linux-kernel - [PATCH AUTOSEL for 4.15 101/102] IB/mlx5: revisit -Wmaybe-uninitialized warning

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Message-ID: <20180303222318.26006-101-alexander.levin@microsoft.com>
Date:   Sat, 3 Mar 2018 22:25:08 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Arnd Bergmann <arnd@...db.de>, Jason Gunthorpe <jgg@...lanox.com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.15 101/102] IB/mlx5: revisit
 -Wmaybe-uninitialized warning

From: Arnd Bergmann <arnd@...db.de>

[ Upstream commit 1b19b95169cd52fe82cd442fec0b279fe25cc838 ]

A warning that I thought I had fixed before occasionally comes
back in rare randconfig builds (I found 7 instances in the last
100000 builds, originally it was much more frequent):

drivers/infiniband/hw/mlx5/mr.c: In function 'mlx5_ib_reg_user_mr':
drivers/infiniband/hw/mlx5/mr.c:1229:5: error: 'order' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  if (order <= mr_cache_max_order(dev)) {
     ^
drivers/infiniband/hw/mlx5/mr.c:1247:8: error: 'ncont' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/infiniband/hw/mlx5/mr.c:1247:8: error: 'page_shift' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/infiniband/hw/mlx5/mr.c:1260:2: error: 'npages' may be used uninitialized in this function [-Werror=maybe-uninitialized]

I've looked at all those findings again and noticed that they are all
with CONFIG_INFINIBAND_USER_MEM=n, which means ib_umem_get() returns
an error unconditionally and we never initialize or use those variables.
This triggers a condition in gcc iff mr_umem_get() is partially but not
entirely inlined, which in turn depends on the exact combination of
optimization settings. This is a known problem with gcc, with no
easy solution in the compiler, so this adds another workaround that
should be more reliable than my previous attempt.

Returning an error from mlx5_ib_reg_user_mr() earlier means that we
can completely bypass the logic that caused the warning, the compiler
can now see that the variable is never accessed.

Fixes: 14ab8896f5d9 ("IB/mlx5: avoid bogus -Wmaybe-uninitialized warning")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Jason Gunthorpe <jgg@...lanox.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 drivers/infiniband/hw/mlx5/mr.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/infiniband/hw/mlx5/mr.c b/drivers/infiniband/hw/mlx5/mr.c
index d109fe8290a7..556e015678de 100644
--- a/drivers/infiniband/hw/mlx5/mr.c
+++ b/drivers/infiniband/hw/mlx5/mr.c
@@ -1206,6 +1206,9 @@ struct ib_mr *mlx5_ib_reg_user_mr(struct ib_pd *pd, u64 start, u64 length,
 	int err;
 	bool use_umr = true;
 
+	if (!IS_ENABLED(CONFIG_INFINIBAND_USER_MEM))
+		return ERR_PTR(-EINVAL);
+
 	mlx5_ib_dbg(dev, "start 0x%llx, virt_addr 0x%llx, length 0x%llx, access_flags 0x%x\n",
 		    start, virt_addr, length, access_flags);
 
-- 
2.14.1