Am 06.06.24 um 20:36 schrieb Stefan Hajnoczi: > On Wed, Jun 05, 2024 at 02:08:48PM +0200, Fiona Ebner wrote: >> The fact that the snapshot_save_job_bh() is scheduled in the main >> loop's qemu_aio_context AioContext means that it might get executed >> during a vCPU thread's aio_poll(). But saving of the VM state cannot >> happen while the guest or devices are active and can lead to assertion >> failures. See issue #2111 for two examples. Avoid the problem by >> scheduling the snapshot_save_job_bh() in the iohandler AioContext, >> which is not polled by vCPU threads. >> >> Solves Issue #2111. >> >> This change also solves the following issue: >> >> Since commit effd60c878 ("monitor: only run coroutine commands in >> qemu_aio_context"), the 'snapshot-save' QMP call would not respond >> right after starting the job anymore, but only after the job finished, >> which can take a long time. The reason is, because after commit >> effd60c878, do_qmp_dispatch_bh() runs in the iohandler AioContext. >> When do_qmp_dispatch_bh() wakes the qmp_dispatch() coroutine, the >> coroutine cannot be entered immediately anymore, but needs to be >> scheduled to the main loop's qemu_aio_context AioContext. But >> snapshot_save_job_bh() was scheduled first to the same AioContext and >> thus gets executed first. >> >> Buglink: https://gitlab.com/qemu-project/qemu/-/issues/2111 >> Signed-off-by: Fiona Ebner <f.eb...@proxmox.com> >> --- >> >> While initial smoke testing seems fine, I'm not familiar enough with >> this to rule out any pitfalls with the approach. Any reason why >> scheduling to the iohandler AioContext could be wrong here? > > If something waits for a BlockJob to finish using aio_poll() from > qemu_aio_context then a deadlock is possible since the iohandler_ctx > won't get a chance to execute. The only suspicious code path I found was > job_completed_txn_abort_locked() -> job_finish_sync_locked() but I'm not > sure whether it triggers this scenario. Please check that code path. >
Sorry, I don't understand. Isn't executing the scheduled BH the only additional progress that the iohandler_ctx needs to make compared to before the patch? How exactly would that cause issues when waiting for a BlockJob? Or do you mean something waiting for the SnapshotJob from qemu_aio_context before snapshot_save_job_bh had the chance to run? Best Regards, Fiona