linux-kernel - Re: [PATCH] x86: Optimize variable_test

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: <20150501162921.GD24151@twins.programming.kicks-ass.net>
Date:	Fri, 1 May 2015 18:29:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: [PATCH] x86: Optimize variable_test_bit()

On Fri, May 01, 2015 at 06:16:54PM +0200, Peter Zijlstra wrote:
> On Fri, May 01, 2015 at 09:03:32AM -0700, Linus Torvalds wrote:
> > On Fri, May 1, 2015 at 8:16 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > Since test_bit() doesn't actually have any output variables, we can use
> > > asm goto without having to add a memory clobber. This reduces the code
> > > to something sensible:
> > 
> > Yes, looks good, except if we have anything that actually wants to use
> > the value rather than branch on it. But a quick grep seems to show
> > that the vast majority of them are all about just directly testing the
> > result.
> > 
> > It worries me a bit that gcc now cannot pick the likely branch any
> > more. It will always branch out for the bit being set. So code like
> > this:
> > 
> >     net/core/dev.c:         if
> > (likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
> > 
> > wouldn't work, but almost all of those seem to be the constant case
> > that doesn't get to this anyway.
> 
> Ah yes, that's another thing we've previously discussed with the GCC
> people (IIRC). The GCC manual states you can use hot and cold attributes
> on the labels (although when we tested that it didn't actually work, it
> might now). But that's no good if the hint is one (or more) layer up
> from the asm goto.
> 
> If would indeed be very good if the likely/unlikely thing would work as
> expected.

Ah, I see what you meant. Yes we're stuck with the 'jc', the compiler
cannot flip that into a jnc.

The best it can do is add unconditional jumps to re-arrange the blocks,
and that's somewhat ugly indeed, although better than nothing at all.
And from experiments back when we did the static_branch stuff that all
didn't work at all.
--
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/