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Date:   Thu, 7 Jan 2021 12:36:04 -0500
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alex Shi <alex.shi@linux.alibaba.com>,
        Minchan Kim <minchan@kernel.org>,
        Michal Hocko <mhocko@suse.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/mmap: replace if (cond) BUG() with BUG_ON()
Message-ID: <X/dGhD/R8r5yeElq@redhat.com>
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On Thu, Jan 07, 2021 at 06:28:29PM +0100, Vlastimil Babka wrote:
> On 1/6/21 9:18 PM, Hugh Dickins wrote:
> > On Wed, 6 Jan 2021, Andrea Arcangeli wrote:
> >> 
> >> I'd be surprised if the kernel can boot with BUG_ON() defined as "do
> >> {}while(0)" so I guess it doesn't make any difference.
> > 
> > I had been afraid of that too, when CONFIG_BUG is not set:
> > but I think it's actually "if (cond) do {} while (0)".
> 
> It's a maze of configs and arch-specific vs generic headers, but I do see this
> in include/asm-generic/bug.h:
> 
> #else /* !CONFIG_BUG */
> #ifndef HAVE_ARCH_BUG
> #define BUG() do {} while (1)
> #endif
> 
> So seems to me there *are* configurations possible where side-effects are indeed
> thrown away, right?

But this not BUG_ON, and that is an infinite loop while(1), not an
optimization away as in while (0) that I was suggesting to just throw
away cond and make it a noop. BUG() is actually the thing to use to
move functional stuff out of BUG_ON so it's not going to be causing
issues if it just loops.

This overall feels mostly an aesthetically issue.

Thanks,
Andrea