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

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Message-ID: <CAFd5g474EYEj1BmqCv=xe6M9JW4L389xL2SU1Ak-evjmpGOvJg@mail.gmail.com>
Date:   Tue, 27 Aug 2019 15:16:56 -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 3:00 PM shuah <shuah@...nel.org> wrote:
>
> On 8/27/19 3:36 PM, Brendan Higgins wrote:
> > 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.
>
> Let me see if we are on the same page first. I am asking if you can
> just include printk.h for vprintk_emit() define for both CONFIG_PRINTK
> and !CONFIG_PRINTK cases.

Ah sorry, I misunderstood you.

No, that doesn't work. I tried including linux/printk.h, and I get the
same error.

The reason for this is that vprintk_emit() is only defined when CONFIG_PRINTK=y:

https://elixir.bootlin.com/linux/latest/ident/vprintk_emit

> I am not asking you to use printk() in place of vprintk_emit().
> It is perfectly fine to use vprintk_emit()

Okay, cool.

> >
> > So it appears that we have to weigh the following trade-offs:
> >
> > Using vprintk_emit:
> >
> > Pros:
> >   - That's what dev_printk uses.
>
> Not sure what you mean by this. I am suggesting if you can just
> call vprintk_emit() and include printk.h and not have to ifdef
> around all the other callers of kunit_vprintk_emit()

Oh, I was just saying that I heavily based my implementation of
kunit_printk on dev_printk. So I have a high degree of confidence that
it is okay to use it the way that I am using it.

> Yes. There is the other issue of why do you need the complexity
> of having kunit_vprintk_emit() at all.

Right, and the problem with the alternative, is there is no good
kernel API for logging with the log level set dynamically. printk
prefers to have it as a string prefix on the format string, but I
cannot do that because I need to add my own prefix to the format
string.

So, I guess I should just go ahead and address the earlier comments on
this patch?