On Fri, Sep 21, 2007 at 12:11:15PM +0200, Nadia Derbey wrote:
> Jarek Poplawski wrote:
...
> >2. I'm not sure this refcounting with ipc_rcu_getref/putref is SMP
> >safe (memory barriers): it's not atomic, so locking is needed, but
> >e.g. in do_msgsnd() kern_ipc_perm lock is used for this, while
> >freeque() calls ipc_rcu_putref() with ipc_ids mutex only.
>
> OK, but freeque() freeary() and shm_destroy() are special cases:
> we have the following path:
>
> mutex_lock(ipc_ids.mutex)
> ...
> ipc_lock(ipcp)
> ... do whatever cleaning is needed ...
> ipc_rmid(ipcp)
> ipc_unlock(ipcp)
> ....
> ipc_rcu_putref(ipcp)
>
> Once the rmid has been done the ipc structure is considered as not
> visible anymore from the user side ==> any syscall called with the
> corresponding id will return invalid.
> The only thing that could happen is that this structure be reused for a
> newly allocated ipc structure. But this too cannot happen since we are
> under the ipc_ids mutex lock.
>
> Am I wrong?
I hope not! But, then it would be probably another logical trick:
ipc_rcu_getref/putref() seems to prevent kfreeing of a structure, so
if it's used in do_msgsnd() there should be a risk something can do
this kfree at this moment, and it seems freeque() is the only one,
which both: can do this and cares for this refcount. Then, e.g., if
any of them does ipc_rcu_getref() a bit later and sees old (cached)
value - kfree can be skipped forever. On the other hand, if there is
no such possibility because of other reasons, this all refcounting
looks like a code beautifier only...
Thanks,
Jarek P.
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