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Subject: Re: [BUG] net: mellanox: mlx4: possible deadlock in mlx4_xdp_set()
 and mlx4_en_reset_config()
To:     Tariq Toukan <ttoukan.linux@gmail.com>, tariqt@nvidia.com,
        davem@davemloft.net, kuba@kernel.org
Cc:     netdev@vger.kernel.org, linux-rdma@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <4a850d04-ed6a-5802-7038-a94ad0d466c5@gmail.com>
 <bde548df-cb80-966a-599c-3e1cb8639716@gmail.com>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <10008665-3a91-f95e-7ecb-994b8e8bcf55@gmail.com>
Date:   Wed, 9 Feb 2022 19:32:31 +0800
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On 2022/2/9 18:21, Tariq Toukan wrote:
>
>
> On 2/7/2022 5:16 PM, Jia-Ju Bai wrote:
>> Hello,
>>
>> My static analysis tool reports a possible deadlock in the mlx4 
>> driver in Linux 5.16:
>>
>
> Hi Jia-Ju,
> Thanks for your email.
>
> Which static analysis tool do you use? Is it standard one?

Hi Tariq,

Thanks for the reply and explanation :)
I developed this tool by myself, based on LLVM.

>
>> mlx4_xdp_set()
>>    mutex_lock(&mdev->state_lock); --> Line 2778 (Lock A)
>>    mlx4_en_try_alloc_resources()
>>      mlx4_en_alloc_resources()
>>        mlx4_en_destroy_tx_ring()
>>          mlx4_qp_free()
>>            wait_for_completion(&qp->free); --> Line 528 (Wait X)
>
> The refcount_dec_and_test(&qp->refcount)) in mlx4_qp_free() pairs with 
> refcount_set(&qp->refcount, 1); in mlx4_qp_alloc.
> mlx4_qp_event increases and decreasing the refcount while running 
> qp->event(qp, event_type); to protect it from being freed.
>
>>
>> mlx4_en_reset_config()
>>    mutex_lock(&mdev->state_lock); --> Line 3522 (Lock A)
>>    mlx4_en_try_alloc_resources()
>>      mlx4_en_alloc_resources()
>>        mlx4_en_destroy_tx_ring()
>>          mlx4_qp_free()
>>            complete(&qp->free); --> Line 527 (Wake X)
>>
>> When mlx4_xdp_set() is executed, "Wait X" is performed by holding 
>> "Lock A". If mlx4_en_reset_config() is executed at this time, "Wake 
>> X" cannot be performed to wake up "Wait X" in mlx4_xdp_set(), because 
>> "Lock A" has been already hold by mlx4_xdp_set(), causing a possible 
>> deadlock.
>>
>> I am not quite sure whether this possible problem is real and how to 
>> fix it if it is real.
>> Any feedback would be appreciated, thanks :)
>>
>
> Not possible.
> These are two different qps, maintaining two different instances of 
> refcount and complete, following the behavior I described above.

Okay, "there are two different qps" should be the reason of this false 
positive, and my tool cannot identify this reason in static analysis...


Best wishes,
Jia-Ju Bai