linux-kernel - Re: [PATCH] mm: thp: avoid uninitialized variable use

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Message-ID: <8d5476e2-5f87-1134-62d4-9f649c4e709a@alibaba-inc.com>
Date:   Sat, 16 Dec 2017 02:01:08 +0800
From:   "Yang Shi" <yang.s@...baba-inc.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Ingo Molnar <mingo@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: avoid uninitialized variable use



On 12/15/17 4:51 AM, Arnd Bergmann wrote:
> When the down_read_trylock() fails, 'vma' has not been initialized
> yet, which gcc now warns about:
> 
> mm/khugepaged.c: In function 'khugepaged':
> mm/khugepaged.c:1659:25: error: 'vma' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Arnd,

Thanks for catching this. I'm wondering why my test didn't catch it. It 
might be because my gcc is old. I'm using gcc 4.8.5 on centos 7.

Regards,
Yang

> 
> Presumable we are not supposed to call find_vma() without the mmap_sem
> either, so setting it to NULL for this case seems appropriate.
> 
> Fixes: 0951b59acf3a ("mm: thp: use down_read_trylock() in khugepaged to avoid long block")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> I'm not completely sure this patch is sufficient, it gets rid of
> the warning, but it would be good to have the code reviewed better
> to see if other problems remain that result from down_read_trylock()
> patch.
> ---
>   mm/khugepaged.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 521b908f9600..b7e2268dfc9a 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1677,11 +1677,10 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
>   	 * Don't wait for semaphore (to avoid long wait times).  Just move to
>   	 * the next mm on the list.
>   	 */
> +	vma = NULL;
>   	if (unlikely(!down_read_trylock(&mm->mmap_sem)))
>   		goto breakouterloop_mmap_sem;
> -	if (unlikely(khugepaged_test_exit(mm)))
> -		vma = NULL;
> -	else
> +	if (likely(!khugepaged_test_exit(mm)))
>   		vma = find_vma(mm, khugepaged_scan.address);
>   
>   	progress++;
>