linux-kernel - Possible race in mlx5

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Date:   Fri, 18 Aug 2017 18:17:15 +0300
From:   Anton Volkov <avolkov@...ras.ru>
To:     matanb@...lanox.com, leonro@...lanox.com
Cc:     linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Possible race in mlx5_ib.ko

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?

Thank you for your time.

-- Anton Volkov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: avolkov@...ras.ru