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

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Message-ID: <20181217223524.GH2218@hirez.programming.kicks-ass.net>
Date:   Mon, 17 Dec 2018 23:35:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: objtool warnings for kernel/trace/trace_selftest_dynamic.o

On Mon, Dec 17, 2018 at 12:55:35PM -0800, Andi Kleen wrote:
> On Mon, Dec 17, 2018 at 08:29:38PM +0100, Peter Zijlstra wrote:
> > When/if we get the LTO trainwreck sorted -- which very much includes
> > getting that memory-order-consume fixed -- we can revisit all that.
> 
> What do you mean? I'm not aware of any LTO problems with memory-order-consume?

The compiler is basically allowed to break RCU (and anything else that
depends on read-read dependencies). LTO makes it _far_ more likely this
happens.

We need guarantees (and possible switches) from the compiler folks that
this will not happen before I'll retract my NAK from any LTO enabling.