <zwisler@google.com>
Fixes: 9c1c251d670bc ("tracing: Allow boot instances to have snapshot buffers")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ed1d1093f5e9..4686473b8497 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -10393,19 +10393,20 @@ __init static int tracer_alloc_buffers(void)
 
 void __init ftrace_boot_snapshot(void)
 {
+#ifdef CONFIG_TRACER_MAX_TRACE
 	struct trace_array *tr;
 
-	if (snapshot_at_boot) {
-		tracing_snapshot();
-		internal_trace_puts("** Boot snapshot taken **\n");
-	}
+	if (!snapshot_at_boot)
+		return;
 
 	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
-		if (tr == &global_trace)
+		if (!tr->allocated_snapshot)
 			continue;
-		trace_array_puts(tr, "** Boot snapshot taken **\n");
+
 		tracing_snapshot_instance(tr);
+		trace_array_puts(tr, "** Boot snapshot taken **\n");
 	}
+#endif
 }
 
 void __init early_trace_init(void)
-- 
2.39.2

On Wed, 05 Apr 2023 09:58:15 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> The kernel command line ftrace_boot_snapshot by itself is supposed to
> trigger a snapshot at the end of boot up of the main top level trace
> buffer. A ftrace_boot_snapshot=foo will do the same for an instance called
> foo that was created by trace_instance=foo,...
> 
> The logic was broken where if ftrace_boot_snapshot was by itself, it would
> trigger a snapshot for all instances that had tracing enabled, regardless
> if it asked for a snapshot or not.
> 
> When a snapshot is requested for a buffer, the buffer's
> tr->allocated_snapshot is set to true. Use that to know if a trace buffer
> wants a snapshot at boot up or not.
> 
> Since the top level buffer is part of the ftrace_trace_arrays list,
> there's no reason to treat it differently than the other buffers. Just
> iterate the list if ftrace_boot_snapshot was specified.
> 
> Link: https://lkml.kernel.org/r/20230405022341.895334039@goodmis.org
> 
> Cc: stable@vger.kernel.org
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ross Zwisler <zwisler@google.com>
> Fixes: 9c1c251d670bc ("tracing: Allow boot instances to have snapshot buffers")

I guess I didn't need to Cc stable here, as the commit it fixed was added
in the 6.3 merge window.

 $ git describe --contains 9c1c251d670bc
 v6.3-rc1~126^2~8

Oh well.

-- Steve


> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/trace.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index ed1d1093f5e9..4686473b8497 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -10393,19 +10393,20 @@ __init static int tracer_alloc_buffers(void)
>  
>  void __init ftrace_boot_snapshot(void)
>  {
> +#ifdef CONFIG_TRACER_MAX_TRACE
>  	struct trace_array *tr;
>  
> -	if (snapshot_at_boot) {
> -		tracing_snapshot();
> -		internal_trace_puts("** Boot snapshot taken **\n");
> -	}
> +	if (!snapshot_at_boot)
> +		return;
>  
>  	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> -		if (tr == &global_trace)
> +		if (!tr->allocated_snapshot)
>  			continue;
> -		trace_array_puts(tr, "** Boot snapshot taken **\n");
> +
>  		tracing_snapshot_instance(tr);
> +		trace_array_puts(tr, "** Boot snapshot taken **\n");
>  	}
> +#endif
>  }
>  
>  void __init early_trace_init(void)