linux-kernel - Re: [PATCH v2] shmem: avoid maybe-uninitialized warning

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Date:   Tue, 25 Oct 2016 10:56:28 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Arnd Bergmann <arnd@...db.de>, Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Andreas Gruenbacher <agruenba@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] shmem: avoid maybe-uninitialized warning

On 10/24/2016 10:57 PM, 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]
>
> 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 and lets us remove two later initializations.
>
> Note that the function is so hard to read that it not only confuses
> the compiler, but also most readers and without this patch it could\
> easily break if one of the 'goto's changed.
>
> Link: https://www.spinics.net/lists/kernel/msg2368133.html
> Acked-by: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/shmem.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index ad7813d73ea7..95c4bb690f98 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;
>
> @@ -1835,7 +1834,6 @@ alloc_nohuge:		page = shmem_alloc_and_acct_page(gfp, info, sbinfo,
>  		put_page(page);
>  	}
>  	if (error == -ENOSPC && !once++) {
> -		info = SHMEM_I(inode);
>  		spin_lock_irq(&info->lock);
>  		shmem_recalc_inode(inode);
>  		spin_unlock_irq(&info->lock);
>