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Re: [Qemu-devel] [big lock] Discussion about the convention of device's
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock |
Date: |
Wed, 19 Sep 2012 12:07:36 +0300 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 |
On 09/19/2012 12:00 PM, liu ping fan wrote:
> On Wed, Sep 19, 2012 at 4:06 PM, Avi Kivity <address@hidden> 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).
Why not? devA will drop its local lock, devX will retake the big lock
recursively, devB will take its local lock. In the end, we have biglock
-> devB.
> I just want to classify and post these cases to discuss. Maybe we can
> achieve without async.
--
error compiling committee.c: too many arguments to function
- [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, liu ping fan, 2012/09/18
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, Avi Kivity, 2012/09/19
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, liu ping fan, 2012/09/19
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock,
Avi Kivity <=
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, liu ping fan, 2012/09/19
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, Paolo Bonzini, 2012/09/19
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, liu ping fan, 2012/09/19
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, Avi Kivity, 2012/09/19
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, Jan Kiszka, 2012/09/19
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, Jan Kiszka, 2012/09/19
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, liu ping fan, 2012/09/20
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, Paolo Bonzini, 2012/09/20
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, liu ping fan, 2012/09/20
- Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock, Paolo Bonzini, 2012/09/20