linux-kernel - Re: trace: fix multiple use of __print_flags and __print

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Date:	Wed, 3 Jun 2009 09:23:11 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Steven Whitehouse <swhiteho@...hat.com>
cc:	Li Zefan <lizf@...fujitsu.com>, linux-kernel@...r.kernel.org,
	cluster-devel@...hat.com, Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: trace: fix multiple use of __print_flags and __print_symbolic


On Wed, 3 Jun 2009, Steven Whitehouse wrote:

> 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,

Ah, I did not account for having multiple instances of print_symbol of 
print_flags in a single print. Your patch is partialy correct. Add a:

	trace_seq_init(p);

after the "p = &get_cpu_var(ftrace_event_seq);".

-- Steve

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