Re: Use of mmap_sem in __qbuf_userptr()

[Hans Verkuil <hverkuil@xs4all.nl>]

Hi Jan,

On 03/05/2015 03:27 PM, Jan Kara wrote:
>   Hello,
> 
>   so after a long pause I've got back to my simplification patches around
> get_user_pages(). After the simplification done by commit f035eb4e976ef5
> (videobuf2: fix lockdep warning) it seems unnecessary to take mmap_sem
> already when calling __qbuf_userptr(). As far as I understand what
> __qbuf_userptr() does, the only thing where mmap_sem is needed is for
> get_userptr and possibly put_userptr memops. So it should be possible to
> push mmap_sem locking down into these memops, shouldn't it? Or am I missing
> something in __qbuf_userptr() for which mmap_sem is also necessary?

No, you are correct. The mmap_sem can be pushed down, either to __qbuf_userptr
or all the way to the videobuf2-dma/vmalloc get/put_userptr ops.

> If I'm right, I can prepare patches to do that (and then on top of those
> rebase patches which will make v4l2 core use some mm helper functions so
> they don't have to care about details of mm locking, vmas, etc.).

That would be really nice.

Regards,

	Hans