On 2012-08-29 19:23, Avi Kivity wrote: > On 08/28/2012 02:42 AM, Jan Kiszka wrote: >> >> Let's not talk about devices or MMIO dispatching. I think the problem is >> way more generic, and we will face it multiple times in QEMU. > > The problem exists outside qemu as well. It is one of the reasons for > the popularity of garbage collection IMO, and the use of reference > counting when GC is not available. > > This pattern is even documented in > Documentation/DocBook/kernel-locking.tmpl: > > @@ -104,12 +114,11 @@ > struct object *cache_find(int id) > { > struct object *obj; > - unsigned long flags; > > - spin_lock_irqsave(&cache_lock, flags); > + rcu_read_lock(); > obj = __cache_find(id); > if (obj) > object_get(obj); > - spin_unlock_irqrestore(&cache_lock, flags); > + rcu_read_unlock(); > > > Of course that doesn't mean we should use it, but at least it indicates > that it is a standard pattern. With MemoryRegion the pattern is > changed, since MemoryRegion is a thunk, not the object we're really > dispatching to.
We are dispatching according to the memory region (parameters, op handlers, opaques). If we end up in device object is not decided at this level. A memory region describes a dispatchable area - not to confuse with a device that may only partially be able to receive such requests. > >> Besides >> MMIO and PIO dispatching, it will haunt us for file or event handlers, >> any kind of callbacks etc. >> >> Context A Context B >> --------- --------- >> object = lookup() >> deregister(object) >> modify(object) -> invalid state >> ... use(object) >> modify(object) -> valid state >> register(object) >> >> And with "object" I'm not talking about QOM but any data structure. >> > > > Context B > --------- > rcu_read_lock() > object = lookup() > if (object) { > ref(object) > } > rcu_read_unlock() > > use(object) > > unref(object) > > (substitute locking scheme to conform to taste and/or dietary > restrictions) > + wait for refcount(object) == 0 in deregister(object). That's what I'm proposing. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux