linux-kernel - Re: [PATCH] nvdimm: avoid bogus -Wmaybe-uninitialized warning

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Message-ID: <20170801222347.GF20061@linux.intel.com>
Date:   Tue, 1 Aug 2017 16:23:47 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Toshi Kani <toshi.kani@....com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvdimm: avoid bogus -Wmaybe-uninitialized warning

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@...db.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.