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

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Date:   Mon, 17 Dec 2018 20:49:16 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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 04:06:18PM -0800, Andi Kleen wrote:
> 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")

Sorry for suggesting this prematurely, my email client stopped syncing
and I missed your later replies to Peter about this.

> > Should it be reverted, or just remove the noclone, and keep the
> > noinline?
> 
> It should not be touched for now, until it is properly debugged.
> 
> IMHO Josh's explanation doesn't make much sense and there
> was a lot of handwaving 
> 
> And just fixing one case isn't good enough because there are other
> noclone functions in the tree.
> 
> It the problem is the plugin the plugin needs to be fixed.
> 
> If the problem is gcc we need a gcc test case and bug, with 
> some analysis, and then based on that select the proper workaround.

The plugin is only used for older versions of GCC.  Newer versions have
the same functionality builtin with -fsanitize-coverage=trace-pc.

So the problem is GCC.  We're using a function attribute which at least
oneGCC developer doesn't recommend.  If you want to keep the LTO support
then '__used' seems like a much better choice.

-- 
Josh