tjh.dev / kernel
Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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tracing: Make sure trace_printk() can output as soon as it can be used

Currently trace_printk() can be used as soon as early_trace_init() is
called from start_kernel(). But if a crash happens, and
"ftrace_dump_on_oops" is set on the kernel command line, all you get will
be:

[ 0.456075] <idle>-0 0dN.2. 347519us : Unknown type 6
[ 0.456075] <idle>-0 0dN.2. 353141us : Unknown type 6
[ 0.456075] <idle>-0 0dN.2. 358684us : Unknown type 6

This is because the trace_printk() event (type 6) hasn't been registered
yet. That gets done via an early_initcall(), which may be early, but not
early enough.

Instead of registering the trace_printk() event (and other ftrace events,
which are not trace events) via an early_initcall(), have them registered at
the same time that trace_printk() can be used. This way, if there is a
crash before early_initcall(), then the trace_printk()s will actually be
useful.

Link: https://lkml.kernel.org/r/20230104161412.019f6c55@gandalf.local.home

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: e725c731e3bb1 ("tracing: Split tracing initialization into two for early initialization")
Reported-by: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Steven Rostedt (Google) 3bb06eb6 8be9fbd5

+4 -2
+2
kernel/trace/trace.c
··· 10295 10295 static_key_enable(&tracepoint_printk_key.key); 10296 10296 } 10297 10297 tracer_alloc_buffers(); 10298 + 10299 + init_events(); 10298 10300 } 10299 10301 10300 10302 void __init trace_init(void)
+1
kernel/trace/trace.h
··· 1490 1490 extern void trace_event_enable_tgid_record(bool enable); 1491 1491 1492 1492 extern int event_trace_init(void); 1493 + extern int init_events(void); 1493 1494 extern int event_trace_add_tracer(struct dentry *parent, struct trace_array *tr); 1494 1495 extern int event_trace_del_tracer(struct trace_array *tr); 1495 1496 extern void __trace_early_add_events(struct trace_array *tr);
+1 -2
kernel/trace/trace_output.c
··· 1535 1535 NULL 1536 1536 }; 1537 1537 1538 - __init static int init_events(void) 1538 + __init int init_events(void) 1539 1539 { 1540 1540 struct trace_event *event; 1541 1541 int i, ret; ··· 1548 1548 1549 1549 return 0; 1550 1550 } 1551 - early_initcall(init_events);