linux-kernel - Re: Possible race in mlx5

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Date:   Sun, 20 Aug 2017 13:15:48 +0300
From:   Leon Romanovsky <leonro@...lanox.com>
To:     Anton Volkov <avolkov@...ras.ru>
Cc:     matanb@...lanox.com, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, ldv-project@...uxtesting.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Re: Possible race in mlx5_ib.ko

On Fri, Aug 18, 2017 at 06:17:15PM +0300, Anton Volkov wrote:
> Hello.
>
> While searching for races in the Linux kernel I've come across
> "drivers/infiniband/hw/mlx5/mlx5_ib.ko" module. Here are questions that I
> came up with while analyzing results. Lines are given using the info from
> Linux v4.12.
>
> Consider the following case:
>
> Thread 1:              Thread 2:
> size_write
> ->remove_keys          limit_write
>     ent->cur--;          if (ent->cur < ent->limit)
>     (mr.c: line 234)     (mr.c: line 335)
>                             err = add_keys(... ent->limit - ent->cur);
>
> If size_write and limit_write are able to work concurrently with the same
> ent then there is a possibility of a race between the accesses to ent->cur.
> In worst case in limit_write new keys wouldn't be added. Is it feasible from
> your point of view? If so, is it a benign race or a serious one?

Technically yes, it can race, but it gives a little value to protect
the ent->check with lock, because the limit_eize instruction is triggered
by debugfs write and is used for the debug.

Thanks

>
> Thank you for your time.
>
> -- Anton Volkov
> Linux Verification Center, ISPRAS
> web: http://linuxtesting.org
> e-mail: avolkov@...ras.ru
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