Re: [PATCH 4.9] dm bufio: avoid false-positive Wmaybe-uninitialized warning

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Jun 11, 2018 at 09:05:33AM -0700, Nathan Chancellor wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> commit 590347e4000356f55eb10b03ced2686bd74dab40 upstream.
> 
> gcc-6.3 and earlier show a new warning after a seemingly unrelated
> change to the arm64 PAGE_KERNEL definition:
> 
> In file included from drivers/md/dm-bufio.c:14:0:
> drivers/md/dm-bufio.c: In function 'alloc_buffer':
> include/linux/sched/mm.h:182:56: warning: 'noio_flag' may be used uninitialized in this function [-Wmaybe-uninitialized]
>   current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flags;
>                                                         ^
> 
> The same warning happened earlier on linux-3.18 for MIPS and I did a
> workaround for that, but now it's come back.
> 
> gcc-7 and newer are apparently smart enough to figure this out, and
> other architectures don't show it, so the best I could come up with is
> to rework the caller slightly in a way that makes it obvious enough to
> all arm64 compilers what is happening here.
> 
> Fixes: 41acec624087 ("arm64: kpti: Make use of nG dependent on arm64_kernel_unmapped_at_el0()")
> Link: https://patchwork.kernel.org/patch/9692829/
> Cc: stable@vger.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> [snitzer: moved declarations inside conditional, altered vmalloc return]
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> [nc: Backport to 4.9, adjust context for lack of 19809c2da28a]
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
> 
> Hi Greg,
> 
> Resending this with a proper prefix and message, in case the other one
> fell through the cracks. I came across this warning when building with
> Google's stock GCC 4.9 toolchain on the OnePlus 6. Context was adjusted
> around lack of commit 19809c2da28a ("mm, vmalloc: use __GFP_HIGHMEM
> implicitly") in 4.9. Please apply when you get a chance.

Now queued up, thanks.  The other one was still in my queue, give me a
chance to catch up :)

greg k-h