On Tue, 2005-11-29 at 23:15 -0500, Jeff Moyer wrote:
> The patch only drops the semaphore if d_lookup finds the dentry and the
> dentry has a revalidate routine. I don't follow how you can end up with
> multiple dentries for the same file in this case.
>
> Sorry if I'm missing something obvious.
The inode->i_sem is what ensures that nobody can insert a new dentry
between the lookup of the cached dentry by d_lookup() and the call to
->lookup() (which instantiates a new dentry).
Imagine you have two separate processes that are doing a __lookup_hash()
of the same cached dentry. Now imagine that d_revalidate() of the dentry
fails.
Process 1 Process 2
__lookup_hash("/foo", "bar") __lookup_hash("/foo", "bar")
... ....
down(&inode->i_sem)
....
<enter cached_lookup("/foo","bar")>
....
dentry = d_lookup("/foo", "bar")
up(&inode->i_sem)
down(&inode->i_sem)
....
<enters cached_lookup("/foo","bar")>
d_revalidate(dentry) (fails) dentry = d_lookup("/foo","bar")
up(&inode->i_sem)
down(&inode->i_sem) d_revalidate(dentry); (fails)
...
<returns NULL to __lookup_hash>
....
dentry = d_alloc(parent,name)
->lookup(dentry) (instantiates "dentry")
up(&inode->i_sem)
down(&inode->i_sem)
....
<returns NULL to __lookup_hash>
....
dentry = d_alloc(parent,name)
->lookup(dentry) (instantiates "dentry")
....
Whoops. Suddenly you have called ->lookup() for 2 dentries that
represent the same filename in the same directory.
Cheers,
Trond
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