linux-kernel - Re: [RFC 13/13] Char: nozomi, cleanup read and write

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20071112075452.GD9771@stusta.de>
Date:	Mon, 12 Nov 2007 08:54:52 +0100
From:	Adrian Bunk <bunk@...nel.org>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	linux-kernel@...r.kernel.org, Frank Seidel <fseidel@...e.de>
Subject: Re: [RFC 13/13] Char: nozomi, cleanup read and write

On Sat, Nov 10, 2007 at 11:04:41PM +0100, Jiri Slaby wrote:
> On 11/10/2007 05:15 PM, Adrian Bunk wrote:
> > On Fri, Nov 09, 2007 at 06:51:35PM -0500, Jiri Slaby wrote:
> >> ...
> >> --- a/drivers/char/nozomi.c
> >> +++ b/drivers/char/nozomi.c
> >> ...
> >> -		if (size_bytes - i == 2) {
> >> +		if (unlikely(size_bytes - i == 2)) {
> >> ...
> > 
> > Please don't add likely/unlikely in drivers unless it brings a 
> > measurable improvement.
> 
> Why? Anyway I think this is the case. The body of the then branch is executed at
> most once, while the else branch each time but last. If you write/read 1002
> bytes, it means 250:1. ...and it's invoked from interrupt too...

AFAIK there is no well defined semantics whether likely/unlikely means 
10:1 or 10000:1 that is guaranteed to not change during the next
10 years.

Unless there's a measurable difference, it's best to write readable 
C code and simply leave all optimizations to the compiler.

> regards,

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/