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Message-ID: <bf354eb3-2149-5faa-8ada-baa4b2f4ad09@meizu.com>
Date:   Wed, 18 May 2022 09:55:37 +0800
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Subject: Re: [PATCH] RDMA: remove null check after call container_of()
To:     Jason Gunthorpe <jgg@ziepe.ca>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>
CC:     Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        "Leon Romanovsky" <leon@kernel.org>, <linux-rdma@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
References: <1652751208-23211-1-git-send-email-baihaowen@meizu.com>
 <20220517121646.GE63055@ziepe.ca>
 <142a9c03-574f-adcf-bc4d-bb2a25c01e88@wanadoo.fr>
 <20220517180303.GK63055@ziepe.ca>
From:   baihaowen <baihaowen@meizu.com>
In-Reply-To: <20220517180303.GK63055@ziepe.ca>
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在 2022/5/18 上午2:03, Jason Gunthorpe 写道:
> On Tue, May 17, 2022 at 07:54:38PM +0200, Christophe JAILLET wrote:
>> Le 17/05/2022 à 14:16, Jason Gunthorpe a écrit :
>>> On Tue, May 17, 2022 at 09:33:28AM +0800, Haowen Bai wrote:
>>>> container_of() will never return NULL, so remove useless code.
>>> It is confusing here, but it can be null.
>> Hi,
>>
>> out of curiosity, can you elaborate how it can be NULL?
> It is guarented/required that that container_of is a 0 offset. The
> normal usage of the ib_alloc_device macro:
>
> #define ib_alloc_device(drv_struct, member)                                    \
> 	container_of(_ib_alloc_device(sizeof(struct drv_struct) +              \
> 				      BUILD_BUG_ON_ZERO(offsetof(              \
> 					      struct drv_struct, member))),    \
> 		     struct drv_struct, member)
>
> Enforces this property with a BUILD_BUG_ON
>
> So, if the input pointer to container_of is reliably NULL or ERR_PTR
> then the output pointer will be the same.
>
> The rvt code here open codes the call because it is a mid-layer and
> the sizeof() calculation above is not correct for it.
>
> Jason
Thank you for the explanation.   : )

-- 
Haowen Bai