[PATCH 4.14 15/43] Revert "compiler.h: update definition of unreachable()"

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

This reverts commit 82017e26e51596ee577171a33f357377ec6513b5, which is
upstream commit fe0640eb30b7da261ae84d252ed9ed3c7e68dfd8.

On Fri, Jun 28, 2019 at 8:53 AM Tony Battersby <tonyb@cybernetics.com> wrote:
>
> Old versions of gcc cannot compile 4.14 since 4.14.113:
>
> ./include/asm-generic/fixmap.h:37: error: implicit declaration of function ‘__builtin_unreachable’
>
> The stable commit that caused the problem is 82017e26e515 ("compiler.h:
> update definition of unreachable()") (upstream commit fe0640eb30b7).
> Reverting the commit fixes the problem.
>
> Kernel 4.17 dropped support for older versions of gcc in upstream commit
> cafa0010cd51 ("Raise the minimum required gcc version to 4.6").  This
> was not backported to 4.14 since that would go against the stable kernel
> rules.
>
> Upstream commit 815f0ddb346c ("include/linux/compiler*.h: make
> compiler-*.h mutually exclusive") was a fix for cafa0010cd51.  This was
> not backported to 4.14.
>
> Upstream commit fe0640eb30b7 ("compiler.h: update definition of
> unreachable()") was a fix for 815f0ddb346c.  This is the commit that was
> backported to 4.14.  But it only fixed a problem introduced in the other
> commits, and without those commits, it ends up introducing a problem
> instead of fixing one.  So I recommend reverting that patch in 4.14,
> which will enable old gcc to compile 4.14 again.  If I understand
> correctly, I believe that clang will still be able to compile 4.14 with
> the patch reverted, although I haven't tried to compile with clang.
>
> The problematic commit is not present in 4.9.x, 4.4.x, 3.18.x, or 3.16.x.

CC: Nick Desaulniers <ndesaulniers@google.com>
CC: Tony Battersby <tonyb@cybernetics.com>,
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/compiler.h | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 67c3934fb9ed..a704d032713b 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -119,10 +119,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 # define ASM_UNREACHABLE
 #endif
 #ifndef unreachable
-# define unreachable() do {		\
-	annotate_unreachable();		\
-	__builtin_unreachable();	\
-} while (0)
+# define unreachable() do { annotate_reachable(); do { } while (1); } while (0)
 #endif
 
 /*
-- 
2.20.1