Patch "IB/mlx5: revisit -Wmaybe-uninitialized warning" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

IB/mlx5: revisit -Wmaybe-uninitialized warning

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ib-mlx5-revisit-wmaybe-uninitialized-warning.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Arnd Bergmann <[email protected]>
Date: Mon, 11 Dec 2017 12:45:44 +0100
Subject: IB/mlx5: revisit -Wmaybe-uninitialized warning

From: Arnd Bergmann <[email protected]>

[ 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.

---
 drivers/infiniband/hw/mlx5/mr.c |    3 +++
 1 file changed, 3 insertions(+)

@@ -1206,6 +1206,9 @@ struct ib_mr *mlx5_ib_reg_user_mr(struct
 	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);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/ib-mlx5-revisit-wmaybe-uninitialized-warning.patch queue-4.14/earlycon-add-reg-offset-to-physical-address-before-mapping.patch