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

[Paolo Bonzini]

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).

Paolo