From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1753245AbeAQONV (ORCPT <rfc822;ralf@linux-mips.org> + 1 other);
        Wed, 17 Jan 2018 09:13:21 -0500
Received: from mx1.redhat.com ([209.132.183.28]:53270 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1752574AbeAQONU (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 17 Jan 2018 09:13:20 -0500
Date: Wed, 17 Jan 2018 08:13:15 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Woodhouse <dwmw2@infradead.org>,
        linux-kernel@vger.kernel.org, Dave Hansen <dave.hansen@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>
Subject: Re: [PATCH v2 11/10] objtool: Even more complex static block checks
Message-ID: <20180117141315.nk4cszdo4tw7zaxk@treble>
References: <20180116142825.376986833@infradead.org>
 <20180116194917.GH2228@hirez.programming.kicks-ass.net>
 <20180117031232.y7t25srlfsym5len@treble>
 <20180117081309.GJ2228@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20180117081309.GJ2228@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.6.0.1 (2016-04-01)
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Wed, 17 Jan 2018 14:13:19 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Return-Path: <linux-kernel-owner@vger.kernel.org>

On Wed, Jan 17, 2018 at 09:13:09AM +0100, Peter Zijlstra wrote:
> On Tue, Jan 16, 2018 at 09:12:32PM -0600, Josh Poimboeuf wrote:
> > On Tue, Jan 16, 2018 at 08:49:17PM +0100, Peter Zijlstra wrote:
> > > Subject: objtool: Even more complex static block checks
> > > From: Peter Zijlstra <peterz@infradead.org>
> > > Date: Tue Jan 16 20:17:01 CET 2018
> > > 
> > > I've observed GCC transform:
> > > 
> > >   f()
> > >   {
> > > 	  if (!static_branch_unlikely())
> > > 		  return;
> > > 
> > > 	  static_assert();
> > > 	  A;
> > >   }
> > > 
> > >   g()
> > >   {
> > > 	  f();
> > >   }
> > > 
> > > Into:
> > > 
> > >   f()
> > >   {
> > > 	  static_assert();
> > > 	  A;
> > >   }
> > > 
> > >   g()
> > >   {
> > > 	  if (static_branch_unlikely())
> > > 		  f();
> > >   }
> > > 
> > > Which results in the assertion landing at f+0. The transformation is
> > > valid and useful; it avoids a pointless CALL+RET sequence, so we'll
> > > have to teach objtool how to deal with this.
> > > 
> > > Do this by marking all CALL destinations with static_call when called
> > > from a static_block and non_static_call when called outside a
> > > static_block. This allows us to identify functions called exclusively
> > > from a static_block and start them with a static_block.
> > 
> > Ew... where'd you place the assertion to trigger this?
> 
> Its the patch I pastebin'ed you earlier, also see below.

Ah, I remembered you mentioning the problem, just forgot you showed me
the patch.

> > It's late and my brain has already clocked out, so I'll need to revisit
> > this tomorrow.  But now I'm wondering if my basic block idea would be a
> > better way to solve this.
> 
> I would think basic-blocks are inside functions, and this patch goes
> across functions, something you'd still need even if you had basic
> blocks.

Right, but I was thinking the patch would be a lot simpler with basic
blocks.

> Also, basic blocks are non-trivial because they can overlap.

Hm, I thought a basic block only has one entry point and one exit point.
How could they overlap?

-- 
Josh