Re: [RFC][PATCH 14/27] tricky: elevate write count files are open()ed

[Dave Hansen <haveblue@us.ibm.com>]

On Thu, 2006-06-08 at 12:54 +0200, Herbert Poetzl wrote:
> > There is also an elevated count around the vfs_create()
> > call in open_namei().  The count does not need to be
> > kept elevated all the way into the may_open() call because
> > after creation, the write bits of the acc_mode are cleared.
> > This keeps may_open() from ever failing.  Howver, this may
> > open one potential race where a change from a r/w to a r/o
> > mount could occur between the mnt_drop_write() and may_open()
> > allowing a user to obtain a r/w file on what is now a r/w
> 
> probably means 'read only mount'
> 
> what about using atomic_dec_and_test() to avoid
> having such cases (or a lock if required)? 

I thought about that, but it isn't that simple.  Such a case needs to
have controlled transitions away from when the counter is at '0'.  It is
a similar problem as i_writecount and put/deny_write_access(), which end
up having to use a spinlock.  I don't think we can do it with simple
atomics.  

-- Dave