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[PATCH v2] shmem: avoid maybe-uninitialized warning

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  [PATCH v2] shmem: avoid maybe-uninitialized warning Arnd Bergmann <arnd@arndb.de> - 2016-10-24 23:00 +0200
    Re: [PATCH v2] shmem: avoid maybe-uninitialized warning "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-10-25 01:50 +0200
    Re: [PATCH v2] shmem: avoid maybe-uninitialized warning Vlastimil Babka <vbabka@suse.cz> - 2016-10-25 11:00 +0200

#1507730 — [PATCH v2] shmem: avoid maybe-uninitialized warning

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@suse.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 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);
-- 
2.9.0

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#1507833

On Mon, Oct 24, 2016 at 10:57:09PM +0200, 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@suse.com>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov

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#1508084

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@suse.com>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Vlastimil Babka <vbabka@suse.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);
>

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