Re: [PATCH] RDMA: remove null check after call container_of()

[Christophe JAILLET]

Le 17/05/2022 à 20:03, Jason Gunthorpe a écrit :
> 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

Crystal clear.
Thank you for the explanation.

CJ