Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock

[Avi Kivity <avi@redhat.com>]

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?

-- 
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