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

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Date:   Tue, 17 Nov 2020 11:31:47 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        linux-kernel@...r.kernel.org,
        Mikulas Patocka <mpatocka@...hat.com>,
        Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, linux-s390@...r.kernel.org
Subject: Re: md: dm-writeback: add __noreturn to BUG-ging function

On Mon, Nov 16 2020 at  6:00pm -0500,
Randy Dunlap <rdunlap@...radead.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).