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

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Message-ID: <75e15c5d66ecd6964231be0f5b3cf702ac242bc1.camel@perches.com>
Date:   Fri, 21 Dec 2018 15:44:41 -0800
From:   Joe Perches <joe@...ches.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, 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.

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

> 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.

cheers, Joe