Re: struct file reference at VFS level

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

On Fri, Apr 22, 2016 at 10:42:10AM -0400, Greg Troxel wrote:
> I still don't understand why this is about FUSE.  What if a file were
> opened without O_NONBLOCK and then the same file were opened with?  Why
> isn't that the same situation, and the same bug?

The filesystem below NetBSD VFS cannot make the difference between 
operations on the two file descriptors. Generic code above VFS 
implements the blocking / non blocking behavior and the filesystem 
does not have to handle that.

Now let us see the locking scenario:
- process 1 opens file foo O_RDWR
- process 2 opens file foo O_RDWR|O_NONBLOCK
- process 1 locks a part of the file
- process 2 atteemps to write to the locked part

With advisory locks (what we have in our filesystems),  the code above
VFS has no reason to block process 2 write operation. The filesystem
could notice there is a lock on the written area, but it cannot enforce
it because it does not distinguish the file descriptors. Hence the write
on locked area succeeds, but this is not a huge problem since the
locks are adivsory.

With mandatory locks, the filesystem must block process 2 when it
writes. In order to do that it must distinguish the file descriptors, 
but NetBSD VFS does not let it do that.

-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost