Messages in this thread | | | From | Vlastimil Babka <> | Subject | Re: [PATCH] mm/mmap: replace if (cond) BUG() with BUG_ON() | Date | Thu, 7 Jan 2021 18:28:29 +0100 |
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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?
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