linux-kernel - Re: [PATCH v1] kunit: fix failure to build without printk

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Message-ID: <CAFd5g44A5vtgxBYErP4mCGwDgOHEMYUXCDSF=d9bYB3ktpW5jA@mail.gmail.com>
Date:   Tue, 27 Aug 2019 14:36:49 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     shuah <shuah@...nel.org>
Cc:     kunit-dev@...glegroups.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH v1] kunit: fix failure to build without printk

On Tue, Aug 27, 2019 at 2:09 PM Brendan Higgins
<brendanhiggins@...gle.com> wrote:
>
> On Tue, Aug 27, 2019 at 2:03 PM Brendan Higgins
> <brendanhiggins@...gle.com> wrote:
> >
> > On Tue, Aug 27, 2019 at 1:21 PM shuah <shuah@...nel.org> wrote:
> > >
> > > On 8/27/19 11:49 AM, Brendan Higgins wrote:
> > > > Previously KUnit assumed that printk would always be present, which is
> > > > not a valid assumption to make. Fix that by ifdefing out functions which
> > > > directly depend on printk core functions similar to what dev_printk
> > > > does.
> > > >
> > > > Reported-by: Randy Dunlap <rdunlap@...radead.org>
> > > > Link: https://lore.kernel.org/linux-kselftest/0352fae9-564f-4a97-715a-fabe016259df@kernel.org/T/#t
> > > > Cc: Stephen Rothwell <sfr@...b.auug.org.au>
> > > > Signed-off-by: Brendan Higgins <brendanhiggins@...gle.com>
> > > > ---
> > > >   include/kunit/test.h |  7 +++++++
> > > >   kunit/test.c         | 41 ++++++++++++++++++++++++-----------------
> > > >   2 files changed, 31 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/include/kunit/test.h b/include/kunit/test.h
> > > > index 8b7eb03d4971..339af5f95c4a 100644
> > > > --- a/include/kunit/test.h
> > > > +++ b/include/kunit/test.h
> > > > @@ -339,9 +339,16 @@ static inline void *kunit_kzalloc(struct kunit *test, size_t size, gfp_t gfp)
> > [...]
> > > Okay after reviewing this, I am not sure why you need to do all
> > > this.
> > >
> > > Why can't you just change the root function that throws the warn:
> > >
> > >   static int kunit_vprintk_emit(int level, const char *fmt, va_list args)
> > > {
> > >          return vprintk_emit(0, level, NULL, 0, fmt, args);
> > > }
> > >
> > > You aren'r really doing anything extra here, other than calling
> > > vprintk_emit()
> >
> > Yeah, I did that a while ago. I think it was a combination of trying
> > to avoid an extra layer of adding and then removing the log level, and
> > that's what dev_printk and friends did.
> >
> > But I think you are probably right. It's a lot of maintenance overhead
> > to get rid of that. Probably best to just use what the printk people
> > have.
> >
> > > Unless I am missing something, can't you solve this problem by including
> > > printk.h and let it handle the !CONFIG_PRINTK case?
> >
> > Randy, I hope you don't mind, but I am going to ask you to re-ack my
> > next revision since it basically addresses the problem in a totally
> > different way.
>
> Actually, scratch that. Checkpatch doesn't like me calling printk
> without using a KERN_<LEVEL>.
>
> Now that I am thinking back to when I wrote this. I think it also
> might not like using a dynamic KERN_<LEVEL> either (printk("%s my
> message", KERN_INFO)).
>
> I am going to have to do some more investigation.

Alright, I am pretty sure it is safe to do printk("%smessage", KERN_<LEVEL>);

Looking at the printk implementation, it appears to do the format
before it checks the log level:

https://elixir.bootlin.com/linux/v5.2.10/source/kernel/printk/printk.c#L1907

So I am pretty sure we can do it either with the vprintk_emit or with printk.

So it appears that we have to weigh the following trade-offs:

Using vprintk_emit:

Pros:
 - That's what dev_printk uses.
 - No checkpatch warnings.

Cons:
 - Harder to maintain (requires ifdefery).

Using printk:

Pros:
 - Easier to maintain.

Cons:
 - I am less confident that it is correct (I am not 100% certain that
printk is intended to be used this way).
 - Checkpatch warnings.
 - Extra layer of string formatting.

My preference is to go the vprintk_emit route since I am more
confident that it is right, but I don't have a strong preference.