Re: [PATCH v2 0/5] kexec: use IS_ENABLED(CONFIG_KEXEC_CORE) instead of #ifdef

[Baoquan He]

On 01/18/22 at 10:13pm, Jisheng Zhang wrote:
> On Sun, Jan 16, 2022 at 09:38:47PM +0800, Baoquan He wrote:
> > Hi Jisheng,
> 
> Hi Baoquan,
> 
> > 
> > On 12/07/21 at 12:05am, Jisheng Zhang wrote:
> > > Replace the conditional compilation using "#ifdef CONFIG_KEXEC_CORE"
> > > by a check for "IS_ENABLED(CONFIG_KEXEC_CORE)", to simplify the code
> > > and increase compile coverage.
> > 
> > I go through this patchset, You mention the benefits it brings are
> > 1) simplity the code;
> > 2) increase compile coverage;
> > 
> > For benefit 1), it mainly removes the dummy function in x86, arm and
> > arm64, right?
> 
> Another benefit: remove those #ifdef #else #endif usage. Recently, I
> fixed a bug due to lots of "#ifdefs":
> http://lists.infradead.org/pipermail/linux-riscv/2021-December/010607.html

Glad to know the fix. While, sometime the ifdeffery is necessary. I am
sorry about the one in riscv and you have fixed, it's truly a bug . But,
the increasing compile coverage at below you tried to make, it may cause
issue. Please see below my comment.

> 
> > 
> > For benefit 2), increasing compile coverage, could you tell more how it
> > achieves and why it matters? What if people disables CONFIG_KEXEC_CORE in
> > purpose? Please forgive my poor compiling knowledge.
> 
> Just my humble opinion, let's compare the code::
> 
> #ifdef CONFIG_KEXEC_CORE
> 
> code block A;
> 
> #endif
> 
> If KEXEC_CORE is disabled, code block A won't be compiled at all, the
> preprocessor will remove code block A;
> 
> If we convert the code to:
> 
> if (IS_ENABLED(CONFIG_KEXEC_CORE)) {
> 	code block A;
> }
> 
> Even if KEXEC_CORE is disabled, code block A is still compiled.

This is what I am worried about. Before, if CONFIG_KEXEC_CORE is
unset, those relevant codes are not compiled in. I can't see what
benefit is brought in if compiled in the unneeded code block. Do I miss
anything?