Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752706AbdHAVpg (ORCPT ); Tue, 1 Aug 2017 17:45:36 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:60304 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752053AbdHAVpf (ORCPT ); Tue, 1 Aug 2017 17:45:35 -0400 Date: Tue, 1 Aug 2017 14:45:34 -0700 From: Andrew Morton To: Arnd Bergmann Cc: 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: <20170801144534.2a1e1def29e68eb6c83e203c@linux-foundation.org> In-Reply-To: <20170801114926.1171418-1-arnd@arndb.de> References: <20170801114926.1171418-1-arnd@arndb.de> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2285 Lines: 61 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? --- 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; } _