Re: md: dm-writeback: add __noreturn to BUG-ging function

Mike Snitzer
2020-11-17T16:31:47Z

On Mon, Nov 16 2020 at 6:00pm -0500,
Randy Dunlap <rdunlap@infradead.org> wrote:

> On 11/15/20 11:30 PM, Christian Borntraeger wrote:
> >
> >
> > On 13.11.20 23:52, Randy Dunlap wrote:
> >> Building on arch/s390/ flags this as an error, so add the
> >> __noreturn attribute modifier to prevent the build error.
> >>
> >> cc1: some warnings being treated as errors
> >> ../drivers/md/dm-writecache.c: In function 'persistent_memory_claim':
> >> ../drivers/md/dm-writecache.c:323:1: error: no return statement in function returning non-void [-Werror=return-type]
> >
> > ok with me, but I am asking why
> >
> > the unreachable macro is not good enough. For x86 it obviously is.
> >
> > form arch/s390/include/asm/bug.h
> > #define BUG() do { \
> > __EMIT_BUG(0); \
> > unreachable(); \
> > } while (0)
> >
>
> Hi Christian,
>
> Good question.
> I don't see any guidance about when to use one or the other etc.
>
> I see __noreturn being used 109 times and unreachable();
> being used 33 times, but only now that I look at them.
> That had nothing to do with why I used __noreturn in the patch.

But doesn't that speak to the proper fix being needed in unreachable()?
Or at a minimum the fix is needed to arch/s390/include/asm/bug.h's BUG.

I really don't think we should be papering over that by sprinkling
__noreturn around the kernel's BUG() callers.

Maybe switch arch/s390/include/asm/bug.h's BUG to be like
arch/mips/include/asm/bug.h? It itself uses __noreturn with a 'static
inline' function definition rather than #define.

Does that fix the issue?

Thanks,
Mike

p.s. you modified dm-writecache.c (not dm-writeback, wich doesn't
exist).