On 2012-08-29 19:13, Avi Kivity wrote: > On 08/27/2012 06:01 PM, Jan Kiszka wrote: >> On 2012-08-27 22:53, Avi Kivity wrote: >>> On 08/27/2012 12:38 PM, Jan Kiszka wrote: >>>>>> Even worse, apply >>>>>> restrictions on how the dispatched objects, the regions, have to be >>>>>> treated because of this. >>>>> >>>>> Please elaborate. >>>> >>>> The fact that you can't manipulate a memory region object arbitrarily >>>> after removing it from the mapping because you track the references to >>>> the object that the region points to, not the region itself. The region >>>> remains in use by the dispatching layer and potentially the called >>>> device, even after deregistration. >>> >>> That object will be a container_of() the region, usually literally but >>> sometimes only in spirit. Reference counting the regions means they >>> cannot be embedded into other objects any more. >> >> I cannot follow the logic of the last sentence. Reference counting just >> means that we track if there are users left, not necessarily that we >> perform asynchronous releases. We can simply wait for those users to >> complete. > > I don't see how. Suppose you add a reference count to MemoryRegion. > How do you delay its containing object's destructor from running? Do > you iterate over all member MemoryRegion and examine their reference counts?
memory_region_transaction_commit (or calls that trigger this) will have to wait. That is required as the caller may start fiddling with the region right afterward. > > Usually a reference count controls the lifetime of the reference counted > object, what are you suggesting here? To synchronize on reference count going to zero. Or all readers leaving the read-side critical sections. > >>> >>> We can probably figure out a way to flush out accesses. After switching >>> to rcu, for example, all we need is synchronize_rcu() in a >>> non-deadlocking place. But my bet is that it will not be needed. >> >> If you properly flush out accesses, you don't need to track at device >> level anymore - and mess with abstraction layers. That's my whole point. > > To flush out an access you need either rwlock_write_lock() or > synchronize_rcu() (depending on the implementation). But neither of > these can be run from an rcu read-side critical section or > rwlock_read_lock(). > > You could defer the change to a bottom half, but if the hardware demands > that the change be complete before returning, that doesn't work. Right, therefore synchronous transactions. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux
