[PATCH] mm: fix uninitialized variable warnings

[PATCH] mm: fix uninitialized variable warnings

[Arnd Bergmann]

In a rare randconfig build, I got a warning about possibly uninitialized
variables:

mm/page-writeback.c: In function 'balance_dirty_pages':
mm/page-writeback.c:1623:16: error: 'writeback' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    mdtc->dirty += writeback;
                ^~
mm/page-writeback.c:1624:4: error: 'filepages' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    mdtc_calc_avail(mdtc, filepages, headroom);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/page-writeback.c:1624:4: error: 'headroom' may be used uninitialized in this function [-Werror=maybe-uninitialized]

The compiler evidently fails to notice that the usage is in dead code
after 'mdtc' is set to NULL when CONFIG_CGROUP_WRITEBACK is disabled.
Adding an IS_ENABLED() check makes this clear to the compiler.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 mm/page-writeback.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 3f690bae6b78..f02535b7731a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1611,7 +1611,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 			bg_thresh = gdtc->bg_thresh;
 		}
 
-		if (mdtc) {
+		if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
 			unsigned long filepages, headroom, writeback;
 
 			/*
@@ -1944,7 +1944,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	    wb_calc_thresh(gdtc->wb, gdtc->bg_thresh))
 		return true;
 
-	if (mdtc) {
+	if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
 		unsigned long filepages, headroom, writeback;
 
 		mem_cgroup_wb_stats(wb, &filepages, &headroom, &mdtc->dirty,
-- 
2.18.0

Re: [PATCH] mm: fix uninitialized variable warnings

[Jan Kara]

On Fri 02-11-18 16:31:06, Arnd Bergmann wrote:
> In a rare randconfig build, I got a warning about possibly uninitialized
> variables:
> 
> mm/page-writeback.c: In function 'balance_dirty_pages':
> mm/page-writeback.c:1623:16: error: 'writeback' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>     mdtc->dirty += writeback;
>                 ^~
> mm/page-writeback.c:1624:4: error: 'filepages' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>     mdtc_calc_avail(mdtc, filepages, headroom);
>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/page-writeback.c:1624:4: error: 'headroom' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> 
> The compiler evidently fails to notice that the usage is in dead code
> after 'mdtc' is set to NULL when CONFIG_CGROUP_WRITEBACK is disabled.
> Adding an IS_ENABLED() check makes this clear to the compiler.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

I'm surprised the compiler was not able to infer this since:

struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
                                                     &mdtc_stor : NULL;

and if CONFIG_CGROUP_WRITEBACK is disabled, mdtc_valid() is defined to
'false'.  But possibly the function is just too big and the problematic
condition is in the loop so maybe it all confuses the compiler too much.

> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3f690bae6b78..f02535b7731a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1611,7 +1611,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  			bg_thresh = gdtc->bg_thresh;
>  		}
>  
> -		if (mdtc) {
> +		if (IS_ENABLED(CONFIG_CGROUP_WRITEBACK) && mdtc) {
>  			unsigned long filepages, headroom, writeback;

Honestly, I don't like the IS_ENABLED(CONFIG_CGROUP_WRITEBACK) check here.
It just looks too arbitrary. Could we perhaps change the code like

struct dirty_throttle_control * const mdtc = &mdtc_stor;

And then replace checks for !mtdc in the function to !mdtc_valid(mdtc)?
That is the same thing as currently and it should make it obvious to the
compiler as well as human what is going on... Tejun?

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] mm: fix uninitialized variable warnings

[Arnd Bergmann]

On 11/5/18, Jan Kara <jack@suse.cz> wrote:
> On Fri 02-11-18 16:31:06, Arnd Bergmann wrote:
>> In a rare randconfig build, I got a warning about possibly uninitialized
>> variables:
>>
>> mm/page-writeback.c: In function 'balance_dirty_pages':
>> mm/page-writeback.c:1623:16: error: 'writeback' may be used uninitialized
>> in this function [-Werror=maybe-uninitialized]
>>     mdtc->dirty += writeback;
>>                 ^~
>> mm/page-writeback.c:1624:4: error: 'filepages' may be used uninitialized
>> in this function [-Werror=maybe-uninitialized]
>>     mdtc_calc_avail(mdtc, filepages, headroom);
>>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> mm/page-writeback.c:1624:4: error: 'headroom' may be used uninitialized in
>> this function [-Werror=maybe-uninitialized]
>>
>> The compiler evidently fails to notice that the usage is in dead code
>> after 'mdtc' is set to NULL when CONFIG_CGROUP_WRITEBACK is disabled.
>> Adding an IS_ENABLED() check makes this clear to the compiler.
>>
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>
> I'm surprised the compiler was not able to infer this since:
>
> struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
>                                                      &mdtc_stor : NULL;
>
> and if CONFIG_CGROUP_WRITEBACK is disabled, mdtc_valid() is defined to
> 'false'.  But possibly the function is just too big and the problematic
> condition is in the loop so maybe it all confuses the compiler too much.

On second thought, I suspect this started with the introduction of
CONFIG_NO_AUTO_INLINE in linux-next. That also caused a similar
issue in 28 other files that I patched later. I wrote this patch before I
saw the others, and then didn't make the connection.

Let's drop the patch for now, and decide what we want to do for the
others. I fixed those by adding 'inline' markers for whatever
function needed it.

       Arnd