Messages in this thread | | | Date | Mon, 9 Sep 2013 09:45:52 -0700 | Subject | Re: linux-next: build warning after merge of the final tree (Linus' tree related - vai vfs tree) | From | Olof Johansson <> |
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On Mon, Sep 9, 2013 at 7:39 AM, Jan Kara <jack@suse.cz> 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@canb.auug.org.au> 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@suse.cz> > > 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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