Re: [Qemu-devel] [PATCH 40/40] memory: add reference counting to FlatView

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-05-07 21:44, Paolo Bonzini wrote:
> Il 07/05/2013 20:00, Jan Kiszka ha scritto:
>> On 2013-05-07 16:17, Paolo Bonzini wrote:
>>> With this change, a FlatView can be used even after a concurrent
>>> update has replaced it.  Because we do not have RCU, we use a
>>> mutex to protect the small critical sections that read/write the
>>> as->current_map pointer.  Accesses to the FlatView can be done
>>> outside the mutex.
>>>
>>> If a MemoryRegion will be used after the FlatView is unref-ed (or after
>>> a MemoryListener callback is returned), a reference has to be added to
>>> that MemoryRegion.  For example, memory_region_find adds a reference to
>>> the MemoryRegion that it returns.
>>
>> For my understanding: Every lookup, e.g. triggered by address_space_rw,
>> will briefly reference the FlatView of that address space and drop that
>> reference again after referencing the target memory region.
>>
>> Provided that is true: If we run that lookup on an address space that
>> happens to be modified in parallel, the lookup may actually cause the
>> final deref and, thus, release of the FlatView - even if the target
>> memory region was totally unrelated to the ongoing change. That could
>> make a hot-path pay the price of an action a slow path caused. Not
>> really a beautiful concept. Even if the FlatView finalization is a
>> simple free() (that is the minimum), we would pull the memory allocator
>> into code paths that might try hard to keep a safe distance for the sake
>> of predictability.
> 
> All this is correct.  But I hope to get RCU in 1.6, otherwise we can use
> a bottom half.  In any case, this is obviously no worse than current
> code that requires the BQL (hence the lookup would serialize against the
> free).

Sure, not a regression. But I'd like to avoid that we are moving in the
potentially wrong direction /wrt final goal. And I'm very interested in
having increments that can be used in RT demonstration scenarios without
having to hack too much on the code.

Jan

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