netdev - Re: BUILD_BUG_ON error in mlx5/core/pagealloc.c

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Date:   Mon, 10 Oct 2016 20:58:09 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: BUILD_BUG_ON error in mlx5/core/pagealloc.c

On Mon, Oct 10, 2016 at 8:17 PM, Saeed Mahameed
<saeedm@....mellanox.co.il> wrote:
>
>
> On Tuesday, October 11, 2016, Tom Herbert <tom@...bertland.com> wrote:
>>
>> On Mon, Oct 10, 2016 at 4:41 PM, Tom Herbert <tom@...bertland.com> wrote:
>> > I am hitting this in mlx5
>> >
>> > drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c: In function
>> > ‘reclaim_pages_cmd.clone.0’:
>> > drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c:346: error: call
>> > to ‘__compiletime_assert_346’ declared with attribute error:
>> > BUILD_BUG_ON failed: __mlx5_bit_off(manage_pages_out, pas[i]) % 64
>> > drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c: In function
>> > ‘give_pages’:
>> > drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c:291: error: call
>> > to ‘__compiletime_assert_291’ declared with attribute error:
>> > BUILD_BUG_ON failed: __mlx5_bit_off(manage_pages_in, pas[i]) % 64
>> >
>>
>> The expression in BUILD_BUG_ON expands to "((unsigned)(unsigned
>> long)(&(((struct mlx5_ifc_manage_pages_in_bits *)0)->pas[i]))) % 64".
>> The variable array index to pas makes this expression non-constant, I
>> imagine that is where the problem lies.
>>
>> > Bisecting points to:
>> >
>> > commit a533ed5e179cd15512d40282617909d3482a771c
>> > Author: Saeed Mahameed <saeedm@...lanox.com>
>> > Date:   Sun Jul 17 13:27:25 2016 +0300
>> >
>> >     net/mlx5: Pages management commands via mlx5 ifc
>
>
> Hi Tom,
> We know about this issue, it happens only with gcc4.4  or gcc 4.2 i don't
> remember exactly which.
> Most likely it is a gcc issue.
>
I don't believe that is the case. As I pointed out this is giving
BUILD_BUG_ON a non-constant expression. To fix this should just be a
matter of using a constant argument for the BUILD_BUG_ON. AFAICT
there's only two instances of MLX5_SET64 where this can be a problem.
I will post a fix shortly.

Tom

> Anyway, we are working on the issue and we will provide a fix or a W/A
> hopefully next week. This week most of the team are on vacation.
>
> Thanks
> Saeed.