Re: [PATCH] waitqueue: fix clang -Wuninitialized warnings

[Nathan Chancellor <natechancellor@gmail.com>]

On Fri, Jul 12, 2019 at 09:45:06AM +0200, Arnd Bergmann wrote:
> On Fri, Jul 12, 2019 at 2:49 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> > On Wed,  3 Jul 2019 10:10:55 +0200 Arnd Bergmann <arnd@arndb.de> wrote:
> 
> > <scratches head>
> >
> > Surely clang is being extraordinarily dumb here?
> >
> > DECLARE_WAIT_QUEUE_HEAD_ONSTACK() is effectively doing
> >
> >         struct wait_queue_head name = ({ __init_waitqueue_head(&name) ; name; })
> >
> > which is perfectly legitimate!  clang has no business assuming that
> > __init_waitqueue_head() will do any reads from the pointer which it was
> > passed, nor can clang assume that __init_waitqueue_head() leaves any of
> > *name uninitialized.
> >
> > Does it also warn if code does this?
> >
> >         struct wait_queue_head name;
> >         __init_waitqueue_head(&name);
> >         name = name;
> >
> > which is equivalent, isn't it?
> 
> No, it does not warn for this.
> 
> I've tried a few more variants here: https://godbolt.org/z/ykSX0r
> 
> What I think is going on here is a result of clang and gcc fundamentally
> treating -Wuninitialized warnings differently. gcc tries to make the warnings
> as helpful as possible, but given the NP-complete nature of this problem
> it won't always get it right, and it traditionally allowed this syntax as a
> workaround.
> 
> int f(void)
> {
>     int i = i; // tell gcc not to warn
>     return i;
> }
> 
> clang apparently implements the warnings in a way that is as
> completely predictable (and won't warn in cases that it
> doesn't completely understand), but decided as a result that the
> gcc 'int i = i' syntax is bogus and it always warns about a variable
> used in its own declaration that is later referenced, without looking
> at whether the declaration does initialize it or not.
> 
> > The proposed solution is, effectively, to open-code
> > __init_waitqueue_head() at each DECLARE_WAIT_QUEUE_HEAD_ONSTACK()
> > callsite.  That's pretty unpleasant and calls for an explanatory
> > comment at the __WAIT_QUEUE_HEAD_INIT_ONSTACK() definition site as well
> > as a cautionary comment at the __init_waitqueue_head() definition so we
> > can keep the two versions in sync as code evolves.
> 
> Yes, makes sense.
> 
> > Hopefully clang will soon be hit with the cluebat (yes?) and this
> > change becomes obsolete in the quite short term.  Surely 6-12 months
> > from now nobody will be using the uncluebatted version of clang on
> > contemporary kernel sources so we get to remove this nastiness again.
> > Which makes me wonder whether we should merge it at all.
> 
> Would it make you feel better to keep the current code but have an alternative
> version guarded with e.g. "#if defined(__clang__ && (__clang_major__ <= 9)"?
> 
> While it is probably a good idea to fix clang here, this is one of the last
> issues that causes a significant difference between gcc and clang in build
> testing with kernelci:
> https://kernelci.org/build/next/branch/master/kernel/next-20190709/
> I'm trying to get all the warnings fixed there so we can spot build-time
> regressions more easily.
> 
>       Arnd

I'm just spitballing here since I am about to go to sleep but could we
do something like you did for bee20031772a ("disable -Wattribute-alias
warning for SYSCALL_DEFINEx()") and disable the warning in
DECLARE_WAIT_QUEUE_HEAD_ONSTACK only since we know it is not going to
be a problem? That way, if/when Clang is fixed, we can just have the
warning be disabled for older versions?

Cheers,
Nathan