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

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Date:   Fri, 20 Nov 2020 09:08:20 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-kernel@...r.kernel.org, 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 Wed, Nov 18 2020 at  4:24pm -0500,
Mikulas Patocka <mpatocka@...hat.com> wrote:

> 
> 
> On Wed, 18 Nov 2020, Mike Snitzer wrote:
> 
> > On Wed, Nov 18 2020 at 10:49am -0500,
> > Mike Snitzer <snitzer@...hat.com> wrote:
> > 
> > > 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().
> > 
> > Decided to go with this, now staged for 5.11:
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.11&id=a1e4865b4dda7071f3707f7e551289ead66e38b1
> 
> Hi
> 
> I would just use "return -EOPNOTSUPP;" and drop the "#ifdef 
> DM_WRITECACHE_HAS_PMEM" that you added.
> 
> That BUG/return -EOPNOTSUPP code can't happen at all - if 
> DM_WRITECACHE_HAS_PMEM is not defined, WC_MODE_PMEM(wc) always returns 
> false - so persistent_memory_claim and BUG() can't ever be called. And if 
> it can't be called, you don't need to add a code that prints an error in 
> that case.
> 
> If we don't have DM_WRITECACHE_HAS_PMEM, the compiler optimizer will 
> remove all the code guarded with if (WC_MODE_PMEM(wc)) as unreachable.
> 
> Mikulas

Fair enough.