Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752168AbdHAWXt (ORCPT ); Tue, 1 Aug 2017 18:23:49 -0400 Received: from mga11.intel.com ([192.55.52.93]:54900 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751656AbdHAWXs (ORCPT ); Tue, 1 Aug 2017 18:23:48 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.41,308,1498546800"; d="scan'208";a="885264022" Date: Tue, 1 Aug 2017 16:23:47 -0600 From: Ross Zwisler To: Andrew Morton Cc: Arnd Bergmann , Ross Zwisler , Dan Williams , Christoph Hellwig , Vishal Verma , Toshi Kani , Johannes Thumshirn , linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] nvdimm: avoid bogus -Wmaybe-uninitialized warning Message-ID: <20170801222347.GF20061@linux.intel.com> References: <20170801114926.1171418-1-arnd@arndb.de> <20170801144534.2a1e1def29e68eb6c83e203c@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170801144534.2a1e1def29e68eb6c83e203c@linux-foundation.org> User-Agent: Mutt/1.8.3 (2017-05-23) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2316 Lines: 54 On Tue, Aug 01, 2017 at 02:45:34PM -0700, Andrew Morton wrote: > On Tue, 1 Aug 2017 13:48:48 +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. > > > > ... > > > > --- 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.