On 10/24/2012 09:29 AM, Paolo Bonzini wrote: > Il 23/10/2012 18:09, Avi Kivity ha scritto: >>> But our interfaces had better support asynchronicity, and indeed they >>> do: after you write to the "eject" register, the "up" will show the >>> device as present until after destroy is done. This can be changed to >>> show the device as present only until after step 4 is done. >> >> Let's say we want to eject the hotplug hardware itself (just as an >> example). With refcounts, the callback that updates "up" will hold on >> to to it via refcounts. With stop_machine(), you need to cancel that >> callback, or wait for it somehow, or it can arrive after the >> stop_machine() and bite you. > > The callback that updates "up" is for the parent of the hotplug > hardware. There is nothing that has to be updated in the hotplug > hardware itself.
I meant, as an unrealistic example, hot-unplugging the bridge itself. So we have a callback that updates information in the bridge (up register state) being called asynchronously. A more realistic example would be hot-unplug of an HBA, then the block layer callback comes back to update the device. So stop_machine() would need to cancel all I/O and wait for I/O that cannot be cancelled. > > Updating the "up" register is the final part of isolate(), and runs > before the stop_machine(). The steps above can be further refined like > this: > > 4a. close all backends (also cancel or complete all pending I/O) ^ long latency > 4b. notify parent that we're done > 4ba. parent removes device from its bus > 4bb. parent notifies guest > 4bc. parent schedules stop_machine(qdev_free(child)) > 5. a bottom half calls stop_machine(qdev_free(child)) > > If unplugging a whole sub-tree, the parent can notify its own parent at > the end of 4b. Because the only purpose of stop_machine is to quiesce > subsystems not affected by step 4 (timer+memory, typically), > destructions can be done in any order and even intermixed with > executions of 4b for the parent. > > In the beginning the only asynchronous step would be 5. If the need > arises we can use continuation-passing to make all the preceding steps > asynchronous too. > Maybe my worry about long stop_machine latencies is premature. Everyone in the kernel hates it, but the kernel scales a lot more than qemu and is in a much better place wrt threading. -- error compiling committee.c: too many arguments to function
