Messages in this thread Patch in this message | | | Date | Tue, 1 Aug 2017 14:45:34 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] nvdimm: avoid bogus -Wmaybe-uninitialized warning |
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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?
--- 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; } _
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