Re: linux-next: build warning after merge of the final tree (Linus'tree related - vai vfs tree)

[Olof Johansson]

On Mon, Sep 9, 2013 at 7:39 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Sun 08-09-13 20:21:54, Olof Johansson wrote:
>> On Fri, Sep 06, 2013 at 10:52:52AM +0200, Geert Uytterhoeven wrote:
>> > On Fri, Sep 6, 2013 at 9:19 AM, Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> wrote:
>> > > After merging the final tree, today's linux-next build (arm defconfig)
>> > > produced this warning:
>> > >
>> > > fs/direct-io.c: In function 'sb_init_dio_done_wq':
>> > > fs/direct-io.c:557:2: warning: value computed is not used [-Wunused-value]
>> > >
>> > > This is:
>> > >
>> > >         cmpxchg(&sb->s_dio_done_wq, NULL, wq);
>> > >
>> > > Introduced by commit 7b7a8665edd8 ("direct-io: Implement generic deferred
>> > > AIO completions").
>> >
>> > This happens for include/asm-generic/cmpxchg.h and several other
>> > arch-specific implementations that cast the return value of cmpxchg()
>> > like
>> >
>> > #define cmpxchg(ptr, o, n)   ((__typeof__(*(ptr)))__cmpxchg(....
>> >
>> > If the caller of cmpxchg() doesn't use the return value, we get a
>> > compiler warning,
>> > at least with some versions of gcc.
>> >
>> > Any idea how to fix this once and for good?
>>
>> Should it be fixed? Chances are that the caller needs to do actions
>> depending on if the change happened, and checking the value afterwards
>> is inherently racy.
>>
>> For this specific fs/direct-io.c case it seems to be safe since the
>> workqueue is only ever set and never cleared, but it might still be a
>> good idea to do:
>   I'm not against this change - feel free to add my:
> Reviewed-by: Jan Kara <jack@xxxxxxx>
>
>   to it and merge it. However I maintain there are valid usecases where you
> do not care about the return value so warning about it doesn't seem right.

Yes, similar to (some) __must_check-annotated functions.

> OTOH thinking about it some more I agree we have other precedents where
> sometimes-correct-often-bugs constructs are warned about and I can see how
> people can consider cmpxchg() to be that case. But in that case we should:
>   a) be consistent among architectures about the warning
>   b) comment at cmpxchg definition that you are supposed to check its
>      return value. If you really know what you are doing, you can cast the
>      return value to (void) and comment why it's safe.

Unfortunately cmpxchg() is a #define since it needs to use __typeof__.
Marking it __must_check isn't feasible. I'm open for better
suggestions.


-Olof
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