linux-kernel - Re: objtool warnings for kernel/trace/trace_selftest

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Message-ID: <20181218030550.5yxvn6yfwvpmm6ev@treble>
Date:   Mon, 17 Dec 2018 21:05:50 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: objtool warnings for kernel/trace/trace_selftest_dynamic.o

On Mon, Dec 17, 2018 at 05:36:44PM -0500, Steven Rostedt wrote:
> On Mon, 17 Dec 2018 15:31:26 -0600
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > On Mon, Dec 17, 2018 at 08:29:38PM +0100, Peter Zijlstra wrote:
> > > On Mon, Dec 17, 2018 at 12:16:38PM -0600, Josh Poimboeuf wrote:
> > >   
> > > > > Yes LTO causes the to be treated like static functions.
> > > > > 
> > > > > I guess noclone is unlikely to be really needed here because these
> > > > > functions are unlikely to be cloned.
> > > > > 
> > > > > So as a workaround it could be removed.
> > > > > 
> > > > > But note we have other noclone functions in the tree (like in KVM)
> > > > > which actually need it.  
> > > > 
> > > > How about we just use the __used attribute then?  It seems to have the
> > > > same result of preventing IPA optimizations (without the weird side
> > > > effect of missing frame pointers).  
> > > 
> > > AFAIK we don't have any in-tree LTO, so it can all go in the bin.
> > > 
> > > When/if we get the LTO trainwreck sorted -- which very much includes
> > > getting that memory-order-consume fixed -- we can revisit all that.  
> > 
> > Ok, then if there are no objections I'll just send a revert of:
> > 
> >   dd3dad0d716d ("ftrace: Mark function tracer test functions noinline/noclone")
> > 
> 
> Should it be reverted, or just remove the noclone, and keep the
> noinline?

If we want to support out-of-tree LTO, then it should only need "used",
because wouldn't that imply noinline?

If we *don't* want to support out-of-tree LTO, then it shouldn't need
any special attributes, because the functions aren't static so GCC won't
mess with their ABI.

-- 
Josh