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

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Message-ID: <CAJWu+oqzd8MJqusRV0LAK=Xnm7VSRSu3QbNZ-j5h9_MbzcFhhg@mail.gmail.com>
Date:   Tue, 18 Jun 2019 09:40:28 -0400
From:   Joel Fernandes <joelaf@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Roman Penyaev <rpenyaev@...e.de>,
        Uladzislau Rezki <urezki@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, Tejun Heo <tj@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

On Tue, Jun 18, 2019 at 5:27 AM Arnd Bergmann <arnd@...db.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@...db.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