linux-kernel - Re: [PATCH 1/1] ftrace: fix wrong pos computing when read buffer has been fulfilled.

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Date:	Tue, 18 Nov 2008 23:17:21 +0800
From:	walimis <walimisdev@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] ftrace: fix wrong pos computing when read buffer
	has been fulfilled.

On Tue, Nov 18, 2008 at 09:36:07AM -0500, Steven Rostedt wrote:
>
>On Sat, 15 Nov 2008, walimis wrote:
>
>> 
>> hi Steven,
>> 
>> Could you help me to review this patch?
>> 
>> Thanks.
>> 
>> Impact: make output of available_filter_functions complete
>> 
>> phenomenon:
>> 
>> The first value of dyn_ftrace_total_info is not equal with
>> `cat available_filter_functions | wc -l`, but they should be equal.
>> 
>> root cause:
>> 
>> When printing functions with seq_printf in t_show, if the read buffer
>> is just overflowed by current function record, then this function
>> won't be printed to user space through read buffer, it will
>> just be dropped. So we can't see this function printing.
>> So, every time the last function to fill the read buffer, if overflowed,
>> will be dropped.
>> This also applies to set_ftrace_filter if set_ftrace_filter has
>> more bytes than read buffer.
>> 
>> fix:
>> 
>> Through checking return value of seq_printf, if less than 0, we know
>> this function doesn't be printed. Then we decrease position to force
>> this function to be printed next time, in next read buffer.
>
>A lot of the code for seq files I cut and paste from other places that I 
>have written it. The seq files always confuse me, so I like to use stuff 
>that worked before. Some of the things that walimis fixed were not needed 
>for this file, and was overkill.
It also confuse me a lot. I also checked all places using seq files in
trace/ftrace code, to see whether there are similar problem.
  - trace.c: in s_start() it force pos to decrease one such as: 
	l = *pos - 1;
	so it works well.
  - trace_stack.c: t_start, in almost all cases, can be only called
    once, so we don't see the error. But I wonder whether it still works
    well if output message exceeds the size of seq file buffer and call
    t_start twice. If needed, I can test it.
  - trace_branch.c : the same with above.


walimis
>
>> 
>> another little fix is to show correct allocating pages count.
>
>Ack.
>
>> 
>> Signed-off-by: walimis <walimisdev@...il.com>
>
>Acked-by: Steven Rostedt <srostedt@...hat.com>
>
>-- Steve
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