# git rev-parse -q --verify d936eb23874433caa3e3d841cfa16f5434b85dcf^{commit}
d936eb23874433caa3e3d841cfa16f5434b85dcf
already have revision, skipping fetch
# git checkout -q -f -B kisskb d936eb23874433caa3e3d841cfa16f5434b85dcf
# git clean -qxdf
# < git log -1
# commit d936eb23874433caa3e3d841cfa16f5434b85dcf
# Author: Linus Torvalds <torvalds@linux-foundation.org>
# Date:   Thu Jul 15 18:05:31 2021 -0700
# 
#     Revert "Makefile: Enable -Wimplicit-fallthrough for Clang"
#     
#     This reverts commit b7eb335e26a9c7f258c96b3962c283c379d3ede0.
#     
#     It turns out that the problem with the clang -Wimplicit-fallthrough
#     warning is not about the kernel source code, but about clang itself, and
#     that the warning is unusable until clang fixes its broken ways.
#     
#     In particular, when you enable this warning for clang, you not only get
#     warnings about implicit fallthroughs.  You also get this:
#     
#        warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
#     
#     which is completely broken becasue it
#     
#      (a) doesn't even tell you where the problem is (seriously: no line
#          numbers, no filename, no nothing).
#     
#      (b) is fundamentally broken anyway, because there are perfectly valid
#          reasons to have a fallthrough statement even if it turns out that
#          it can perhaps not be reached.
#     
#     In the kernel, an example of that second case is code in the scheduler:
#     
#                     switch (state) {
#                     case cpuset:
#                             if (IS_ENABLED(CONFIG_CPUSETS)) {
#                                     cpuset_cpus_allowed_fallback(p);
#                                     state = possible;
#                                     break;
#                             }
#                             fallthrough;
#                     case possible:
#     
#     where if CONFIG_CPUSETS is enabled you actually never hit the
#     fallthrough case at all.  But that in no way makes the fallthrough
#     wrong.
#     
#     So the warning is completely broken, and enabling it for clang is a very
#     bad idea.
#     
#     In the meantime, we can keep the gcc option enabled, and make the gcc
#     build use
#     
#         -Wimplicit-fallthrough=5
#     
#     which means that we will at least continue to require a proper
#     fallthrough statement, and that gcc won't silently accept the magic
#     comment versions. Because gcc does this all correctly, and while the odd
#     "=5" part is kind of obscure, it's documented in [1]:
#     
#       "-Wimplicit-fallthrough=5 doesn’t recognize any comments as
#        fallthrough comments, only attributes disable the warning"
#     
#     so if clang ever fixes its bad behavior we can try enabling it there again.
#     
#     Link: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html [1]
#     Cc: Kees Cook <keescook@chromium.org>
#     Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
#     Cc: Nathan Chancellor <nathan@kernel.org>
#     Cc: Nick Desaulniers <ndesaulniers@google.com>
#     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/kisskb/korg/gcc-4.9.4-nolibc/ia64-linux/bin/ia64-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-4.9.4-nolibc/ia64-linux/bin/ia64-linux-ld --version
# < git log --format=%s --max-count=1 d936eb23874433caa3e3d841cfa16f5434b85dcf
# < make -s -j 160 ARCH=ia64 O=/kisskb/build/linus_ia64-defconfig_ia64-gcc4.9 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-4.9.4-nolibc/ia64-linux/bin/ia64-linux-  defconfig
# < make -s -j 160 ARCH=ia64 O=/kisskb/build/linus_ia64-defconfig_ia64-gcc4.9 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-4.9.4-nolibc/ia64-linux/bin/ia64-linux-  help
# make -s -j 160 ARCH=ia64 O=/kisskb/build/linus_ia64-defconfig_ia64-gcc4.9 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-4.9.4-nolibc/ia64-linux/bin/ia64-linux-  olddefconfig
# make -s -j 160 ARCH=ia64 O=/kisskb/build/linus_ia64-defconfig_ia64-gcc4.9 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-4.9.4-nolibc/ia64-linux/bin/ia64-linux-  
<stdin>:1515:2: warning: #warning syscall clone3 not implemented [-Wcpp]
/kisskb/src/net/sched/sch_frag.c: In function 'sch_fragment':
/kisskb/src/net/sched/sch_frag.c:93:10: warning: missing braces around initializer [-Wmissing-braces]
   struct rtable sch_frag_rt = { 0 };
          ^
/kisskb/src/net/sched/sch_frag.c:93:10: warning: (near initialization for 'sch_frag_rt.dst') [-Wmissing-braces]
No errors detected in 22740 functions.
Completed OK
# rm -rf /kisskb/build/linus_ia64-defconfig_ia64-gcc4.9
# Build took: 0:00:49.478404