From: Nick Desaulniers <ndesaulniers@google.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Nathan Chancellor <natechancellor@gmail.com>
Subject: Re: [PATCH] waitqueue: fix clang -Wuninitialized warnings
Date: Fri, 12 Jul 2019 09:48:08 -0700 [thread overview]
Message-ID: <CAKwvOdkOG_fcJLc-2mFUUp9N1CrJ6EcnZjpraHsvfTsmiKX4SQ@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a2ZRw9B=X76yL-bRzC+01z6VaHDzPAhQQw7V9MXtkp+Jg@mail.gmail.com>
On Fri, Jul 12, 2019 at 12:45 AM Arnd Bergmann <arnd@arndb.de> 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.
So I think this is just a bug in Clang, where it's getting tripped up
due to the GNU C statement expression. See the example I put in this
bug report: https://bugs.llvm.org/show_bug.cgi?id=42604
Clang is warning for this pattern of struct assignment, but not for
non-aggregate (integral) assignment.
(I think that pattern is pretty cool; it makes it more straightforward
to provide macro's that properly construct aggregates in C; in
particular I feel like I've been looking for something like this to
simply the use of __attribute__((__cleanup__)) to do RAII in C and
make smart pointers, fd's, etc.).
Let's fix Clang, drop the kernel workaround, and thanks for the discussion.
--
Thanks,
~Nick Desaulniers
prev parent reply other threads:[~2019-07-12 16:48 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-03 8:10 [PATCH] waitqueue: fix clang -Wuninitialized warnings Arnd Bergmann
2019-07-03 17:58 ` Nathan Chancellor
2019-07-09 19:27 ` Arnd Bergmann
2019-07-12 7:28 ` Peter Zijlstra
2019-07-12 0:49 ` Andrew Morton
2019-07-12 7:45 ` Arnd Bergmann
2019-07-12 7:54 ` Nathan Chancellor
2019-07-12 14:50 ` Arnd Bergmann
2019-07-12 16:48 ` Nick Desaulniers [this message]
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