Re: Mapping range locking and related stuff

[Dave Chinner <david@fromorbit.com>]

On Fri, Sep 27, 2013 at 10:42:14PM +0200, Jan Kara wrote:
>   Hello,
> 
>   so recently I've spent some time rummaging in get_user_pages(), fault
> code etc. The use of mmap_sem is really messy in some places (like V4L
> drivers, infiniband,...). It is held over a deep & wide call chains and
> it's not clear what's protected by it, just in the middle of that is a call
> to get_user_pages(). Anyway that's mostly a side note.
> 
> The main issue I found is with the range locking itself. Consider someone
> doing:
>   fd = open("foo", O_RDWR);
>   base = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
>   write(fd, base, 4096);
> 
> The write() is an interesting way to do nothing but if the mapping range
> lock will be acquired early (like in generic_file_aio_write()), then this
> would deadlock because generic_perform_write() will try to fault in
> destination buffer and that will try to get the range lock for the same
> range again.

Quite frankly, I'd like to see EFAULT or EDEADLOCK returned to the
caller doing something like this. It's a stupid thing to do, and
while I beleive in giving people enough rope to hang themselves,
the contortions we are going through here to provide that rope
doesn't seem worthwhile at all.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com