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

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Message-ID: <20170801180017.GC20061@linux.intel.com>
Date:   Tue, 1 Aug 2017 12:00:17 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 01:48:48PM +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.
> 
> Fixes: 503a5e89b1de ("drivers/nvdimm/btt.c: remove btt_rw_page()")
> Fixes: 58100d6e735e ("drivers/nvdimm/pmem.c: remove pmem_rw_page()")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

This change looks fine:

Reviewed-by: Ross Zwisler <ross.zwisler@...ux.intel.com>

I believe the patches removing the btt_rw_page() and btt_rw_page() are on hold
until I can get some performance numbers to justify them.

Dan, do you want to take this as is, or do you want me to include it in my
larger rw_page() series if/when that gets revived?