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

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Message-Id: <20170801114926.1171418-1-arnd@arndb.de>
Date:   Tue,  1 Aug 2017 13:48:48 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        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: [PATCH] nvdimm: avoid bogus -Wmaybe-uninitialized warning

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>
---
 drivers/nvdimm/nd.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/nvdimm/nd.h b/drivers/nvdimm/nd.h
index e1b5715bd91f..64f79a156456 100644
--- 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),
-- 
2.9.0