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Message-ID: <bde548df-cb80-966a-599c-3e1cb8639716@gmail.com>
Date:   Wed, 9 Feb 2022 12:21:24 +0200
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Subject: Re: [BUG] net: mellanox: mlx4: possible deadlock in mlx4_xdp_set()
 and mlx4_en_reset_config()
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To:     Jia-Ju Bai <baijiaju1990@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>
From:   Tariq Toukan <ttoukan.linux@gmail.com>
In-Reply-To: <4a850d04-ed6a-5802-7038-a94ad0d466c5@gmail.com>
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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?

> 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.
> 
> Best wishes,
> Jia-Ju Bai

Thanks,
Tariq