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 -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux
