linux-kernel - Re: [PATCH v3] string.h: Add str_has

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Message-ID: <20181221190039.73016323@gandalf.local.home>
Date:   Fri, 21 Dec 2018 19:00:39 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joe Perches <joe@...ches.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>
Subject: Re: [PATCH v3] string.h: Add str_has_prefix() helper

On Fri, 21 Dec 2018 15:44:41 -0800
Joe Perches <joe@...ches.com> wrote:

> On Fri, 2018-12-21 at 18:25 -0500, Steven Rostedt wrote:
> > On Fri, 21 Dec 2018 15:19:33 -0800
> > Joe Perches <joe@...ches.com> wrote:
> >   
> > > I believe this should be bool.
> > > 
> > > I don't find a use for non-zero assigned len value in the kernel
> > > for strncmp and I believe the function should simply be:
> > > 
> > > static inline bool str_has_prefix(const char *str, const char prefix[])
> > > {
> > > 	return !strncmp(str, prefix, strlen(prefix));
> > > }  
> > 
> > diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c  
> []
> > @@ -172,8 +172,8 @@ cache_type_store(struct device *dev, struct device_attribute *attr,
> >  		 * it's not worth the risk */
> >  		return -EINVAL;
> >  
> > -	if (strncmp(buf, temp, sizeof(temp) - 1) == 0) {
> > -		buf += sizeof(temp) - 1;
> > +	if ((len = str_has_prefix(buf, temp))) {
> > +		buf += len;  
> 
> That's not really a use of the non-zero strncmp return value.
> 
> You are attempting an optimization not already done.
> I also wonder if it's actually an optimization as the
> return value may not be precomputed.

Note, temp is this:

	static const char temp[] = "temporary ";

> 
> Also the assignment in the test isn't preferred style.

We could have two helper functions:

static __always_inline bool
str_has_prefix(const char *str, const char *prefix)
{
	return strncmp(str, prefix, strlen(prefix));
}

and a 

static __always_inline bool
str_has_prefix_len(const char *str, const char *prefix, unsigned int *len)
{
	*len = strlen(prefix);
	return strncmp(str, prefix, *len);
}

This was my original thought with the first patches. But when Linus
suggested changing the style from the strncmp() I thought it was a way
to encapsulate the two.

Either way, but yes, I do want a way to do the compare and calculate
the length all in one function. That even makes checking options easier
to get to:

	if (str_has_prefix_len(cmdline, "param=", &len)) {
		value = cmdline + len;


> 
> > And there's more places like this.  
> 
> Any where the non-zero return value is actually used?
> 
> > > It's hard to believe __always_inline vs inline matters 
> > > for any single line function.  
> > 
> > I've been burnt by gcc deciding to not inline single functions before.  
> 
> Complex single functions sure, but single line inlines?
> I haven't seen that externed anywhere.
> 
> Today no inline function is marked __always_inline in
> string.h
> 
> I don't doubt there should be some standardization
> of inline vs __always_inline in the kernel, but this
> right now seems different just for difference sake.

I got burnt by some crazy gcc config options making local_irq_save()
become a out of line function, and that cause crazy crap to happen with
the function tracer.

Now inlining here is just for guaranteeing that strlen() gets turned
into a constant for constant strings and wont do anything harmful if
that doesn't happen (but slightly slow things down). But again, it
doesn't hurt to have the __always_inline. Why are you so dead against
it? You haven't stated your rational for that.

-- Steve