linux-kernel - Re: [PATCH] jump label: fix passing kbuild_cflags when checking for asm goto support

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Message-ID: <20170310173952.GW27556@glebfm.cloud.tilaa.com>
Date:   Fri, 10 Mar 2017 20:39:52 +0300
From:   Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        David Lin <dtwlin@...gle.com>,
        "Anton V. Boyarshinov" <boyarsh@...linux.org>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Michal Marek <mmarek@...e.com>, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] jump label: fix passing kbuild_cflags when checking for
 asm goto support

On Fri, Mar 10, 2017 at 12:04:22PM -0500, Steven Rostedt wrote:
> On Fri, 10 Mar 2017 19:24:18 +0300
> Gleb Fotengauer-Malinovskiy <glebfm@...linux.org> wrote:
> 
> > The latest change of asm goto support check added passing of
> > KBUILD_CFLAGS to compiler.  When these flags reference gcc plugins,
> > there is a race condition between the build of these plugins and their
> > use in this asm goto support check.
> > 
> > This change moves the check before all these references are added
> > to KBUILD_CFLAGS.  This is correct because subsequent KBUILD_CFLAGS
> > modifications are not relevant to this check.
> 
> What exactly breaks when the race happens?

Actually, this is not quite a race.  When you run "make bzImage" followed
by "make modules", the kernel is always built with HAVE_JUMP_LABEL
disabled, while the modules are built depending on CONFIG_JUMP_LABEL.
If HAVE_JUMP_LABEL macro happens to be different, modules are built
with undefined references:

ERROR: "static_key_slow_inc" [net/netfilter/xt_TEE.ko] undefined!
ERROR: "static_key_slow_dec" [net/netfilter/xt_TEE.ko] undefined!
ERROR: "static_key_slow_dec" [net/netfilter/nft_meta.ko] undefined!
ERROR: "static_key_slow_inc" [net/netfilter/nft_meta.ko] undefined!
ERROR: "nf_hooks_needed" [net/netfilter/ipvs/ip_vs.ko] undefined!
ERROR: "nf_hooks_needed" [net/ipv6/ipv6.ko] undefined!
ERROR: "static_key_count" [net/ipv6/ipv6.ko] undefined!
ERROR: "static_key_slow_inc" [net/ipv6/ipv6.ko] undefined!
ERROR: "nf_hooks_needed" [net/decnet/decnet.ko] undefined!

-- 
glebfm