nvdimm: avoid bogus -Wmaybe-uninitialized warning
diff mbox

Message ID 20170801114926.1171418-1-arnd@arndb.de
State New, archived
Headers show

Commit Message

Arnd Bergmann Aug. 1, 2017, 11:48 a.m. UTC
Removing the btt_rw_page/pmem_rw_page functions had a surprising
side-effect of introducing a false-positive warning in another
function, due to changed inlining decisions in gcc:

In file included from drivers/nvdimm/pmem.c:36:0:
drivers/nvdimm/pmem.c: In function 'pmem_make_request':
drivers/nvdimm/nd.h:407:2: error: 'start' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/nvdimm/pmem.c:174:16: note: 'start' was declared here
In file included from drivers/nvdimm/btt.c:27:0:
drivers/nvdimm/btt.c: In function 'btt_make_request':
drivers/nvdimm/nd.h:407:2: error: 'start' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/nvdimm/btt.c:1202:16: note: 'start' was declared here

The problem is that gcc fails to track the value of the 'do_acct'
variable here and has to read it back from stack, but it does
remember that 'start' may be uninitialized sometimes.

This shuts up the warning by making nd_iostat_start() always
initialize the 'start' variable. In those cases that gcc successfully
tracks the state of the variable, this will have no effect.

Fixes: 503a5e89b1de ("drivers/nvdimm/btt.c: remove btt_rw_page()")
Fixes: 58100d6e735e ("drivers/nvdimm/pmem.c: remove pmem_rw_page()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/nvdimm/nd.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Comments

Ross Zwisler Aug. 1, 2017, 6 p.m. UTC | #1
On Tue, Aug 01, 2017 at 01:48:48PM +0200, Arnd Bergmann wrote:
> Removing the btt_rw_page/pmem_rw_page functions had a surprising
> side-effect of introducing a false-positive warning in another
> function, due to changed inlining decisions in gcc:
> 
> In file included from drivers/nvdimm/pmem.c:36:0:
> drivers/nvdimm/pmem.c: In function 'pmem_make_request':
> drivers/nvdimm/nd.h:407:2: error: 'start' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> drivers/nvdimm/pmem.c:174:16: note: 'start' was declared here
> In file included from drivers/nvdimm/btt.c:27:0:
> drivers/nvdimm/btt.c: In function 'btt_make_request':
> drivers/nvdimm/nd.h:407:2: error: 'start' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> drivers/nvdimm/btt.c:1202:16: note: 'start' was declared here
> 
> The problem is that gcc fails to track the value of the 'do_acct'
> variable here and has to read it back from stack, but it does
> remember that 'start' may be uninitialized sometimes.
> 
> This shuts up the warning by making nd_iostat_start() always
> initialize the 'start' variable. In those cases that gcc successfully
> tracks the state of the variable, this will have no effect.
> 
> Fixes: 503a5e89b1de ("drivers/nvdimm/btt.c: remove btt_rw_page()")
> Fixes: 58100d6e735e ("drivers/nvdimm/pmem.c: remove pmem_rw_page()")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

This change looks fine:

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>

I believe the patches removing the btt_rw_page() and btt_rw_page() are on hold
until I can get some performance numbers to justify them.

Dan, do you want to take this as is, or do you want me to include it in my
larger rw_page() series if/when that gets revived?
Dan Williams Aug. 1, 2017, 6:07 p.m. UTC | #2
On Tue, Aug 1, 2017 at 11:00 AM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> On Tue, Aug 01, 2017 at 01:48:48PM +0200, Arnd Bergmann wrote:
>> Removing the btt_rw_page/pmem_rw_page functions had a surprising
>> side-effect of introducing a false-positive warning in another
>> function, due to changed inlining decisions in gcc:
>>
>> In file included from drivers/nvdimm/pmem.c:36:0:
>> drivers/nvdimm/pmem.c: In function 'pmem_make_request':
>> drivers/nvdimm/nd.h:407:2: error: 'start' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>> drivers/nvdimm/pmem.c:174:16: note: 'start' was declared here
>> In file included from drivers/nvdimm/btt.c:27:0:
>> drivers/nvdimm/btt.c: In function 'btt_make_request':
>> drivers/nvdimm/nd.h:407:2: error: 'start' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>> drivers/nvdimm/btt.c:1202:16: note: 'start' was declared here
>>
>> The problem is that gcc fails to track the value of the 'do_acct'
>> variable here and has to read it back from stack, but it does
>> remember that 'start' may be uninitialized sometimes.
>>
>> This shuts up the warning by making nd_iostat_start() always
>> initialize the 'start' variable. In those cases that gcc successfully
>> tracks the state of the variable, this will have no effect.
>>
>> Fixes: 503a5e89b1de ("drivers/nvdimm/btt.c: remove btt_rw_page()")
>> Fixes: 58100d6e735e ("drivers/nvdimm/pmem.c: remove pmem_rw_page()")
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>
> This change looks fine:
>
> Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
>
> I believe the patches removing the btt_rw_page() and btt_rw_page() are on hold
> until I can get some performance numbers to justify them.
>
> Dan, do you want to take this as is, or do you want me to include it in my
> larger rw_page() series if/when that gets revived?

I'd say include it with your set.

Patch
diff mbox

diff --git a/drivers/nvdimm/nd.h b/drivers/nvdimm/nd.h
index e1b5715bd91f..64f79a156456 100644
--- a/drivers/nvdimm/nd.h
+++ b/drivers/nvdimm/nd.h
@@ -392,8 +392,10 @@  static inline bool nd_iostat_start(struct bio *bio, unsigned long *start)
 {
 	struct gendisk *disk = bio->bi_bdev->bd_disk;
 
-	if (!blk_queue_io_stat(disk->queue))
+	if (!blk_queue_io_stat(disk->queue)) {
+		*start = 0;
 		return false;
+	}
 
 	*start = jiffies;
 	generic_start_io_acct(bio_data_dir(bio),