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

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Message-ID: <20201118154944.GB545@redhat.com>
Date:   Wed, 18 Nov 2020 10:49:44 -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 Tue, Nov 17 2020 at 11:31am -0500,
Mike Snitzer <snitzer@...hat.com> wrote:

> 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).

I don't think my suggestion will help.. given it'd still leave
persistent_memory_claim() without a return statement.

Think it worthwhile to just add a dummy 'return 0;' after the BUG().

Mike