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?
WARN_ON is different as the result of the "inner" condition should be further
usable for constructing "outer" conditions:
(still in !CONFIG_BUG section)
#ifndef HAVE_ARCH_WARN_ON
#define WARN_ON(condition) ({
int __ret_warn_on = !!(condition);
unlikely(__ret_warn_on);
})
#endif
For completeness let's look at our own extensions when VM_DEBUG is disabled,
which is quite analogical to disabling CONFIG_BUG and thus it should better be
consistent with the generic stuff.
#define VM_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
where BUILD_BUG_ON_INVALID generates no code, so it's consistent with BUG_ON()
and !CONFIG_BUG.
#define VM_WARN_ON(cond) BUILD_BUG_ON_INVALID(cond)
... well that's not consistent with WARN_ON. Hmm if you have asked me before I
checked, I would have said that it is, that I checked it already in the past
and/or there was some discussion already about it. Memory is failing me it
seems. We should better fix this?
