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

[Olof Johansson]

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:


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