| Message ID | 20170801144534.2a1e1def29e68eb6c83e203c@linux-foundation.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Tue, Aug 01, 2017 at 02:45:34PM -0700, Andrew Morton wrote: > On Tue, 1 Aug 2017 13:48:48 +0200 Arnd Bergmann <arnd@arndb.de> 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. > > > > ... > > > > --- 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), > > Well that's sad. > > The future of btt-remove-btt_rw_page.patch and friends is shrouded in > mystery, but if we proceed that way then yes, I guess we'll need to > work around such gcc glitches. > > But let's not leave apparently-unneeded code in place without telling > people why it is in fact needed? Maybe it's just cleaner to initialize 'start' in all the callers, so we don't have a mysterious line and have to remember why it's there / comment it? I'll throw a patch like that in my series if/when I repost.
On Wed, Aug 2, 2017 at 12:23 AM, Ross Zwisler <ross.zwisler@linux.intel.com> wrote: > On Tue, Aug 01, 2017 at 02:45:34PM -0700, Andrew Morton wrote: >> On Tue, 1 Aug 2017 13:48:48 +0200 Arnd Bergmann <arnd@arndb.de> wrote: >> > --- 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), >> >> Well that's sad. >> >> The future of btt-remove-btt_rw_page.patch and friends is shrouded in >> mystery, but if we proceed that way then yes, I guess we'll need to >> work around such gcc glitches. >> >> But let's not leave apparently-unneeded code in place without telling >> people why it is in fact needed? > > Maybe it's just cleaner to initialize 'start' in all the callers, so we don't > have a mysterious line and have to remember why it's there / comment it? I considered that but decided that would be worse, since it shuts up more potential warnings about actual uninitialized use of the variable, and is slightly harder for the compiler to optimize away. You also end up having to add a comment in multiple places. Note that Andrew already added a comment when he applied my patch to his mmotm tree. Arnd
--- a/drivers/nvdimm/nd.h~nvdimm-avoid-bogus-wmaybe-uninitialized-warning-fix +++ a/drivers/nvdimm/nd.h @@ -393,7 +393,7 @@ static inline bool nd_iostat_start(struc struct gendisk *disk = bio->bi_bdev->bd_disk; if (!blk_queue_io_stat(disk->queue)) { - *start = 0; + *start = 0; /* Suppress bogus warning */ return false; }