Re: [PATCH] cgroups: fix probable race with put_css_set[_taskexit] and find_css_set

["Paul Menage" <menage@google.com>]

On Tue, Sep 9, 2008 at 10:01 PM, Greg KH <greg@kroah.com> wrote:
>
> What are you trying to solve here with this change?  I agree, it does
> seem a bit "chaotic" :)

There's a place in cgroups that uses kref_put() to release an object;
the release function *then* takes a write-lock and removes the object
from a lookup table; it could race with another thread that searches
the lookup table (while holding a read-lock) and does kref_get() on
the same object.

The current fix is for the release function to recheck inside the lock
that the object's refcount is still zero, and only actually
unlink/free it if so. And actually I've just realised that this isn't
actually even safe, since the thread that just acquired the object
could kref_put() it almost immediately, which would leave two threads
both trying to unlink/free the object.

The two solutions being considered are:

- add a kref_put_and_write_lock(), similar to atomic_dec_and_lock(),
which would ensure that the final refcount on the object was only
released inside the lock

- add a kref_get_if_not_zero(), which would prevent a lookup from
succeeding if another thread had just dropped the last reference on
the object.

Paul