linux-kernel - Re: [PATCH] mm/mmap: replace if (cond) BUG() with BUG

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Message-ID: <e59e109d-0f46-c789-8e2c-52fc6d023a82@suse.cz>
Date:   Thu, 7 Jan 2021 18:45:12 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mmap: replace if (cond) BUG() with BUG_ON()

On 1/7/21 6:36 PM, Andrea Arcangeli wrote:
> 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, 

Oh, you're right, I got lost in the maze.

> and that is an infinite loop while(1), not an

And I overlooked that "1" too.

So that AFAICS means *both* VM_BUG_ON and VM_WARN_ON behave differently wrt
side-effects when disabled than BUG_ON and WARN_ON.

> 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
>