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

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Date:   Sat, 19 Aug 2017 15:34:01 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Boqun Feng <boqun.feng@...il.com>
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 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?
>
> 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.

        Arnd