Re: RFC replace some locking of i_sem wiht atomic_t

[Al Viro <viro@ftp.linux.org.uk>]

On Sun, Apr 02, 2006 at 11:01:30AM -0700, Joshua Hudson wrote:
> Herein lies the problem with the current locking scheme:
> 1. rename locks target if it exists, but target may be created by
> link() immediately
> after the check&lock procedure.
> 2. The target of link() is completely unprotected.
 
3. You have failed to RTFS or RTFM.

> Against ext2, this can result in a corrupted filesystem (two directory
> entries with
> the same name) by a three-way race between two instances of link() and one
> unlink().

Not really.
 
> 1. Both instances of link are started with target being the same name
> in the same directory.
> 2. unlink() is started on a different name in the same directory.
> 3. link() 1 doesn't find a free slot in the first page, moves to the second.
>     *rescheduled before locking second page*
> 4. unlink() finds target in first page, removes it.
> 5. link() 2 finds free slot in first page, creates entry, finishes
> 6. link() 1 continues, finds space in second page, creates entry

And this is BS, since link() _does_ grab ->i_sem on directory it modifies.
So does unlink().