On Thu, Apr 11, 2024, at 09:16, Adrian Hunter wrote: > On 11/04/24 10:04, Arnd Bergmann wrote: >> On Wed, Apr 10, 2024, at 17:32, Adrian Hunter wrote: >>> BUG() does not return, and arch implementations of BUG() use unreachable() >>> or other non-returning code. However with !CONFIG_BUG, the default >>> implementation is often used instead, and that does not do that. x86 always >>> uses its own implementation, but powerpc with !CONFIG_BUG gives a build >>> error: >>> >>> kernel/time/timekeeping.c: In function ‘timekeeping_debug_get_ns’: >>> kernel/time/timekeeping.c:286:1: error: no return statement in function >>> returning non-void [-Werror=return-type] >>> >>> Add unreachable() to default !CONFIG_BUG BUG() implementation. >> >> I'm a bit worried about this patch, since we have had problems >> with unreachable() inside of BUG() in the past, and as far as I >> can remember, the current version was the only one that >> actually did the right thing on all compilers. >> >> One problem with an unreachable() annotation here is that if >> a compiler misanalyses the endless loop, it can decide to >> throw out the entire code path leading up to it and just >> run into undefined behavior instead of printing a BUG() >> message. >> >> Do you know which compiler version show the warning above? > > Original report has a list > It looks like it's all versions of gcc, though no versions of clang show the warnings. I did a few more tests and could not find any differences on actual code generation, but I'd still feel more comfortable changing the caller than the BUG() macro. It's trivial to add a 'return 0' there. Another interesting observation is that clang-11 and earlier versions end up skipping the endless loop, both with and without the __builtin_unreachable, see https://godbolt.org/z/aqa9zqz8x clang-12 and above do work like gcc, so I guess that is good. Arnd
