On Tue, Aug 01, 2017 at 02:45:34PM -0700, Andrew Morton wrote: > On Tue, 1 Aug 2017 13:48:48 +0200 Arnd Bergmann <[email protected]> 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.
