Re: [PATCH 07/10] trace_syscalls: init print_fmt

Steven Rostedt
2009-12-11T03:04:37Z

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@cn.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
> >