linux-kernel - Re: [PATCH 20/24] objtool: Another static block fail

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Message-ID: <20180129225252.bi2etgk3eqprcv3x@treble>
Date:   Mon, 29 Jan 2018 16:52:53 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 20/24] objtool: Another static block fail

On Tue, Jan 23, 2018 at 04:25:59PM +0100, Peter Zijlstra wrote:
> I've observed GCC generate:
> 
>   sym:
>      NOP/JMP 1f	(static_branch)
>      JMP 2f
>   1: /* crud */
>      JMP 3f
>   2: /* other crud */
> 
>   3: RETQ
> 
> 
> This means we need to follow unconditional jumps; be conservative and
> only follow if its a unique jump.
> 
> (I've not yet figured out which CONFIG option is responsible for this,
>  a normal defconfig build does not generate crap like this)
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Any chance we can just add a compiler barrier to the assertion macro and
avoid all this grow_static_blocks() mess?  It seems a bit... fragile.

-- 
Josh