linux-kernel - Re: [PATCH v8 09/14] lockdep: Apply crossrelease to completions

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Message-ID: <20170823144140.GK11771@tardis>
Date:   Wed, 23 Aug 2017 22:43:37 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Byungchul Park <byungchul.park@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michel Lespinasse <walken@...gle.com>, kirill@...temov.name,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>, willy@...radead.org,
        Nicholas Piggin <npiggin@...il.com>, kernel-team@....com
Subject: Re: [PATCH v8 09/14] lockdep: Apply crossrelease to completions

On Sat, Aug 19, 2017 at 03:34:01PM +0200, Arnd Bergmann wrote:
> On Sat, Aug 19, 2017 at 2:51 PM, Arnd Bergmann <arnd@...db.de> wrote:
> 
> >> --- a/include/linux/completion.h
> >> +++ b/include/linux/completion.h
> >> @@ -74,7 +74,7 @@ static inline void complete_release_commit(struct completion *x) {}
> >>  #endif
> >>
> >>  #define COMPLETION_INITIALIZER_ONSTACK(work) \
> >> -       ({ init_completion(&work); work; })
> >> +       (*({ init_completion(&work); &work; }))
> >>
> >>  /**
> >>   * DECLARE_COMPLETION - declare and initialize a completion structure
> >
> > Nice hack. Any idea why that's different to the compiler?
> >

So I find this link:

	https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

it says:

"In G++, the result value of a statement expression undergoes array and
function pointer decay, and is returned by value to the enclosing
expression. "

I think this is why the temporary variable is constructed(or at least
allocated). Lemme put this in my commit log.

> > I've applied that one to my test tree now, and reverted my own patch,
> > will let you know if anything else shows up. I think we probably want
> > to merge both patches to mainline.
> 
> There is apparently one user of COMPLETION_INITIALIZER_ONSTACK
> that causes a regression with the patch above:
> 
> drivers/acpi/nfit/core.c: In function 'acpi_nfit_flush_probe':
> include/linux/completion.h:77:3: error: value computed is not used
> [-Werror=unused-value]
>   (*({ init_completion(&work); &work; }))
> 
> It would be trivial to convert to init_completion(), which seems to be
> what was intended there.
> 

Thanks. Will send the conversion as a separate patch along with my
patch.

Regards,
Boqun

>         Arnd

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