Re: [Qemu-devel] [PATCH V3 10/11] vcpu: introduce lockmap

[Avi Kivity]

On 09/11/2012 12:44 PM, liu ping fan wrote:
> On Tue, Sep 11, 2012 at 4:35 PM, Avi Kivity <address@hidden> wrote:
>> On 09/11/2012 10:51 AM, Liu Ping Fan wrote:
>>> From: Liu Ping Fan <address@hidden>
>>>
>>> The func call chain can suffer from recursively hold
>>> qemu_mutex_lock_iothread. We introduce lockmap to record the
>>> lock depth.
>>
>> What is the root cause?  io handlers initiating I/O?
>>
> cpu_physical_memory_rw() can be called nested, and when called, it can
> be protected by no-lock/device lock/ big-lock.

Then we should look for a solution that is local to exec.c (and the
nested dispatch problem).  I think we can identify and fix all nested
dispatches (converting them to either async dma, or letting the memory
core do a single dispatch without indirection through I/O handlers).

> I think without big lock, io-dispatcher should face the same issue.
> As to main-loop, have not carefully consider, but at least, dma-helper
> will call cpu_physical_memory_rw() with big-lock.

DMA is inherently asynchronous, so we already drop the lock between
initiation and completion; we need to find a way to make it easy to use
the API without taking the lock while the transfer takes place.

-- 
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