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

[Jan Kiszka]

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.

Jan

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