linux-kernel - Re: [PATCH] shmem: avoid maybe-uninitialized warning

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Date:   Mon, 24 Oct 2016 18:22:44 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] shmem: avoid maybe-uninitialized warning

On Mon 24-10-16 17:25:03, Arnd Bergmann wrote:
> After enabling -Wmaybe-uninitialized warnings, we get a false-postive
> warning for shmem:
> 
> mm/shmem.c: In function ‘shmem_getpage_gfp’:
> include/linux/spinlock.h:332:21: error: ‘info’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

Is this really a false positive? If we goto clear and then 
        if (sgp <= SGP_CACHE &&
            ((loff_t)index << PAGE_SHIFT) >= i_size_read(inode)) {
                if (alloced) {

we could really take a spinlock on an unitialized variable. But maybe
there is something that prevents from that... Anyway the whole
shmem_getpage_gfp is really hard to follow due to gotos and labels
proliferation.

> This can be easily avoided, since the correct 'info' pointer is known
> at the time we first enter the function, so we can simply move the
> initialization up. Moving it before the first label avoids the
> warning.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Looks good to me.
Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/shmem.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index ad7813d73ea7..69e6777096a3 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1537,7 +1537,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
>  	struct mm_struct *fault_mm, int *fault_type)
>  {
>  	struct address_space *mapping = inode->i_mapping;
> -	struct shmem_inode_info *info;
> +	struct shmem_inode_info *info = SHMEM_I(inode);
>  	struct shmem_sb_info *sbinfo;
>  	struct mm_struct *charge_mm;
>  	struct mem_cgroup *memcg;
> @@ -1587,7 +1587,6 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
>  	 * Fast cache lookup did not find it:
>  	 * bring it back from swap or allocate.
>  	 */
> -	info = SHMEM_I(inode);
>  	sbinfo = SHMEM_SB(inode->i_sb);
>  	charge_mm = fault_mm ? : current->mm;
>  
> -- 
> 2.9.0
> 

-- 
Michal Hocko
SUSE Labs