linux-kernel - Re: [PATCH] TRACING: Fix a copmile warning

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Message-ID: <1311688552.3526.75.camel@gandalf.stny.rr.com>
Date:	Tue, 26 Jul 2011 09:55:52 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Paulo Marques <pmarques@...popie.com>
Cc:	Jesper Juhl <jj@...osbits.net>, stufever@...il.com,
	linux-kernel@...r.kernel.org,
	Wang Shaoyan <wangshaoyan.pt@...bao.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] TRACING: Fix a copmile warning

On Tue, 2011-07-26 at 14:32 +0100, Paulo Marques wrote:
> Jesper Juhl wrote:

> >> tb_fmt = kmalloc(sizeof(*tb_fmt), GFP_KERNEL);
> >> if (tb_fmt) {
> >>         fmt = kmalloc(strlen(*iter) + 1, GFP_KERNEL);
> >> 	if (fmt) {
> >>         	list_add_tail(&tb_fmt->list, &trace_bprintk_fmt_list);
> >>         	strcpy(fmt, *iter);
> >>         	tb_fmt->fmt = fmt;
> >>         	*iter = tb_fmt->fmt;
> >>         } else {
> >> 	        kfree(tb_fmt);
> >>         	*iter = NULL;
> >>         }
> >> } else {
> >>         *iter = NULL;
> >> }
> >>
> >> The downside is that the "*iter = NULL" gets repeated twice...
> >>
> > 
> > You could avoid that like this:
> > 
> > *iter = NULL;
> > tb_fmt = kmalloc(sizeof(*tb_fmt), GFP_KERNEL);
> > if (tb_fmt) {
> >       fmt = kmalloc(strlen(*iter) + 1, GFP_KERNEL);
> >       if (fmt) {
> >               list_add_tail(&tb_fmt->list, &trace_bprintk_fmt_list);
> >               strcpy(fmt, *iter);
> >               tb_fmt->fmt = fmt;
> >               *iter = tb_fmt->fmt;
> >         } else {
> >               kfree(tb_fmt);
> >         }
> > }
> 
> Yes, but this way you always set *iter to NULL, whereas in the previous
> version that was the very unlikely case (kmalloc returning NULL).
> 
> Probably gcc is smart enough to generate the same code for both
> versions, to avoid setting *iter twice for the likely case (and even if
> it doesn't, then the cache will be hot and probably not written back
> yet, yadda, yadda)...

gcc may not be allowed to optimize it as iter points to a global, and
external functions are called (kmalloc).

But what would work is:

static
void hold_module_trace_bprintk_format(const char **start, const char **end)
{
	const char **iter;
	char *fmt;

	mutex_lock(&btrace_mutex);
	for (iter = start; iter < end; iter++) {
		struct trace_bprintk_fmt *tb_fmt = lookup_format(*iter);
		if (tb_fmt) {
			*iter = tb_fmt->fmt;
			continue;
		}

		fmt = NULL;
		tb_fmt = kmalloc(sizeof(*tb_fmt), GFP_KERNEL);
		if (tb_fmt) {
			fmt = kmalloc(strlen(*iter) + 1, GFP_KERNEL);
			if (fmt) {
				list_add_tail(&tb_fmt->list, &trace_bprintk_fmt_list);
				strcpy(fmt, *iter);
				tb_fmt->fmt = fmt;
			} else
				kfree(tb_fmt);
		}
		*iter = fmt;
	}
	mutex_unlock(&btrace_mutex);
}

The above is easier to read, removes the false warning, and uses local
variables that gcc can optimize.

-- Steve



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