On 09/19/2012 06:02 AM, liu ping fan wrote:
> Currently, cpu_physical_memory_rw() can be used directly or indirectly
> by mmio-dispatcher to access other devices' memory region. This can
> cause some problem when adopting device's private lock.
>
> Back ground refer to:
> http://lists.gnu.org/archive/html/qemu-devel/2012-09/msg01481.html
> For lazy, just refer to:
> http://lists.gnu.org/archive/html/qemu-devel/2012-09/msg01878.html
>
>
> --1st. the recursive lock of biglock.
> If we leave c_p_m_rw() as it is, ie, no lock inside. Then we can have
> the following (section of the whole call chain, and with
> private_lockA):
> lockA-mmio-dispatcher --> hold biglock -- >c_p_m_rw() --- >
> Before c_p_m_rw(), we drop private_lockA to anti the possibly of
> deadlock. But we can not anti the nested of this chain or calling to
> another lockB-mmio-dispatcher. So we can not avoid the possibility of
> nested lock of biglock. And another important factor is that we break
> the lock sequence: private_lock-->biglock.
> All of these require us to push biglock's holding into c_p_m_rw(), the
> wrapper can not give help.
I agree that this is unavoidable.
>
> --2nd. c_p_m_rw(), sync or async?
>
> IF we convert all of the device to be protected by refcount, then we can have
> //no big lock
> c_p_m_rw()
> {
> devB->ref++;
> {
> --------------------------------------->pushed onto another thread.
> lock_privatelock
> mr->ops->write();
> unlock_privatelock
> }
> wait_for_completion();
> devB->ref--;
> }
> This model can help c_p_m_rw() present as a SYNC API. But currently,
> we mix biglock and private lock together, and wait_for_completion()
> maybe block the release of big lock, which finally causes deadlock. So
> we can not simply rely on this model.
> Instead, we need to classify the calling scene into three cases:
> case1. lockA--dispatcher ---> lockB-dispatcher //can use
> async+completion model
> case2. lockA--dispatcher ---> biglock-dispatcher // sync, but can
> cause the nested lock of biglock
> case3. biglock-dispacher ---> lockB-dispatcher // async to avoid
> the lock sequence problem, (as to completion, it need to be placed
> outside the top level biglock, and it is hard to do so. Suggest to
> change to case 1. Or at present, just leave it async)
>
> This new model will require the biglock can be nested.
I think changing to an async model is too complicated. It's difficult
enough already. Isn't dropping private locks + recursive big locks
sufficient?
--
error compiling committee.c: too many arguments to function
