On Wed, Sep 19, 2012 at 4:06 PM, Avi Kivity <a...@redhat.com> wrote: > 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? > I think that "dropping private locks + recursive big locks" just cover case 2. And most of the important, it dont describe case3 which break the rule of lock sequence "private-lock --> biglock". Scene: devA_lock-->(devX_with-biglock--->devB_lock). I just want to classify and post these cases to discuss. Maybe we can achieve without async.
Thanks and regards, pingfan > -- > error compiling committee.c: too many arguments to function
