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    Subject[PATCH AUTOSEL for 4.15 101/102] IB/mlx5: revisit -Wmaybe-uninitialized warning
    Date
    From: Arnd Bergmann <arnd@arndb.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@arndb.de>
    Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.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
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