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Subject: Re: trace: fix multiple use of __print_flags and __print_symbolic
From: Steven Whitehouse <swhiteho@redhat.com>
To: Li Zefan <lizf@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       cluster-devel@redhat.com, Christoph Hellwig <hch@infradead.org>,
       Ingo Molnar <mingo@elte.hu>
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Date: Wed, 03 Jun 2009 13:57:55 +0100
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Hi

On Wed, 2009-06-03 at 17:50 +0800, Li Zefan wrote:
> Steven Whitehouse wrote:
> > When using multiple calls to __print_symbolic and __print_flags in
> > the same tracer, I noticed that the output was incorrect. I think
> > the following patch is the correct fix (it works for me) but please
> > check it carefully since I'm not that familiar with this code,
> > and I may well have made a mistake somewhere.
> > 
> 
> I don't see there's bug in __print_symbolic() or __print_flags():
> 
> enum print_line_t
> ftrace_raw_output_##call(struct trace_iterator *iter, int flags)
> {
> 	...
> 	p = &get_cpu_var(ftrace_event_seq);
> 	/* here we call ftrace_print_flags_seq(p, ...) */
> 	ret = trace_seq_printf(s, #call ": " print);
> 	put_cpu();
> 	...
> }
> 
> ftrace_event_seq is percpu data, and here is used with preempt-disabled,
> so there shouldn't be problem of concurrent accessing.
> 
I've double checked this now, and I'm seeing results like:


glock_workqueue-2482  [001]   809.956985: gfs2_glock_state_change: 8.7 glock 2:33119 state EX => EX tgt:EX dmt:EX flags:EX
glock_workqueue-2482  [001]   809.957155: gfs2_glock_state_change: 8.7 glock 2:33119 state NL => NL tgt:NL dmt:NL flags:NL
glock_workqueue-2482  [001]   809.959473: gfs2_glock_state_change: 8.7 glock 1:2 state NL => NL tgt:NL dmt:NL flags:NL
glock_workqueue-2482  [001]   809.959556: gfs2_glock_state_change: 8.7 glock 2:33119 state EX => EX tgt:EX dmt:EX flags:EX
glock_workqueue-2482  [001]   810.008773: gfs2_glock_state_change: 8.7 glock 1:0 state EX => EX tgt:EX dmt:EX flags:EX


without the patch, which is clearly wrong,

Steve.