linux-kernel - Re: [PATCH 07/10] trace_syscalls: init print

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Date:	Thu, 10 Dec 2009 22:04:30 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Jason Baron <jbaron@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/10] trace_syscalls: init print_fmt

On Fri, 2009-12-11 at 10:41 +0800, Lai Jiangshan wrote:
> Steven Rostedt wrote:
> > On Wed, 2009-12-09 at 15:15 +0800, Lai Jiangshan wrote:
> >> Init print_fmt for trace_syscalls.
> >> It will be used for replacing ->show_format().
> > 
> > This needs more explanation too.
> > 
> >> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> >> ---
> >> diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
> >> index 75289f3..dcd8699 100644
> >> --- a/kernel/trace/trace_syscalls.c
> >> +++ b/kernel/trace/trace_syscalls.c
> >> @@ -191,6 +191,61 @@ int syscall_enter_format(struct ftrace_event_call *call, struct trace_seq *s)
> >>  	return trace_seq_putc(s, '\n');
> >>  }
> >>  
> >> +static
> >> +int  __set_enter_print_fmt(struct syscall_metadata *entry, char *buf, int len)
> >> +{
> >> +	int i;
> >> +	int pos = 0;
> >> +
> >> +	pos += snprintf(buf + pos, len > pos ? len - pos : 0, "\"");
> >> +	for (i = 0; i < entry->nb_args; i++) {
> >> +		pos += snprintf(buf + pos, len > pos ? len - pos : 0,
> >> +				"%s: 0x%%0%zulx%s", entry->args[i],
> >> +				sizeof(unsigned long),
> >> +				i == entry->nb_args - 1 ? "" : ", ");
> >> +	}
> >> +	pos += snprintf(buf + pos, len > pos ? len - pos : 0, "\"");
> >> +
> >> +	for (i = 0; i < entry->nb_args; i++) {
> >> +		pos += snprintf(buf + pos, len > pos ? len - pos : 0,
> >> +				", ((unsigned long)(REC->%s))", entry->args[i]);
> > 
> > Yuck! 4 snprintf(buf + pos, len > pos ? len - pos: 0 ...
> > 
> > Please make a wrapper for that.
> 
> Do you mind if I use a macro?
> Readability are still the same... 
> 
> static
> int  __set_enter_print_fmt(struct syscall_metadata *entry, char *buf, int len)
> {
> 	int i;
> 	int pos = 0;
> 
> #define BUF_PRINTF(fmt...)						\
> do {									\
> 	pos += snprintf(buf + pos, len > pos ? len - pos : 0, fmt);	\
> } while (0)
> 
> 	BUF_PRINTF("\"");
> 	for (i = 0; i < entry->nb_args; i++) {
> 		BUF_PRINTF("%s: 0x%%0%zulx%s", entry->args[i],
> 				sizeof(unsigned long),
> 				i == entry->nb_args - 1 ? "\"" : ", ");
> 	}
> 
> 	for (i = 0; i < entry->nb_args; i++)
> 		BUF_PRINTF(", ((unsigned long)(REC->%s))", entry->args[i]);
> 
> #under BUF_PRINTF
> 
> 	/* return the length of print_fmt */
> 	return pos;
> }
> 
> > 
> > Actually, this could use the new trace_seq code if I separate the
> > buffer. You would just need to do:
> > 
> > 	struct trace_seq s;
> 
> I think trace_seq can not help.
> __set_enter_print_fmt() is called twice.
> We calculate the length at the first time(don't write any thing),
> then allocate memory and then really "snprintf" string into the buffer.
> 

Oh God, I see what you are doing. Too many side effects here.

This code needs major comments. Anyway, you already calculated the
length that is needed, you can just do "len ? len - pos : 0".

First, please comment what it is doing (that it gets called twice, once
with 0 length to calculate the needed length and once with the
calculated length).

Then just make the test a macro:

#define LEN_OR_ZERO (len ? len - pos : 0)

	pos += snprintf(buf + pos, LEN_OR_ZERO, fmt);

...

#undef LEN_OR_ZERO


With comments explaining what is happening and this macro, it can make
reading the code a bit easier to understand what is going on.

-- Steve




> -- Lai
> 
> > 
> > 	trace_seq_init(&s, buf, len);
> > 
> > 	trace_seq_putc(&s, '"');
> > 	for (i = 0; i < entry->nb_args; i++)
> > 		trace_seq_printf(&s, "%s: 0x%%0%zulx%s", entry->args[i],
> > 				sizeof(unsigned long),
> > 				i == entry->nb_args - 1 ? "" : ", ");
> > 
> > 	trace_seq_putc(&s, '"');
> > 
> > 	for (i = 0; i < entry->nb_args; i++)
> > 		trace_seq_printf(&s, ", ((unsigned long)(REC->%s)",
> > 				entry->args[i]);
> > 
> > 	return s.len;
> > 
> > 
> > Looks much better, and less error prone.
> > 
> > -- Steve
> > 


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