On 02/04/2015 18:26, Stefan Hajnoczi wrote:
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.
Laszlo hit something very similar fairly easily with virtio-scsi (but
not virtio-blk!) on aarch64 hosts. Any attempt to debug it (ranging
from compilation with -O0 to tracing) made it disappear. A reliable
reproducer with qemu-io would be a dream...
Paolo