John Snow has reported that qemu-io can hang when the host is under heavy load. He made the following observations in gdb:
1. The program is sitting in aio_poll() (called by bdrv_prwv_co()) waiting for request completion. 2. The thread pool has a ThreadPoolElement with ->state == THREAD_DONE. The ThreadPoolElement should have been reaped by thread_pool_completion_bh() and its callback invoked. For some reason this didn't happen and the program is blocked in poll(2) waiting. This suggests a race condition in thread-pool.c or qemu_bh_schedule() (used to complete ThreadPoolElement from a QEMU event loop). I don't have a good theory why this happens yet. Just wanted to share in case someone else hits this problem. Stefan
