Messages in this thread | | | Date | Tue, 9 Sep 2008 22:31:24 -0700 | From | "Paul Menage" <> | Subject | Re: [PATCH] cgroups: fix probable race with put_css_set[_taskexit] and find_css_set |
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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
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