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    SubjectRe: 2.6.23-rc6-mm1: IPC: sleeping function called ...
    Jarek Poplawski wrote:
    > On 18-09-2007 16:55, Nadia Derbey wrote:
    > ...
    >
    >>Well, reviewing the code I found another place where the
    >>rcu_read_unlock() was missing.
    >>I'm so sorry for the inconvenience. It's true that I should have tested
    >>with CONFIG_PREEMPT=y :-(
    >>Now, the ltp tests pass even with this option set...
    >>
    >>In attachment you'll find a patch thhat
    >>1) adds the missing rcu_read_unlock()
    >>2) replaces Andrew's fix with a new one: the rcu_read_lock() is now
    >>taken in ipc_lock() / ipc_lock_by_ptr() and released in ipc_unlock(),
    >>exactly as it was done in the ref code.
    >
    >
    > BTW, probably I miss something, but I wonder, how this RCU is working
    > here. E.g. in msg.c do_msgsnd() there is:
    >
    > msq = msg_lock_check(ns, msqid);
    > ...
    >
    > msg_unlock(msq);
    > schedule();
    >
    > ipc_lock_by_ptr(&msq->q_perm);
    >
    > Since msq_lock_check() gets msq with ipc_lock_check() under
    > rcu_read_lock(), and then goes msg_unlock(msq) (i.e. ipc_unlock())
    > with rcu_read_unlock(), is it valid to use this with
    > ipc_lock_by_ptr() yet?

    Before Calling msg_unlock() they call ipc_rcu_getref() that increments a
    refcount in the rcu header for the msg structure. This guarantees that
    the the structure won't be freed before they relock it. Once the
    structure is relocked by ipc_lock_by_ptr(), they do the symmetric
    operation i.e. ipc_rcu_putref().

    Regards,
    Nadia
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