Re: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

[Joel Fernandes <joelaf@google.com>]

On Tue, Jun 18, 2019 at 5:27 AM Arnd Bergmann <arnd@arndb.de> wrote:
>
> gcc gets confused in pcpu_get_vm_areas() because there are too many
> branches that affect whether 'lva' was initialized before it gets
> used:
>
> mm/vmalloc.c: In function 'pcpu_get_vm_areas':
> mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>     insert_vmap_area_augment(lva, &va->rb_node,
>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>      &free_vmap_area_root, &free_vmap_area_list);
>      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/vmalloc.c:916:20: note: 'lva' was declared here
>   struct vmap_area *lva;
>                     ^~~
>
> Add an intialization to NULL, and check whether this has changed
> before the first use.
>
> Fixes: 68ad4a330433 ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  mm/vmalloc.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index a9213fc3802d..42a6f795c3ee 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -913,7 +913,12 @@ adjust_va_to_fit_type(struct vmap_area *va,
>         unsigned long nva_start_addr, unsigned long size,
>         enum fit_type type)
>  {
> -       struct vmap_area *lva;
> +       /*
> +        * GCC cannot always keep track of whether this variable
> +        * was initialized across many branches, therefore set
> +        * it NULL here to avoid a warning.
> +        */
> +       struct vmap_area *lva = NULL;

Fair enough, but is this 5-line comment really needed here?

- Joel